Fix resolving symbols for section-relative relocations

Also fixes MIPS `j` handling when jumping within the function.

Reworks `ObjReloc` struct to be a little more sensible.

.github/workflows/build.yaml

@@ -110,11 +110,6 @@ jobs:
             name: linux-aarch64
             build: zigbuild
             features: default
-          - platform: ubuntu-latest
-            target: armv7-unknown-linux-musleabi
-            name: linux-armv7l
-            build: zigbuild
-            features: default
           - platform: windows-latest
             target: i686-pc-windows-msvc
             name: windows-x86
@@ -147,7 +142,11 @@ jobs:
         uses: actions/checkout@v4
       - name: Install cargo-zigbuild
         if: matrix.build == 'zigbuild'
-        run: pip install ziglang==0.13.0 cargo-zigbuild==0.19.1
+        run: |
+          python3 -m venv .venv
+          . .venv/bin/activate
+          echo PATH=$PATH >> $GITHUB_ENV
+          pip install ziglang==0.13.0 cargo-zigbuild==0.19.1
       - name: Setup Rust toolchain
         uses: dtolnay/rust-toolchain@stable
         with:

Cargo.toml

@@ -13,7 +13,7 @@ strip = "debuginfo"
 codegen-units = 1
 
 [workspace.package]
-version = "2.1.0"
+version = "2.3.2"
 authors = ["Luke Street <luke@street.dev>"]
 edition = "2021"
 license = "MIT OR Apache-2.0"

config.schema.json

@@ -133,6 +133,13 @@
         },
         "metadata": {
           "ref": "#/$defs/metadata"
+        },
+        "symbol_mappings": {
+          "type": "object",
+          "description": "Manual symbol mappings from target to base.",
+          "additionalProperties": {
+            "type": "string"
+          }
         }
       }
     },

deny.toml

@@ -70,7 +70,6 @@ feature-depth = 1
 # A list of advisory IDs to ignore. Note that ignored advisories will still
 # output a note when they are encountered.
 ignore = [
-    "RUSTSEC-2024-0370",
     #{ id = "RUSTSEC-0000-0000", reason = "you can specify a reason the advisory is ignored" },
     #"a-crate-that-is-yanked@0.1.1", # you can also ignore yanked crate versions if you wish
     #{ crate = "a-crate-that-is-yanked@0.1.1", reason = "you can specify why you are ignoring the yanked crate" },
@@ -240,7 +239,7 @@ allow-git = []
 
 [sources.allow-org]
 # github.com organizations to allow git sources for
-github = ["encounter"]
+github = ["notify-rs"]
 # gitlab.com organizations to allow git sources for
 gitlab = []
 # bitbucket.org organizations to allow git sources for

objdiff-cli/Cargo.toml

@@ -28,3 +28,6 @@ supports-color = "3.0"
 time = { version = "0.3", features = ["formatting", "local-offset"] }
 tracing = "0.1"
 tracing-subscriber = { version = "0.3", features = ["env-filter"] }
+
+[target.'cfg(target_env = "musl")'.dependencies]
+mimalloc = "0.1"

objdiff-cli/src/cmd/diff.rs

@@ -102,26 +102,32 @@ pub fn run(args: Args) -> Result<()> {
                     let unit_path =
                         PathBuf::from_str(u).ok().and_then(|p| fs::canonicalize(p).ok());
 
-                    let Some(object) = project_config.objects.iter_mut().find_map(|obj| {
-                        if obj.name.as_deref() == Some(u) {
+                    let Some(object) = project_config
+                        .units
+                        .as_deref_mut()
+                        .unwrap_or_default()
+                        .iter_mut()
+                        .find_map(|obj| {
+                            if obj.name.as_deref() == Some(u) {
+                                resolve_paths(obj);
+                                return Some(obj);
+                            }
+
+                            let up = unit_path.as_deref()?;
+
                             resolve_paths(obj);
-                            return Some(obj);
-                        }
 
-                        let up = unit_path.as_deref()?;
+                            if [&obj.base_path, &obj.target_path]
+                                .into_iter()
+                                .filter_map(|p| p.as_ref().and_then(|p| p.canonicalize().ok()))
+                                .any(|p| p == up)
+                            {
+                                return Some(obj);
+                            }
 
-                        resolve_paths(obj);
-
-                        if [&obj.base_path, &obj.target_path]
-                            .into_iter()
-                            .filter_map(|p| p.as_ref().and_then(|p| p.canonicalize().ok()))
-                            .any(|p| p == up)
-                        {
-                            return Some(obj);
-                        }
-
-                        None
-                    }) else {
+                            None
+                        })
+                    else {
                         bail!("Unit not found: {}", u)
                     };
 
@@ -129,7 +135,13 @@ pub fn run(args: Args) -> Result<()> {
                 } else if let Some(symbol_name) = &args.symbol {
                     let mut idx = None;
                     let mut count = 0usize;
-                    for (i, obj) in project_config.objects.iter_mut().enumerate() {
+                    for (i, obj) in project_config
+                        .units
+                        .as_deref_mut()
+                        .unwrap_or_default()
+                        .iter_mut()
+                        .enumerate()
+                    {
                         resolve_paths(obj);
 
                         if obj
@@ -148,7 +160,7 @@ pub fn run(args: Args) -> Result<()> {
                     }
                     match (count, idx) {
                         (0, None) => bail!("Symbol not found: {}", symbol_name),
-                        (1, Some(i)) => &mut project_config.objects[i],
+                        (1, Some(i)) => &mut project_config.units_mut()[i],
                         (2.., Some(_)) => bail!(
                             "Multiple instances of {} were found, try specifying a unit",
                             symbol_name
@@ -303,7 +315,7 @@ fn find_function(obj: &ObjInfo, name: &str) -> Option<SymbolRef> {
     None
 }
 
-#[allow(dead_code)]
+#[expect(dead_code)]
 struct FunctionDiffUi {
     relax_reloc_diffs: bool,
     left_highlight: HighlightKind,
@@ -758,7 +770,7 @@ impl FunctionDiffUi {
         self.scroll_y += self.per_page / if half { 2 } else { 1 };
     }
 
-    #[allow(clippy::too_many_arguments)]
+    #[expect(clippy::too_many_arguments)]
     fn print_sym(
         &self,
         out: &mut Text<'static>,

objdiff-cli/src/cmd/report.rs

@@ -94,7 +94,7 @@ fn generate(args: GenerateArgs) -> Result<()> {
     };
     info!(
         "Generating report for {} units (using {} threads)",
-        project.objects.len(),
+        project.units().len(),
         if args.deduplicate { 1 } else { rayon::current_num_threads() }
     );
 
@@ -103,7 +103,7 @@ fn generate(args: GenerateArgs) -> Result<()> {
     let mut existing_functions: HashSet<String> = HashSet::new();
     if args.deduplicate {
         // If deduplicating, we need to run single-threaded
-        for object in &mut project.objects {
+        for object in project.units.as_deref_mut().unwrap_or_default() {
             if let Some(unit) = report_object(
                 object,
                 project_dir,
@@ -116,7 +116,9 @@ fn generate(args: GenerateArgs) -> Result<()> {
         }
     } else {
         let vec = project
-            .objects
+            .units
+            .as_deref_mut()
+            .unwrap_or_default()
             .par_iter_mut()
             .map(|object| {
                 report_object(
@@ -132,7 +134,7 @@ fn generate(args: GenerateArgs) -> Result<()> {
     }
     let measures = units.iter().flat_map(|u| u.measures.into_iter()).collect();
     let mut categories = Vec::new();
-    for category in &project.progress_categories {
+    for category in project.progress_categories() {
         categories.push(ReportCategory {
             id: category.id.clone(),
             name: category.name.clone(),
@@ -199,7 +201,7 @@ fn report_object(
             .unwrap_or_default(),
         auto_generated: object.metadata.as_ref().and_then(|m| m.auto_generated),
     };
-    let mut measures = Measures::default();
+    let mut measures = Measures { total_units: 1, ..Default::default() };
     let mut sections = vec![];
     let mut functions = vec![];
 
@@ -280,6 +282,7 @@ fn report_object(
     if metadata.complete.unwrap_or(false) {
         measures.complete_code = measures.total_code;
         measures.complete_data = measures.total_data;
+        measures.complete_units = 1;
     }
     measures.calc_fuzzy_match_percent();
     measures.calc_matched_percent();

objdiff-cli/src/main.rs

@@ -2,6 +2,12 @@ mod argp_version;
 mod cmd;
 mod util;
 
+// musl's allocator is very slow, so use mimalloc when targeting musl.
+// Otherwise, use the system allocator to avoid extra code size.
+#[cfg(target_env = "musl")]
+#[global_allocator]
+static ALLOC: mimalloc::MiMalloc = mimalloc::MiMalloc;
+
 use std::{env, ffi::OsStr, fmt::Display, path::PathBuf, str::FromStr};
 
 use anyhow::{Error, Result};

objdiff-core/Cargo.toml

@@ -17,8 +17,8 @@ crate-type = ["cdylib", "rlib"]
 
 [features]
 all = ["config", "dwarf", "mips", "ppc", "x86", "arm", "bindings"]
-any-arch = [] # Implicit, used to check if any arch is enabled
-config = ["globset", "semver", "serde_json", "serde_yaml"]
+any-arch = ["bimap"] # Implicit, used to check if any arch is enabled
+config = ["bimap", "globset", "semver", "serde_json", "serde_yaml"]
 dwarf = ["gimli"]
 mips = ["any-arch", "rabbitizer"]
 ppc = ["any-arch", "cwdemangle", "cwextab", "ppc750cl"]
@@ -32,6 +32,7 @@ features = ["all"]
 
 [dependencies]
 anyhow = "1.0"
+bimap = { version = "0.6", features = ["serde"], optional = true }
 byteorder = "1.5"
 filetime = "0.2"
 flagset = "0.4"
@@ -60,7 +61,7 @@ gimli = { version = "0.31", default-features = false, features = ["read-all"], o
 
 # ppc
 cwdemangle = { version = "1.0", optional = true }
-cwextab = { version = "0.3", optional = true }
+cwextab = { version = "1.0.2", optional = true }
 ppc750cl = { version = "0.3", optional = true }
 
 # mips

objdiff-core/protos/report.proto

@@ -32,6 +32,10 @@ message Measures {
   uint64 complete_data = 13;
   // Completed (or "linked") data percent
   float complete_data_percent = 14;
+  // Total number of units
+  uint32 total_units = 15;
+  // Completed (or "linked") units
+  uint32 complete_units = 16;
 }
 
 // Project progress report

objdiff-core/src/arch/mips.rs

@@ -83,7 +83,7 @@ impl ObjArch for ObjArchMips {
         &self,
         address: u64,
         code: &[u8],
-        _section_index: usize,
+        section_index: usize,
         relocations: &[ObjReloc],
         line_info: &BTreeMap<u64, u32>,
         config: &DiffObjConfig,
@@ -140,11 +140,18 @@ impl ObjArch for ObjArchMips {
                     | OperandType::cpu_label
                     | OperandType::cpu_branch_target_label => {
                         if let Some(reloc) = reloc {
-                            if matches!(&reloc.target_section, Some(s) if s == ".text")
-                                && reloc.target.address > start_address
-                                && reloc.target.address < end_address
+                            // If the relocation target is within the current function, we can
+                            // convert it into a relative branch target. Note that we check
+                            // target_address > start_address instead of >= so that recursive
+                            // tail calls are not considered branch targets.
+                            let target_address =
+                                reloc.target.address.checked_add_signed(reloc.addend);
+                            if reloc.target.orig_section_index == Some(section_index)
+                                && matches!(target_address, Some(addr) if addr > start_address && addr < end_address)
                             {
-                                args.push(ObjInsArg::BranchDest(reloc.target.address));
+                                let target_address = target_address.unwrap();
+                                args.push(ObjInsArg::BranchDest(target_address));
+                                branch_dest = Some(target_address);
                             } else {
                                 push_reloc(&mut args, reloc)?;
                                 branch_dest = None;

objdiff-core/src/bindings/diff.rs

@@ -70,9 +70,13 @@ impl FunctionDiff {
         //     let (_section, symbol) = object.section_symbol(symbol_ref);
         //     Symbol::from(symbol)
         // });
-        let instructions = symbol_diff.instructions.iter().map(InstructionDiff::from).collect();
+        let instructions = symbol_diff
+            .instructions
+            .iter()
+            .map(|ins_diff| InstructionDiff::new(object, ins_diff))
+            .collect();
         Self {
-            symbol: Some(Symbol::from(symbol)),
+            symbol: Some(Symbol::new(symbol)),
             // diff_symbol,
             instructions,
             match_percent: symbol_diff.match_percent,
@@ -90,8 +94,8 @@ impl DataDiff {
     }
 }
 
-impl<'a> From<&'a ObjSymbol> for Symbol {
-    fn from(value: &'a ObjSymbol) -> Self {
+impl Symbol {
+    pub fn new(value: &ObjSymbol) -> Self {
         Self {
             name: value.name.to_string(),
             demangled_name: value.demangled_name.clone(),
@@ -122,29 +126,29 @@ fn symbol_flags(value: ObjSymbolFlagSet) -> u32 {
     flags
 }
 
-impl<'a> From<&'a ObjIns> for Instruction {
-    fn from(value: &'a ObjIns) -> Self {
+impl Instruction {
+    pub fn new(object: &ObjInfo, instruction: &ObjIns) -> Self {
         Self {
-            address: value.address,
-            size: value.size as u32,
-            opcode: value.op as u32,
-            mnemonic: value.mnemonic.clone(),
-            formatted: value.formatted.clone(),
-            arguments: value.args.iter().map(Argument::from).collect(),
-            relocation: value.reloc.as_ref().map(Relocation::from),
-            branch_dest: value.branch_dest,
-            line_number: value.line,
-            original: value.orig.clone(),
+            address: instruction.address,
+            size: instruction.size as u32,
+            opcode: instruction.op as u32,
+            mnemonic: instruction.mnemonic.clone(),
+            formatted: instruction.formatted.clone(),
+            arguments: instruction.args.iter().map(Argument::new).collect(),
+            relocation: instruction.reloc.as_ref().map(|reloc| Relocation::new(object, reloc)),
+            branch_dest: instruction.branch_dest,
+            line_number: instruction.line,
+            original: instruction.orig.clone(),
         }
     }
 }
 
-impl<'a> From<&'a ObjInsArg> for Argument {
-    fn from(value: &'a ObjInsArg) -> Self {
+impl Argument {
+    pub fn new(value: &ObjInsArg) -> Self {
         Self {
             value: Some(match value {
                 ObjInsArg::PlainText(s) => argument::Value::PlainText(s.to_string()),
-                ObjInsArg::Arg(v) => argument::Value::Argument(ArgumentValue::from(v)),
+                ObjInsArg::Arg(v) => argument::Value::Argument(ArgumentValue::new(v)),
                 ObjInsArg::Reloc => argument::Value::Relocation(ArgumentRelocation {}),
                 ObjInsArg::BranchDest(dest) => argument::Value::BranchDest(*dest),
             }),
@@ -152,8 +156,8 @@ impl<'a> From<&'a ObjInsArg> for Argument {
     }
 }
 
-impl From<&ObjInsArgValue> for ArgumentValue {
-    fn from(value: &ObjInsArgValue) -> Self {
+impl ArgumentValue {
+    pub fn new(value: &ObjInsArgValue) -> Self {
         Self {
             value: Some(match value {
                 ObjInsArgValue::Signed(v) => argument_value::Value::Signed(*v),
@@ -164,42 +168,39 @@ impl From<&ObjInsArgValue> for ArgumentValue {
     }
 }
 
-impl<'a> From<&'a ObjReloc> for Relocation {
-    fn from(value: &ObjReloc) -> Self {
+impl Relocation {
+    pub fn new(object: &ObjInfo, reloc: &ObjReloc) -> Self {
         Self {
-            r#type: match value.flags {
+            r#type: match reloc.flags {
                 object::RelocationFlags::Elf { r_type } => r_type,
                 object::RelocationFlags::MachO { r_type, .. } => r_type as u32,
                 object::RelocationFlags::Coff { typ } => typ as u32,
                 object::RelocationFlags::Xcoff { r_rtype, .. } => r_rtype as u32,
                 _ => unreachable!(),
             },
-            type_name: String::new(), // TODO
-            target: Some(RelocationTarget::from(&value.target)),
+            type_name: object.arch.display_reloc(reloc.flags).into_owned(),
+            target: Some(RelocationTarget {
+                symbol: Some(Symbol::new(&reloc.target)),
+                addend: reloc.addend,
+            }),
         }
     }
 }
 
-impl<'a> From<&'a ObjSymbol> for RelocationTarget {
-    fn from(value: &'a ObjSymbol) -> Self {
-        Self { symbol: Some(Symbol::from(value)), addend: value.addend }
-    }
-}
-
-impl<'a> From<&'a ObjInsDiff> for InstructionDiff {
-    fn from(value: &'a ObjInsDiff) -> Self {
+impl InstructionDiff {
+    pub fn new(object: &ObjInfo, instruction_diff: &ObjInsDiff) -> Self {
         Self {
-            instruction: value.ins.as_ref().map(Instruction::from),
-            diff_kind: DiffKind::from(value.kind) as i32,
-            branch_from: value.branch_from.as_ref().map(InstructionBranchFrom::from),
-            branch_to: value.branch_to.as_ref().map(InstructionBranchTo::from),
-            arg_diff: value.arg_diff.iter().map(ArgumentDiff::from).collect(),
+            instruction: instruction_diff.ins.as_ref().map(|ins| Instruction::new(object, ins)),
+            diff_kind: DiffKind::from(instruction_diff.kind) as i32,
+            branch_from: instruction_diff.branch_from.as_ref().map(InstructionBranchFrom::new),
+            branch_to: instruction_diff.branch_to.as_ref().map(InstructionBranchTo::new),
+            arg_diff: instruction_diff.arg_diff.iter().map(ArgumentDiff::new).collect(),
         }
     }
 }
 
-impl From<&Option<ObjInsArgDiff>> for ArgumentDiff {
-    fn from(value: &Option<ObjInsArgDiff>) -> Self {
+impl ArgumentDiff {
+    pub fn new(value: &Option<ObjInsArgDiff>) -> Self {
         Self { diff_index: value.as_ref().map(|v| v.idx as u32) }
     }
 }
@@ -228,8 +229,8 @@ impl From<ObjDataDiffKind> for DiffKind {
     }
 }
 
-impl<'a> From<&'a ObjInsBranchFrom> for InstructionBranchFrom {
-    fn from(value: &'a ObjInsBranchFrom) -> Self {
+impl InstructionBranchFrom {
+    pub fn new(value: &ObjInsBranchFrom) -> Self {
         Self {
             instruction_index: value.ins_idx.iter().map(|&x| x as u32).collect(),
             branch_index: value.branch_idx as u32,
@@ -237,8 +238,8 @@ impl<'a> From<&'a ObjInsBranchFrom> for InstructionBranchFrom {
     }
 }
 
-impl<'a> From<&'a ObjInsBranchTo> for InstructionBranchTo {
-    fn from(value: &'a ObjInsBranchTo) -> Self {
+impl InstructionBranchTo {
+    pub fn new(value: &ObjInsBranchTo) -> Self {
         Self { instruction_index: value.ins_idx as u32, branch_index: value.branch_idx as u32 }
     }
 }

objdiff-core/src/bindings/report.rs

@@ -8,9 +8,10 @@ use serde_json::error::Category;
 include!(concat!(env!("OUT_DIR"), "/objdiff.report.rs"));
 include!(concat!(env!("OUT_DIR"), "/objdiff.report.serde.rs"));
 
-pub const REPORT_VERSION: u32 = 1;
+pub const REPORT_VERSION: u32 = 2;
 
 impl Report {
+    /// Attempts to parse the report as binary protobuf or JSON.
     pub fn parse(data: &[u8]) -> Result<Self> {
         if data.is_empty() {
             bail!(std::io::Error::from(std::io::ErrorKind::UnexpectedEof));
@@ -25,6 +26,7 @@ impl Report {
         Ok(report)
     }
 
+    /// Attempts to parse the report as JSON, migrating from the legacy report format if necessary.
     fn from_json(bytes: &[u8]) -> Result<Self, serde_json::Error> {
         match serde_json::from_slice::<Self>(bytes) {
             Ok(report) => Ok(report),
@@ -43,16 +45,23 @@ impl Report {
         }
     }
 
+    /// Migrates the report to the latest version.
+    /// Fails if the report version is newer than supported.
     pub fn migrate(&mut self) -> Result<()> {
         if self.version == 0 {
             self.migrate_v0()?;
         }
+        if self.version == 1 {
+            self.migrate_v1()?;
+        }
         if self.version != REPORT_VERSION {
             bail!("Unsupported report version: {}", self.version);
         }
         Ok(())
     }
 
+    /// Adds `complete_code`, `complete_data`, `complete_code_percent`, and `complete_data_percent`
+    /// to measures, and sets `progress_categories` in unit metadata.
     fn migrate_v0(&mut self) -> Result<()> {
         let Some(measures) = &mut self.measures else {
             bail!("Missing measures in report");
@@ -61,15 +70,16 @@ impl Report {
             let Some(unit_measures) = &mut unit.measures else {
                 bail!("Missing measures in report unit");
             };
-            let Some(metadata) = &mut unit.metadata else {
-                bail!("Missing metadata in report unit");
+            let mut complete = false;
+            if let Some(metadata) = &mut unit.metadata {
+                if metadata.module_name.is_some() || metadata.module_id.is_some() {
+                    metadata.progress_categories = vec!["modules".to_string()];
+                } else {
+                    metadata.progress_categories = vec!["dol".to_string()];
+                }
+                complete = metadata.complete.unwrap_or(false);
             };
-            if metadata.module_name.is_some() || metadata.module_id.is_some() {
-                metadata.progress_categories = vec!["modules".to_string()];
-            } else {
-                metadata.progress_categories = vec!["dol".to_string()];
-            }
-            if metadata.complete.unwrap_or(false) {
+            if complete {
                 unit_measures.complete_code = unit_measures.total_code;
                 unit_measures.complete_data = unit_measures.total_data;
                 unit_measures.complete_code_percent = 100.0;
@@ -84,10 +94,42 @@ impl Report {
             measures.complete_data += unit_measures.complete_data;
         }
         measures.calc_matched_percent();
+        self.calculate_progress_categories();
         self.version = 1;
         Ok(())
     }
 
+    /// Adds `total_units` and `complete_units` to measures.
+    fn migrate_v1(&mut self) -> Result<()> {
+        let Some(total_measures) = &mut self.measures else {
+            bail!("Missing measures in report");
+        };
+        for unit in &mut self.units {
+            let Some(measures) = &mut unit.measures else {
+                bail!("Missing measures in report unit");
+            };
+            let complete = unit.metadata.as_ref().and_then(|m| m.complete).unwrap_or(false) as u32;
+            let progress_categories =
+                unit.metadata.as_ref().map(|m| m.progress_categories.as_slice()).unwrap_or(&[]);
+            measures.total_units = 1;
+            measures.complete_units = complete;
+            total_measures.total_units += 1;
+            total_measures.complete_units += complete;
+            for id in progress_categories {
+                if let Some(category) = self.categories.iter_mut().find(|c| &c.id == id) {
+                    let Some(measures) = &mut category.measures else {
+                        bail!("Missing measures in category");
+                    };
+                    measures.total_units += 1;
+                    measures.complete_units += complete;
+                }
+            }
+        }
+        self.version = 2;
+        Ok(())
+    }
+
+    /// Calculate progress categories based on unit metadata.
     pub fn calculate_progress_categories(&mut self) {
         for unit in &self.units {
             let Some(metadata) = unit.metadata.as_ref() else {
@@ -242,6 +284,8 @@ impl AddAssign for Measures {
         self.matched_functions += other.matched_functions;
         self.complete_code += other.complete_code;
         self.complete_data += other.complete_data;
+        self.total_units += other.total_units;
+        self.complete_units += other.complete_units;
     }
 }
 

objdiff-core/src/config/mod.rs

@@ -1,77 +1,100 @@
 use std::{
+    fs,
     fs::File,
-    io::Read,
+    io::{BufReader, BufWriter, Read},
     path::{Path, PathBuf},
 };
 
 use anyhow::{anyhow, Context, Result};
+use bimap::BiBTreeMap;
 use filetime::FileTime;
 use globset::{Glob, GlobSet, GlobSetBuilder};
 
-#[inline]
-fn bool_true() -> bool { true }
-
-#[derive(Default, Clone, serde::Deserialize)]
+#[derive(Default, Clone, serde::Serialize, serde::Deserialize)]
 pub struct ProjectConfig {
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub min_version: Option<String>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub custom_make: Option<String>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub custom_args: Option<Vec<String>>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub target_dir: Option<PathBuf>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub base_dir: Option<PathBuf>,
-    #[serde(default = "bool_true")]
-    pub build_base: bool,
-    #[serde(default)]
-    pub build_target: bool,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub build_base: Option<bool>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub build_target: Option<bool>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub watch_patterns: Option<Vec<Glob>>,
-    #[serde(default, alias = "units")]
-    pub objects: Vec<ProjectObject>,
-    #[serde(default)]
-    pub progress_categories: Vec<ProjectProgressCategory>,
+    #[serde(default, alias = "objects", skip_serializing_if = "Option::is_none")]
+    pub units: Option<Vec<ProjectObject>>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub progress_categories: Option<Vec<ProjectProgressCategory>>,
 }
 
-#[derive(Default, Clone, serde::Deserialize)]
+impl ProjectConfig {
+    #[inline]
+    pub fn units(&self) -> &[ProjectObject] { self.units.as_deref().unwrap_or_default() }
+
+    #[inline]
+    pub fn units_mut(&mut self) -> &mut Vec<ProjectObject> {
+        self.units.get_or_insert_with(Vec::new)
+    }
+
+    #[inline]
+    pub fn progress_categories(&self) -> &[ProjectProgressCategory] {
+        self.progress_categories.as_deref().unwrap_or_default()
+    }
+
+    #[inline]
+    pub fn progress_categories_mut(&mut self) -> &mut Vec<ProjectProgressCategory> {
+        self.progress_categories.get_or_insert_with(Vec::new)
+    }
+}
+
+#[derive(Default, Clone, serde::Serialize, serde::Deserialize)]
 pub struct ProjectObject {
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub name: Option<String>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub path: Option<PathBuf>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub target_path: Option<PathBuf>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub base_path: Option<PathBuf>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     #[deprecated(note = "Use metadata.reverse_fn_order")]
     pub reverse_fn_order: Option<bool>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     #[deprecated(note = "Use metadata.complete")]
     pub complete: Option<bool>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub scratch: Option<ScratchConfig>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub metadata: Option<ProjectObjectMetadata>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub symbol_mappings: Option<SymbolMappings>,
 }
 
-#[derive(Default, Clone, serde::Deserialize)]
+pub type SymbolMappings = BiBTreeMap<String, String>;
+
+#[derive(Default, Clone, serde::Serialize, serde::Deserialize)]
 pub struct ProjectObjectMetadata {
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub complete: Option<bool>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub reverse_fn_order: Option<bool>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub source_path: Option<String>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub progress_categories: Option<Vec<String>>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub auto_generated: Option<bool>,
 }
 
-#[derive(Default, Clone, serde::Deserialize)]
+#[derive(Default, Clone, serde::Serialize, serde::Deserialize)]
 pub struct ProjectProgressCategory {
     #[serde(default)]
     pub id: String,
@@ -112,32 +135,36 @@ impl ProjectObject {
     }
 
     pub fn complete(&self) -> Option<bool> {
-        #[allow(deprecated)]
+        #[expect(deprecated)]
         self.metadata.as_ref().and_then(|m| m.complete).or(self.complete)
     }
 
     pub fn reverse_fn_order(&self) -> Option<bool> {
-        #[allow(deprecated)]
+        #[expect(deprecated)]
         self.metadata.as_ref().and_then(|m| m.reverse_fn_order).or(self.reverse_fn_order)
     }
 
     pub fn hidden(&self) -> bool {
         self.metadata.as_ref().and_then(|m| m.auto_generated).unwrap_or(false)
     }
+
+    pub fn source_path(&self) -> Option<&String> {
+        self.metadata.as_ref().and_then(|m| m.source_path.as_ref())
+    }
 }
 
 #[derive(Default, Clone, Eq, PartialEq, serde::Deserialize, serde::Serialize)]
 pub struct ScratchConfig {
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub platform: Option<String>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub compiler: Option<String>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub c_flags: Option<String>,
-    #[serde(default)]
+    #[serde(default, skip_serializing_if = "Option::is_none")]
     pub ctx_path: Option<PathBuf>,
-    #[serde(default)]
-    pub build_ctx: bool,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub build_ctx: Option<bool>,
 }
 
 pub const CONFIG_FILENAMES: [&str; 3] = ["objdiff.json", "objdiff.yml", "objdiff.yaml"];
@@ -150,13 +177,13 @@ pub const DEFAULT_WATCH_PATTERNS: &[&str] = &[
 #[derive(Clone, Eq, PartialEq)]
 pub struct ProjectConfigInfo {
     pub path: PathBuf,
-    pub timestamp: FileTime,
+    pub timestamp: Option<FileTime>,
 }
 
 pub fn try_project_config(dir: &Path) -> Option<(Result<ProjectConfig>, ProjectConfigInfo)> {
     for filename in CONFIG_FILENAMES.iter() {
         let config_path = dir.join(filename);
-        let Ok(mut file) = File::open(&config_path) else {
+        let Ok(file) = File::open(&config_path) else {
             continue;
         };
         let metadata = file.metadata();
@@ -165,9 +192,10 @@ pub fn try_project_config(dir: &Path) -> Option<(Result<ProjectConfig>, ProjectC
                 continue;
             }
             let ts = FileTime::from_last_modification_time(&metadata);
+            let mut reader = BufReader::new(file);
             let mut result = match filename.contains("json") {
-                true => read_json_config(&mut file),
-                false => read_yml_config(&mut file),
+                true => read_json_config(&mut reader),
+                false => read_yml_config(&mut reader),
             };
             if let Ok(config) = &result {
                 // Validate min_version if present
@@ -175,12 +203,41 @@ pub fn try_project_config(dir: &Path) -> Option<(Result<ProjectConfig>, ProjectC
                     result = Err(e);
                 }
             }
-            return Some((result, ProjectConfigInfo { path: config_path, timestamp: ts }));
+            return Some((result, ProjectConfigInfo { path: config_path, timestamp: Some(ts) }));
         }
     }
     None
 }
 
+pub fn save_project_config(
+    config: &ProjectConfig,
+    info: &ProjectConfigInfo,
+) -> Result<ProjectConfigInfo> {
+    if let Some(last_ts) = info.timestamp {
+        // Check if the file has changed since we last read it
+        if let Ok(metadata) = fs::metadata(&info.path) {
+            let ts = FileTime::from_last_modification_time(&metadata);
+            if ts != last_ts {
+                return Err(anyhow!("Config file has changed since last read"));
+            }
+        }
+    }
+    let mut writer =
+        BufWriter::new(File::create(&info.path).context("Failed to create config file")?);
+    let ext = info.path.extension().and_then(|ext| ext.to_str()).unwrap_or("json");
+    match ext {
+        "json" => serde_json::to_writer_pretty(&mut writer, config).context("Failed to write JSON"),
+        "yml" | "yaml" => {
+            serde_yaml::to_writer(&mut writer, config).context("Failed to write YAML")
+        }
+        _ => Err(anyhow!("Unknown config file extension: {ext}")),
+    }?;
+    let file = writer.into_inner().context("Failed to flush file")?;
+    let metadata = file.metadata().context("Failed to get file metadata")?;
+    let ts = FileTime::from_last_modification_time(&metadata);
+    Ok(ProjectConfigInfo { path: info.path.clone(), timestamp: Some(ts) })
+}
+
 fn validate_min_version(config: &ProjectConfig) -> Result<()> {
     let Some(min_version) = &config.min_version else { return Ok(()) };
     let version = semver::Version::parse(env!("CARGO_PKG_VERSION"))

objdiff-core/src/diff/code.rs

@@ -9,7 +9,7 @@ use crate::{
         DiffObjConfig, ObjInsArgDiff, ObjInsBranchFrom, ObjInsBranchTo, ObjInsDiff, ObjInsDiffKind,
         ObjSymbolDiff,
     },
-    obj::{ObjInfo, ObjInsArg, ObjReloc, ObjSymbol, ObjSymbolFlags, SymbolRef},
+    obj::{ObjInfo, ObjInsArg, ObjReloc, ObjSymbolFlags, SymbolRef},
 };
 
 pub fn process_code_symbol(
@@ -41,10 +41,12 @@ pub fn no_diff_code(out: &ProcessCodeResult, symbol_ref: SymbolRef) -> Result<Ob
         });
     }
     resolve_branches(&mut diff);
-    Ok(ObjSymbolDiff { symbol_ref, diff_symbol: None, instructions: diff, match_percent: None })
+    Ok(ObjSymbolDiff { symbol_ref, target_symbol: None, instructions: diff, match_percent: None })
 }
 
 pub fn diff_code(
+    left_obj: &ObjInfo,
+    right_obj: &ObjInfo,
     left_out: &ProcessCodeResult,
     right_out: &ProcessCodeResult,
     left_symbol_ref: SymbolRef,
@@ -60,14 +62,14 @@ pub fn diff_code(
 
     let mut diff_state = InsDiffState::default();
     for (left, right) in left_diff.iter_mut().zip(right_diff.iter_mut()) {
-        let result = compare_ins(config, left, right, &mut diff_state)?;
+        let result = compare_ins(config, left_obj, right_obj, left, right, &mut diff_state)?;
         left.kind = result.kind;
         right.kind = result.kind;
         left.arg_diff = result.left_args_diff;
         right.arg_diff = result.right_args_diff;
     }
 
-    let total = left_out.insts.len();
+    let total = left_out.insts.len().max(right_out.insts.len());
     let percent = if diff_state.diff_count >= total {
         0.0
     } else {
@@ -77,13 +79,13 @@ pub fn diff_code(
     Ok((
         ObjSymbolDiff {
             symbol_ref: left_symbol_ref,
-            diff_symbol: Some(right_symbol_ref),
+            target_symbol: Some(right_symbol_ref),
             instructions: left_diff,
             match_percent: Some(percent),
         },
         ObjSymbolDiff {
             symbol_ref: right_symbol_ref,
-            diff_symbol: Some(left_symbol_ref),
+            target_symbol: Some(left_symbol_ref),
             instructions: right_diff,
             match_percent: Some(percent),
         },
@@ -170,12 +172,33 @@ fn resolve_branches(vec: &mut [ObjInsDiff]) {
     }
 }
 
-fn address_eq(left: &ObjSymbol, right: &ObjSymbol) -> bool {
-    left.address as i64 + left.addend == right.address as i64 + right.addend
+fn address_eq(left: &ObjReloc, right: &ObjReloc) -> bool {
+    left.target.address as i64 + left.addend == right.target.address as i64 + right.addend
+}
+
+fn section_name_eq(
+    left_obj: &ObjInfo,
+    right_obj: &ObjInfo,
+    left_orig_section_index: usize,
+    right_orig_section_index: usize,
+) -> bool {
+    let Some(left_section) =
+        left_obj.sections.iter().find(|s| s.orig_index == left_orig_section_index)
+    else {
+        return false;
+    };
+    let Some(right_section) =
+        right_obj.sections.iter().find(|s| s.orig_index == right_orig_section_index)
+    else {
+        return false;
+    };
+    left_section.name == right_section.name
 }
 
 fn reloc_eq(
     config: &DiffObjConfig,
+    left_obj: &ObjInfo,
+    right_obj: &ObjInfo,
     left_reloc: Option<&ObjReloc>,
     right_reloc: Option<&ObjReloc>,
 ) -> bool {
@@ -189,29 +212,32 @@ fn reloc_eq(
         return true;
     }
 
-    let name_matches = left.target.name == right.target.name;
-    match (&left.target_section, &right.target_section) {
+    let symbol_name_matches = left.target.name == right.target.name;
+    match (&left.target.orig_section_index, &right.target.orig_section_index) {
         (Some(sl), Some(sr)) => {
             // Match if section and name or address match
-            sl == sr && (name_matches || address_eq(&left.target, &right.target))
+            section_name_eq(left_obj, right_obj, *sl, *sr)
+                && (symbol_name_matches || address_eq(left, right))
         }
         (Some(_), None) => false,
         (None, Some(_)) => {
             // Match if possibly stripped weak symbol
-            name_matches && right.target.flags.0.contains(ObjSymbolFlags::Weak)
+            symbol_name_matches && right.target.flags.0.contains(ObjSymbolFlags::Weak)
         }
-        (None, None) => name_matches,
+        (None, None) => symbol_name_matches,
     }
 }
 
 fn arg_eq(
     config: &DiffObjConfig,
+    left_obj: &ObjInfo,
+    right_obj: &ObjInfo,
     left: &ObjInsArg,
     right: &ObjInsArg,
     left_diff: &ObjInsDiff,
     right_diff: &ObjInsDiff,
 ) -> bool {
-    return match left {
+    match left {
         ObjInsArg::PlainText(l) => match right {
             ObjInsArg::PlainText(r) => l == r,
             _ => false,
@@ -227,6 +253,8 @@ fn arg_eq(
             matches!(right, ObjInsArg::Reloc)
                 && reloc_eq(
                     config,
+                    left_obj,
+                    right_obj,
                     left_diff.ins.as_ref().and_then(|i| i.reloc.as_ref()),
                     right_diff.ins.as_ref().and_then(|i| i.reloc.as_ref()),
                 )
@@ -236,7 +264,7 @@ fn arg_eq(
             left_diff.branch_to.as_ref().map(|b| b.ins_idx)
                 == right_diff.branch_to.as_ref().map(|b| b.ins_idx)
         }
-    };
+    }
 }
 
 #[derive(Default)]
@@ -257,6 +285,8 @@ struct InsDiffResult {
 
 fn compare_ins(
     config: &DiffObjConfig,
+    left_obj: &ObjInfo,
+    right_obj: &ObjInfo,
     left: &ObjInsDiff,
     right: &ObjInsDiff,
     state: &mut InsDiffState,
@@ -283,7 +313,7 @@ fn compare_ins(
             state.diff_count += 1;
         }
         for (a, b) in left_ins.args.iter().zip(&right_ins.args) {
-            if arg_eq(config, a, b, left, right) {
+            if arg_eq(config, left_obj, right_obj, a, b, left, right) {
                 result.left_args_diff.push(None);
                 result.right_args_diff.push(None);
             } else {

objdiff-core/src/diff/data.rs

@@ -20,13 +20,13 @@ pub fn diff_bss_symbol(
     Ok((
         ObjSymbolDiff {
             symbol_ref: left_symbol_ref,
-            diff_symbol: Some(right_symbol_ref),
+            target_symbol: Some(right_symbol_ref),
             instructions: vec![],
             match_percent: Some(percent),
         },
         ObjSymbolDiff {
             symbol_ref: right_symbol_ref,
-            diff_symbol: Some(left_symbol_ref),
+            target_symbol: Some(left_symbol_ref),
             instructions: vec![],
             match_percent: Some(percent),
         },
@@ -34,7 +34,7 @@ pub fn diff_bss_symbol(
 }
 
 pub fn no_diff_symbol(_obj: &ObjInfo, symbol_ref: SymbolRef) -> ObjSymbolDiff {
-    ObjSymbolDiff { symbol_ref, diff_symbol: None, instructions: vec![], match_percent: None }
+    ObjSymbolDiff { symbol_ref, target_symbol: None, instructions: vec![], match_percent: None }
 }
 
 /// Compare the data sections of two object files.
@@ -158,13 +158,13 @@ pub fn diff_data_symbol(
     Ok((
         ObjSymbolDiff {
             symbol_ref: left_symbol_ref,
-            diff_symbol: Some(right_symbol_ref),
+            target_symbol: Some(right_symbol_ref),
             instructions: vec![],
             match_percent: Some(match_percent),
         },
         ObjSymbolDiff {
             symbol_ref: right_symbol_ref,
-            diff_symbol: Some(left_symbol_ref),
+            target_symbol: Some(left_symbol_ref),
             instructions: vec![],
             match_percent: Some(match_percent),
         },

objdiff-core/src/diff/display.rs

@@ -29,7 +29,7 @@ pub enum DiffText<'a> {
     Eol,
 }
 
-#[derive(Default, Clone, PartialEq, Eq)]
+#[derive(Debug, Default, Clone, PartialEq, Eq)]
 pub enum HighlightKind {
     #[default]
     None,
@@ -94,9 +94,9 @@ fn display_reloc_name<E>(
     mut cb: impl FnMut(DiffText) -> Result<(), E>,
 ) -> Result<(), E> {
     cb(DiffText::Symbol(&reloc.target))?;
-    match reloc.target.addend.cmp(&0i64) {
-        Ordering::Greater => cb(DiffText::Basic(&format!("+{:#x}", reloc.target.addend))),
-        Ordering::Less => cb(DiffText::Basic(&format!("-{:#x}", -reloc.target.addend))),
+    match reloc.addend.cmp(&0i64) {
+        Ordering::Greater => cb(DiffText::Basic(&format!("+{:#x}", reloc.addend))),
+        Ordering::Less => cb(DiffText::Basic(&format!("-{:#x}", -reloc.addend))),
         _ => Ok(()),
     }
 }

objdiff-core/src/diff/mod.rs

@@ -3,6 +3,7 @@ use std::collections::HashSet;
 use anyhow::Result;
 
 use crate::{
+    config::SymbolMappings,
     diff::{
         code::{diff_code, no_diff_code, process_code_symbol},
         data::{
@@ -161,6 +162,8 @@ pub struct DiffObjConfig {
     #[serde(default = "default_true")]
     pub space_between_args: bool,
     pub combine_data_sections: bool,
+    #[serde(default)]
+    pub symbol_mappings: MappingConfig,
     // x86
     pub x86_formatter: X86Formatter,
     // MIPS
@@ -182,6 +185,7 @@ impl Default for DiffObjConfig {
             relax_reloc_diffs: false,
             space_between_args: true,
             combine_data_sections: false,
+            symbol_mappings: Default::default(),
             x86_formatter: Default::default(),
             mips_abi: Default::default(),
             mips_instr_category: Default::default(),
@@ -223,8 +227,10 @@ impl ObjSectionDiff {
 
 #[derive(Debug, Clone, Default)]
 pub struct ObjSymbolDiff {
+    /// The symbol ref this object
     pub symbol_ref: SymbolRef,
-    pub diff_symbol: Option<SymbolRef>,
+    /// The symbol ref in the _other_ object that this symbol was diffed against
+    pub target_symbol: Option<SymbolRef>,
     pub instructions: Vec<ObjInsDiff>,
     pub match_percent: Option<f32>,
 }
@@ -294,8 +300,13 @@ pub struct ObjInsBranchTo {
 
 #[derive(Default)]
 pub struct ObjDiff {
+    /// A list of all section diffs in the object.
     pub sections: Vec<ObjSectionDiff>,
+    /// Common BSS symbols don't live in a section, so they're stored separately.
     pub common: Vec<ObjSymbolDiff>,
+    /// If `selecting_left` or `selecting_right` is set, this is the list of symbols
+    /// that are being mapped to the other object.
+    pub mapping_symbols: Vec<ObjSymbolDiff>,
 }
 
 impl ObjDiff {
@@ -303,13 +314,14 @@ impl ObjDiff {
         let mut result = Self {
             sections: Vec::with_capacity(obj.sections.len()),
             common: Vec::with_capacity(obj.common.len()),
+            mapping_symbols: vec![],
         };
         for (section_idx, section) in obj.sections.iter().enumerate() {
             let mut symbols = Vec::with_capacity(section.symbols.len());
             for (symbol_idx, _) in section.symbols.iter().enumerate() {
                 symbols.push(ObjSymbolDiff {
                     symbol_ref: SymbolRef { section_idx, symbol_idx },
-                    diff_symbol: None,
+                    target_symbol: None,
                     instructions: vec![],
                     match_percent: None,
                 });
@@ -328,7 +340,7 @@ impl ObjDiff {
         for (symbol_idx, _) in obj.common.iter().enumerate() {
             result.common.push(ObjSymbolDiff {
                 symbol_ref: SymbolRef { section_idx: obj.sections.len(), symbol_idx },
-                diff_symbol: None,
+                target_symbol: None,
                 instructions: vec![],
                 match_percent: None,
             });
@@ -378,7 +390,7 @@ pub fn diff_objs(
     right: Option<&ObjInfo>,
     prev: Option<&ObjInfo>,
 ) -> Result<DiffObjsResult> {
-    let symbol_matches = matching_symbols(left, right, prev)?;
+    let symbol_matches = matching_symbols(left, right, prev, &config.symbol_mappings)?;
     let section_matches = matching_sections(left, right)?;
     let mut left = left.map(|p| (p, ObjDiff::new_from_obj(p)));
     let mut right = right.map(|p| (p, ObjDiff::new_from_obj(p)));
@@ -399,6 +411,8 @@ pub fn diff_objs(
                         let left_code = process_code_symbol(left_obj, left_symbol_ref, config)?;
                         let right_code = process_code_symbol(right_obj, right_symbol_ref, config)?;
                         let (left_diff, right_diff) = diff_code(
+                            left_obj,
+                            right_obj,
                             &left_code,
                             &right_code,
                             left_symbol_ref,
@@ -412,6 +426,8 @@ pub fn diff_objs(
                             let (prev_obj, prev_out) = prev.as_mut().unwrap();
                             let prev_code = process_code_symbol(prev_obj, prev_symbol_ref, config)?;
                             let (_, prev_diff) = diff_code(
+                                left_obj,
+                                right_obj,
                                 &right_code,
                                 &prev_code,
                                 right_symbol_ref,
@@ -529,6 +545,17 @@ pub fn diff_objs(
         }
     }
 
+    if let (Some((right_obj, right_out)), Some((left_obj, left_out))) =
+        (right.as_mut(), left.as_mut())
+    {
+        if let Some(right_name) = &config.symbol_mappings.selecting_left {
+            generate_mapping_symbols(right_obj, right_name, left_obj, left_out, config)?;
+        }
+        if let Some(left_name) = &config.symbol_mappings.selecting_right {
+            generate_mapping_symbols(left_obj, left_name, right_obj, right_out, config)?;
+        }
+    }
+
     Ok(DiffObjsResult {
         left: left.map(|(_, o)| o),
         right: right.map(|(_, o)| o),
@@ -536,6 +563,65 @@ pub fn diff_objs(
     })
 }
 
+/// When we're selecting a symbol to use as a comparison, we'll create comparisons for all
+/// symbols in the other object that match the selected symbol's section and kind. This allows
+/// us to display match percentages for all symbols in the other object that could be selected.
+fn generate_mapping_symbols(
+    base_obj: &ObjInfo,
+    base_name: &str,
+    target_obj: &ObjInfo,
+    target_out: &mut ObjDiff,
+    config: &DiffObjConfig,
+) -> Result<()> {
+    let Some(base_symbol_ref) = symbol_ref_by_name(base_obj, base_name) else {
+        return Ok(());
+    };
+    let (base_section, base_symbol) = base_obj.section_symbol(base_symbol_ref);
+    let Some(base_section) = base_section else {
+        return Ok(());
+    };
+    let base_code = match base_section.kind {
+        ObjSectionKind::Code => Some(process_code_symbol(base_obj, base_symbol_ref, config)?),
+        _ => None,
+    };
+    for (target_section_index, target_section) in
+        target_obj.sections.iter().enumerate().filter(|(_, s)| s.kind == base_section.kind)
+    {
+        for (target_symbol_index, _target_symbol) in
+            target_section.symbols.iter().enumerate().filter(|(_, s)| s.kind == base_symbol.kind)
+        {
+            let target_symbol_ref =
+                SymbolRef { section_idx: target_section_index, symbol_idx: target_symbol_index };
+            match base_section.kind {
+                ObjSectionKind::Code => {
+                    let target_code = process_code_symbol(target_obj, target_symbol_ref, config)?;
+                    let (left_diff, _right_diff) = diff_code(
+                        target_obj,
+                        base_obj,
+                        &target_code,
+                        base_code.as_ref().unwrap(),
+                        target_symbol_ref,
+                        base_symbol_ref,
+                        config,
+                    )?;
+                    target_out.mapping_symbols.push(left_diff);
+                }
+                ObjSectionKind::Data => {
+                    let (left_diff, _right_diff) =
+                        diff_data_symbol(target_obj, base_obj, target_symbol_ref, base_symbol_ref)?;
+                    target_out.mapping_symbols.push(left_diff);
+                }
+                ObjSectionKind::Bss => {
+                    let (left_diff, _right_diff) =
+                        diff_bss_symbol(target_obj, base_obj, target_symbol_ref, base_symbol_ref)?;
+                    target_out.mapping_symbols.push(left_diff);
+                }
+            }
+        }
+    }
+    Ok(())
+}
+
 #[derive(Copy, Clone, Eq, PartialEq)]
 struct SymbolMatch {
     left: Option<SymbolRef>,
@@ -551,19 +637,115 @@ struct SectionMatch {
     section_kind: ObjSectionKind,
 }
 
+#[derive(Debug, Clone, PartialEq, Eq, Hash, Default, serde::Deserialize, serde::Serialize)]
+pub struct MappingConfig {
+    /// Manual symbol mappings
+    pub mappings: SymbolMappings,
+    /// The right object symbol name that we're selecting a left symbol for
+    pub selecting_left: Option<String>,
+    /// The left object symbol name that we're selecting a right symbol for
+    pub selecting_right: Option<String>,
+}
+
+fn symbol_ref_by_name(obj: &ObjInfo, name: &str) -> Option<SymbolRef> {
+    for (section_idx, section) in obj.sections.iter().enumerate() {
+        for (symbol_idx, symbol) in section.symbols.iter().enumerate() {
+            if symbol.name == name {
+                return Some(SymbolRef { section_idx, symbol_idx });
+            }
+        }
+    }
+    None
+}
+
+fn apply_symbol_mappings(
+    left: &ObjInfo,
+    right: &ObjInfo,
+    mapping_config: &MappingConfig,
+    left_used: &mut HashSet<SymbolRef>,
+    right_used: &mut HashSet<SymbolRef>,
+    matches: &mut Vec<SymbolMatch>,
+) -> Result<()> {
+    // If we're selecting a symbol to use as a comparison, mark it as used
+    // This ensures that we don't match it to another symbol at any point
+    if let Some(left_name) = &mapping_config.selecting_left {
+        if let Some(left_symbol) = symbol_ref_by_name(left, left_name) {
+            left_used.insert(left_symbol);
+        }
+    }
+    if let Some(right_name) = &mapping_config.selecting_right {
+        if let Some(right_symbol) = symbol_ref_by_name(right, right_name) {
+            right_used.insert(right_symbol);
+        }
+    }
+
+    // Apply manual symbol mappings
+    for (left_name, right_name) in &mapping_config.mappings {
+        let Some(left_symbol) = symbol_ref_by_name(left, left_name) else {
+            continue;
+        };
+        if left_used.contains(&left_symbol) {
+            continue;
+        }
+        let Some(right_symbol) = symbol_ref_by_name(right, right_name) else {
+            continue;
+        };
+        if right_used.contains(&right_symbol) {
+            continue;
+        }
+        let left_section = &left.sections[left_symbol.section_idx];
+        let right_section = &right.sections[right_symbol.section_idx];
+        if left_section.kind != right_section.kind {
+            log::warn!(
+                "Symbol section kind mismatch: {} ({:?}) vs {} ({:?})",
+                left_name,
+                left_section.kind,
+                right_name,
+                right_section.kind
+            );
+            continue;
+        }
+        matches.push(SymbolMatch {
+            left: Some(left_symbol),
+            right: Some(right_symbol),
+            prev: None, // TODO
+            section_kind: left_section.kind,
+        });
+        left_used.insert(left_symbol);
+        right_used.insert(right_symbol);
+    }
+    Ok(())
+}
+
 /// Find matching symbols between each object.
 fn matching_symbols(
     left: Option<&ObjInfo>,
     right: Option<&ObjInfo>,
     prev: Option<&ObjInfo>,
+    mappings: &MappingConfig,
 ) -> Result<Vec<SymbolMatch>> {
     let mut matches = Vec::new();
+    let mut left_used = HashSet::new();
     let mut right_used = HashSet::new();
     if let Some(left) = left {
+        if let Some(right) = right {
+            apply_symbol_mappings(
+                left,
+                right,
+                mappings,
+                &mut left_used,
+                &mut right_used,
+                &mut matches,
+            )?;
+        }
         for (section_idx, section) in left.sections.iter().enumerate() {
             for (symbol_idx, symbol) in section.symbols.iter().enumerate() {
+                let symbol_ref = SymbolRef { section_idx, symbol_idx };
+                if left_used.contains(&symbol_ref) {
+                    continue;
+                }
                 let symbol_match = SymbolMatch {
-                    left: Some(SymbolRef { section_idx, symbol_idx }),
+                    left: Some(symbol_ref),
                     right: find_symbol(right, symbol, section, Some(&right_used)),
                     prev: find_symbol(prev, symbol, section, None),
                     section_kind: section.kind,
@@ -575,8 +757,12 @@ fn matching_symbols(
             }
         }
         for (symbol_idx, symbol) in left.common.iter().enumerate() {
+            let symbol_ref = SymbolRef { section_idx: left.sections.len(), symbol_idx };
+            if left_used.contains(&symbol_ref) {
+                continue;
+            }
             let symbol_match = SymbolMatch {
-                left: Some(SymbolRef { section_idx: left.sections.len(), symbol_idx }),
+                left: Some(symbol_ref),
                 right: find_common_symbol(right, symbol),
                 prev: find_common_symbol(prev, symbol),
                 section_kind: ObjSectionKind::Bss,