read: add ObjectSection::relocation_map (#654)

This is intended to handle relocations for DWARF.

src/read/any.rs

@@ -19,9 +19,9 @@ use crate::read::xcoff;
 use crate::read::{
     self, Architecture, BinaryFormat, CodeView, ComdatKind, CompressedData, CompressedFileRange,
     Error, Export, FileFlags, FileKind, Import, Object, ObjectComdat, ObjectKind, ObjectMap,
-    ObjectSection, ObjectSegment, ObjectSymbol, ObjectSymbolTable, ReadRef, Relocation, Result,
-    SectionFlags, SectionIndex, SectionKind, SegmentFlags, SubArchitecture, SymbolFlags,
-    SymbolIndex, SymbolKind, SymbolMap, SymbolMapName, SymbolScope, SymbolSection,
+    ObjectSection, ObjectSegment, ObjectSymbol, ObjectSymbolTable, ReadRef, Relocation,
+    RelocationMap, Result, SectionFlags, SectionIndex, SectionKind, SegmentFlags, SubArchitecture,
+    SymbolFlags, SymbolIndex, SymbolKind, SymbolMap, SymbolMapName, SymbolScope, SymbolSection,
 };
 
 /// Evaluate an expression on the contents of a file format enum.
@@ -787,6 +787,10 @@ impl<'data, 'file, R: ReadRef<'data>> ObjectSection<'data> for Section<'data, 'f
         }
     }
 
+    fn relocation_map(&self) -> Result<RelocationMap> {
+        with_inner!(self.inner, SectionInternal, |x| x.relocation_map())
+    }
+
     fn flags(&self) -> SectionFlags {
         with_inner!(self.inner, SectionInternal, |x| x.flags())
     }

src/read/coff/section.rs

@@ -6,7 +6,7 @@ use crate::pe;
 use crate::read::util::StringTable;
 use crate::read::{
     self, CompressedData, CompressedFileRange, Error, ObjectSection, ObjectSegment, ReadError,
-    ReadRef, Result, SectionFlags, SectionIndex, SectionKind, SegmentFlags,
+    ReadRef, RelocationMap, Result, SectionFlags, SectionIndex, SectionKind, SegmentFlags,
 };
 
 use super::{CoffFile, CoffHeader, CoffRelocationIterator};
@@ -384,6 +384,10 @@ impl<'data, 'file, R: ReadRef<'data>, Coff: CoffHeader> ObjectSection<'data>
         }
     }
 
+    fn relocation_map(&self) -> read::Result<RelocationMap> {
+        RelocationMap::new(self.file, self)
+    }
+
     fn flags(&self) -> SectionFlags {
         SectionFlags::Coff {
             characteristics: self.section.characteristics.get(LE),

src/read/elf/section.rs

@@ -6,7 +6,7 @@ use crate::endian::{self, Endianness, U32Bytes};
 use crate::pod::Pod;
 use crate::read::{
     self, Bytes, CompressedData, CompressedFileRange, CompressionFormat, Error, ObjectSection,
-    ReadError, ReadRef, SectionFlags, SectionIndex, SectionKind, StringTable,
+    ReadError, ReadRef, RelocationMap, SectionFlags, SectionIndex, SectionKind, StringTable,
 };
 
 use super::{
@@ -593,6 +593,10 @@ where
         }
     }
 
+    fn relocation_map(&self) -> read::Result<RelocationMap> {
+        RelocationMap::new(self.file, self)
+    }
+
     fn flags(&self) -> SectionFlags {
         SectionFlags::Elf {
             sh_flags: self.section.sh_flags(self.file.endian).into(),

src/read/macho/section.rs

@@ -5,8 +5,8 @@ use crate::endian::{self, Endianness};
 use crate::macho;
 use crate::pod::Pod;
 use crate::read::{
-    self, CompressedData, CompressedFileRange, ObjectSection, ReadError, ReadRef, Result,
-    SectionFlags, SectionIndex, SectionKind,
+    self, CompressedData, CompressedFileRange, ObjectSection, ReadError, ReadRef, RelocationMap,
+    Result, SectionFlags, SectionIndex, SectionKind,
 };
 
 use super::{MachHeader, MachOFile, MachORelocationIterator};
@@ -199,6 +199,10 @@ where
         }
     }
 
+    fn relocation_map(&self) -> read::Result<RelocationMap> {
+        RelocationMap::new(self.file, self)
+    }
+
     fn flags(&self) -> SectionFlags {
         SectionFlags::MachO {
             flags: self.internal.section.flags(self.file.endian),

src/read/mod.rs

@@ -45,6 +45,11 @@ use alloc::borrow::Cow;
 use alloc::vec::Vec;
 use core::{fmt, result};
 
+#[cfg(not(feature = "std"))]
+use alloc::collections::btree_map::BTreeMap as Map;
+#[cfg(feature = "std")]
+use std::collections::hash_map::HashMap as Map;
+
 pub use crate::common::*;
 
 mod read_ref;
@@ -727,6 +732,90 @@ impl Relocation {
     }
 }
 
+/// A map from section offsets to relocation information.
+///
+/// This can be used to apply relocations to a value at a given section offset.
+/// This is intended for use with DWARF in relocatable object files, and only
+/// supports relocations that are used in DWARF.
+///
+/// Returned by [`ObjectSection::relocation_map`].
+#[derive(Debug, Default)]
+pub struct RelocationMap(Map<u64, RelocationMapEntry>);
+
+impl RelocationMap {
+    /// Construct a new relocation map for a section.
+    ///
+    /// Fails if any relocation cannot be added to the map.
+    /// You can manually use `add` if you need different error handling,
+    /// such as to list all errors or to ignore them.
+    pub fn new<'data, 'file, T>(file: &'file T, section: &T::Section) -> Result<Self>
+    where
+        T: Object<'data, 'file>,
+    {
+        let mut map = RelocationMap(Map::new());
+        for (offset, relocation) in section.relocations() {
+            map.add(file, offset, relocation)?;
+        }
+        Ok(map)
+    }
+
+    /// Add a single relocation to the map.
+    pub fn add<'data: 'file, 'file, T>(
+        &mut self,
+        file: &'file T,
+        offset: u64,
+        relocation: Relocation,
+    ) -> Result<()>
+    where
+        T: Object<'data, 'file>,
+    {
+        let mut entry = RelocationMapEntry {
+            implicit_addend: relocation.has_implicit_addend(),
+            addend: relocation.addend() as u64,
+        };
+        match relocation.kind() {
+            RelocationKind::Absolute => match relocation.target() {
+                RelocationTarget::Symbol(symbol_idx) => {
+                    let symbol = file
+                        .symbol_by_index(symbol_idx)
+                        .read_error("Relocation with invalid symbol")?;
+                    entry.addend = symbol.address().wrapping_add(entry.addend);
+                }
+                _ => {
+                    return Err(Error("Unsupported relocation target"));
+                }
+            },
+            _ => {
+                return Err(Error("Unsupported relocation type"));
+            }
+        }
+        if self.0.insert(offset, entry).is_some() {
+            return Err(Error("Multiple relocations for offset"));
+        }
+        Ok(())
+    }
+
+    /// Relocate a value that was read from the section at the given offset.
+    pub fn relocate(&self, offset: u64, value: u64) -> u64 {
+        if let Some(relocation) = self.0.get(&offset) {
+            if relocation.implicit_addend {
+                // Use the explicit addend too, because it may have the symbol value.
+                value.wrapping_add(relocation.addend)
+            } else {
+                relocation.addend
+            }
+        } else {
+            value
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+struct RelocationMapEntry {
+    implicit_addend: bool,
+    addend: u64,
+}
+
 /// A data compression format.
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 #[non_exhaustive]

src/read/pe/section.rs

@@ -6,7 +6,7 @@ use crate::pe;
 use crate::pe::ImageSectionHeader;
 use crate::read::{
     self, CompressedData, CompressedFileRange, ObjectSection, ObjectSegment, ReadError, ReadRef,
-    Relocation, Result, SectionFlags, SectionIndex, SectionKind, SegmentFlags,
+    Relocation, RelocationMap, Result, SectionFlags, SectionIndex, SectionKind, SegmentFlags,
 };
 
 use super::{ImageNtHeaders, PeFile, SectionTable};
@@ -288,6 +288,10 @@ where
         PeRelocationIterator(PhantomData)
     }
 
+    fn relocation_map(&self) -> read::Result<RelocationMap> {
+        RelocationMap::new(self.file, self)
+    }
+
     fn flags(&self) -> SectionFlags {
         SectionFlags::Coff {
             characteristics: self.section.characteristics.get(LE),

src/read/traits.rs

@@ -4,9 +4,9 @@ use alloc::vec::Vec;
 use crate::endian::Endianness;
 use crate::read::{
     self, Architecture, CodeView, ComdatKind, CompressedData, CompressedFileRange, Export,
-    FileFlags, Import, ObjectKind, ObjectMap, Relocation, Result, SectionFlags, SectionIndex,
-    SectionKind, SegmentFlags, SubArchitecture, SymbolFlags, SymbolIndex, SymbolKind, SymbolMap,
-    SymbolMapName, SymbolScope, SymbolSection,
+    FileFlags, Import, ObjectKind, ObjectMap, Relocation, RelocationMap, Result, SectionFlags,
+    SectionIndex, SectionKind, SegmentFlags, SubArchitecture, SymbolFlags, SymbolIndex, SymbolKind,
+    SymbolMap, SymbolMapName, SymbolScope, SymbolSection,
 };
 
 /// An object file.
@@ -419,6 +419,9 @@ pub trait ObjectSection<'data>: read::private::Sealed {
     /// Get the relocations for this section.
     fn relocations(&self) -> Self::RelocationIterator;
 
+    /// Construct a relocation map for this section.
+    fn relocation_map(&self) -> Result<RelocationMap>;
+
     /// Section flags that are specific to each file format.
     fn flags(&self) -> SectionFlags;
 }

src/read/wasm.rs

@@ -11,9 +11,9 @@ use wasmparser as wp;
 use crate::read::{
     self, Architecture, ComdatKind, CompressedData, CompressedFileRange, Error, Export, FileFlags,
     Import, NoDynamicRelocationIterator, Object, ObjectComdat, ObjectKind, ObjectSection,
-    ObjectSegment, ObjectSymbol, ObjectSymbolTable, ReadError, ReadRef, Relocation, Result,
-    SectionFlags, SectionIndex, SectionKind, SegmentFlags, SymbolFlags, SymbolIndex, SymbolKind,
-    SymbolScope, SymbolSection,
+    ObjectSegment, ObjectSymbol, ObjectSymbolTable, ReadError, ReadRef, Relocation, RelocationMap,
+    Result, SectionFlags, SectionIndex, SectionKind, SegmentFlags, SymbolFlags, SymbolIndex,
+    SymbolKind, SymbolScope, SymbolSection,
 };
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
@@ -737,6 +737,10 @@ impl<'data, 'file, R: ReadRef<'data>> ObjectSection<'data> for WasmSection<'data
         WasmRelocationIterator(PhantomData)
     }
 
+    fn relocation_map(&self) -> read::Result<RelocationMap> {
+        RelocationMap::new(self.file, self)
+    }
+
     #[inline]
     fn flags(&self) -> SectionFlags {
         SectionFlags::None

src/read/xcoff/section.rs

@@ -4,8 +4,8 @@ use core::{iter, result, slice, str};
 use crate::endian::BigEndian as BE;
 use crate::pod::Pod;
 use crate::read::{
-    self, CompressedData, CompressedFileRange, Error, ObjectSection, ReadError, ReadRef, Result,
-    SectionFlags, SectionIndex, SectionKind,
+    self, CompressedData, CompressedFileRange, Error, ObjectSection, ReadError, ReadRef,
+    RelocationMap, Result, SectionFlags, SectionIndex, SectionKind,
 };
 use crate::xcoff;
 
@@ -191,6 +191,10 @@ where
         }
     }
 
+    fn relocation_map(&self) -> read::Result<RelocationMap> {
+        RelocationMap::new(self.file, self)
+    }
+
     fn flags(&self) -> SectionFlags {
         SectionFlags::Xcoff {
             s_flags: self.section.s_flags(),