Bug 1077384 - Make libmozglue a pseudo-LD_PRELOAD on android. r=nfroyd

In order to avoid adding more dlsym overhead than there already is, resolve
symbols directly in the library containing the linker. (GetSymbolPtr is
essentially free ; dlsym makes the system linker compule a ElfHash itself,
and that's quite expensive to do on all symbols)

This also paves the way for direct symbol resolution in all system libraries.

mozglue/linker/BaseElf.cpp

@@ -0,0 +1,55 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "BaseElf.h"
+#include "Elfxx.h"
+#include "Logging.h"
+
+using namespace Elf;
+
+unsigned long
+BaseElf::Hash(const char *symbol)
+{
+  const unsigned char *sym = reinterpret_cast<const unsigned char *>(symbol);
+  unsigned long h = 0, g;
+  while (*sym) {
+    h = (h << 4) + *sym++;
+    g = h & 0xf0000000;
+    h ^= g;
+    h ^= g >> 24;
+  }
+  return h;
+}
+
+void *
+BaseElf::GetSymbolPtr(const char *symbol, unsigned long hash) const
+{
+  const Sym *sym = GetSymbol(symbol, hash);
+  void *ptr = nullptr;
+  if (sym && sym->st_shndx != SHN_UNDEF)
+    ptr = GetPtr(sym->st_value);
+  DEBUG_LOG("BaseElf::GetSymbolPtr(%p [\"%s\"], \"%s\") = %p",
+            reinterpret_cast<const void *>(this), GetPath(), symbol, ptr);
+  return ptr;
+}
+
+const Sym *
+BaseElf::GetSymbol(const char *symbol, unsigned long hash) const
+{
+  /* Search symbol with the buckets and chains tables.
+   * The hash computed from the symbol name gives an index in the buckets
+   * table. The corresponding value in the bucket table is an index in the
+   * symbols table and in the chains table.
+   * If the corresponding symbol in the symbols table matches, we're done.
+   * Otherwise, the corresponding value in the chains table is a new index
+   * in both tables, which corresponding symbol is tested and so on and so
+   * forth */
+  size_t bucket = hash % buckets.numElements();
+  for (size_t y = buckets[bucket]; y != STN_UNDEF; y = chains[y]) {
+    if (strcmp(symbol, strtab.GetStringAt(symtab[y].st_name)))
+      continue;
+    return &symtab[y];
+  }
+  return nullptr;
+}

mozglue/linker/BaseElf.h

@@ -0,0 +1,93 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef BaseElf_h
+#define BaseElf_h
+
+#include "ElfLoader.h"
+#include "Elfxx.h"
+
+
+/**
+ * Base class for ELF libraries. This class includes things that will be
+ * common between SystemElfs and CustomElfs.
+ */
+class BaseElf: public LibHandle
+{
+public:
+  /**
+   * Hash function for symbol lookup, as defined in ELF standard for System V.
+   */
+  static unsigned long Hash(const char *symbol);
+
+  /**
+   * Returns the address corresponding to the given symbol name (with a
+   * pre-computed hash).
+   */
+  void *GetSymbolPtr(const char *symbol, unsigned long hash) const;
+
+  /**
+   * Returns a pointer to the Elf Symbol in the Dynamic Symbol table
+   * corresponding to the given symbol name (with a pre-computed hash).
+   */
+  const Elf::Sym *GetSymbol(const char *symbol, unsigned long hash) const;
+
+  BaseElf(const char *path)
+  : LibHandle(path) { }
+
+protected:
+   /**
+    * Inherited from LibHandle. Those are temporary and are not supposed to
+    * be used.
+    */
+   virtual void *GetSymbolPtr(const char *symbol) const { return NULL; };
+   virtual bool Contains(void *addr) const { return false; };
+   virtual void *GetBase() const { return GetPtr(0); }
+
+#ifdef __ARM_EABI__
+  virtual const void *FindExidx(int *pcount) const { return NULL; };
+#endif
+
+  virtual Mappable *GetMappable() const { return NULL; };
+
+public:
+/* private: */
+  /**
+   * Returns a pointer relative to the base address where the library is
+   * loaded.
+   */
+  void *GetPtr(const Elf::Addr offset) const
+  {
+    if (reinterpret_cast<void *>(offset) > base)
+      return reinterpret_cast<void *>(offset);
+    return base + offset;
+  }
+
+  /**
+   * Like the above, but returns a typed (const) pointer
+   */
+  template <typename T>
+  const T *GetPtr(const Elf::Addr offset) const
+  {
+    if (reinterpret_cast<void *>(offset) > base)
+      return reinterpret_cast<const T *>(offset);
+    return reinterpret_cast<const T *>(base + offset);
+  }
+
+  /* Base address where the library is loaded */
+  MappedPtr base;
+
+  /* Buckets and chains for the System V symbol hash table */
+  Array<Elf::Word> buckets;
+  UnsizedArray<Elf::Word> chains;
+
+/* protected: */
+  /* String table */
+  Elf::Strtab strtab;
+
+  /* Symbol table */
+  UnsizedArray<Elf::Sym> symtab;
+};
+
+#endif /* BaseElf_h */

mozglue/linker/CustomElf.cpp

@@ -7,7 +7,9 @@
 #include <vector>
 #include <dlfcn.h>
 #include <signal.h>
+#include <string.h>
 #include "CustomElf.h"
+#include "BaseElf.h"
 #include "Mappable.h"
 #include "Logging.h"
 
@@ -279,43 +281,10 @@ CustomElf::~CustomElf()
   ElfLoader::Singleton.Forget(this);
 }
 
-namespace {
-
-/**
- * Hash function for symbol lookup, as defined in ELF standard for System V
- */
-unsigned long
-ElfHash(const char *symbol)
-{
-  const unsigned char *sym = reinterpret_cast<const unsigned char *>(symbol);
-  unsigned long h = 0, g;
-  while (*sym) {
-    h = (h << 4) + *sym++;
-    if ((g = h & 0xf0000000))
-      h ^= g >> 24;
-    h &= ~g;
-  }
-  return h;
-}
-
-} /* anonymous namespace */
-
 void *
 CustomElf::GetSymbolPtr(const char *symbol) const
 {
-  return GetSymbolPtr(symbol, ElfHash(symbol));
-}
-
-void *
-CustomElf::GetSymbolPtr(const char *symbol, unsigned long hash) const
-{
-  const Sym *sym = GetSymbol(symbol, hash);
-  void *ptr = nullptr;
-  if (sym && sym->st_shndx != SHN_UNDEF)
-    ptr = GetPtr(sym->st_value);
-  DEBUG_LOG("CustomElf::GetSymbolPtr(%p [\"%s\"], \"%s\") = %p",
-            reinterpret_cast<const void *>(this), GetPath(), symbol, ptr);
-  return ptr;
+  return BaseElf::GetSymbolPtr(symbol, Hash(symbol));
 }
 
 void *
@@ -372,16 +341,17 @@ CustomElf::GetSymbolPtrInDeps(const char *symbol) const
   }
 
   void *sym;
-  /* Search the symbol in the main program. Note this also tries all libraries
-   * the system linker will have loaded RTLD_GLOBAL. Unfortunately, that doesn't
-   * work with bionic, but its linker doesn't normally search the main binary
-   * anyways. Moreover, on android, the main binary is dalvik. */
-#ifdef __GLIBC__
-  sym = dlsym(RTLD_DEFAULT, symbol);
-  DEBUG_LOG("dlsym(RTLD_DEFAULT, \"%s\") = %p", symbol, sym);
-  if (sym)
-    return sym;
-#endif
+
+  unsigned long hash = Hash(symbol);
+
+  /* self_elf should never be NULL, but better safe than sorry. */
+  if (ElfLoader::Singleton.self_elf) {
+    /* We consider the library containing this code a permanent LD_PRELOAD,
+     * so, check if the symbol exists here first. */
+    sym = ElfLoader::Singleton.self_elf->GetSymbolPtr(symbol, hash);
+    if (sym)
+      return sym;
+  }
 
   /* Then search the symbol in our dependencies. Since we already searched in
    * libraries the system linker loaded, skip those (on glibc systems). We
@@ -389,15 +359,13 @@ CustomElf::GetSymbolPtrInDeps(const char *symbol) const
    * directly, not in their own dependent libraries. Building libraries with
    * --no-allow-shlib-undefined ensures such indirect symbol dependency don't
    * happen. */
-  unsigned long hash = ElfHash(symbol);
   for (std::vector<RefPtr<LibHandle> >::const_iterator it = dependencies.begin();
        it < dependencies.end(); ++it) {
     if (!(*it)->IsSystemElf()) {
-      sym = reinterpret_cast<CustomElf *>((*it).get())->GetSymbolPtr(symbol, hash);
-#ifndef __GLIBC__
+      sym = static_cast<BaseElf *>(
+        static_cast<CustomElf *>((*it).get()))->GetSymbolPtr(symbol, hash);
     } else {
       sym = (*it)->GetSymbolPtr(symbol);
-#endif
     }
     if (sym)
       return sym;
@@ -405,26 +373,6 @@ CustomElf::GetSymbolPtrInDeps(const char *symbol) const
   return nullptr;
 }
 
-const Sym *
-CustomElf::GetSymbol(const char *symbol, unsigned long hash) const
-{
-  /* Search symbol with the buckets and chains tables.
-   * The hash computed from the symbol name gives an index in the buckets
-   * table. The corresponding value in the bucket table is an index in the
-   * symbols table and in the chains table.
-   * If the corresponding symbol in the symbols table matches, we're done.
-   * Otherwise, the corresponding value in the chains table is a new index
-   * in both tables, which corresponding symbol is tested and so on and so
-   * forth */
-  size_t bucket = hash % buckets.numElements();
-  for (size_t y = buckets[bucket]; y != STN_UNDEF; y = chains[y]) {
-    if (strcmp(symbol, strtab.GetStringAt(symtab[y].st_name)))
-      continue;
-    return &symtab[y];
-  }
-  return nullptr;
-}
-
 bool
 CustomElf::Contains(void *addr) const
 {

mozglue/linker/CustomElf.h

@@ -6,6 +6,7 @@
 #define CustomElf_h
 
 #include "ElfLoader.h"
+#include "BaseElf.h"
 #include "Logging.h"
 #include "Elfxx.h"
 
@@ -13,7 +14,7 @@
  * Library Handle class for ELF libraries we don't let the system linker
  * handle.
  */
-class CustomElf: public LibHandle, private ElfLoader::link_map
+class CustomElf: public BaseElf, private ElfLoader::link_map
 {
   friend class ElfLoader;
   friend class SEGVHandler;
@@ -31,7 +32,7 @@ public:
                                                const char *path, int flags);
 
   /**
-   * Inherited from LibHandle
+   * Inherited from LibHandle/BaseElf
    */
   virtual ~CustomElf();
   virtual void *GetSymbolPtr(const char *symbol) const;
@@ -54,18 +55,6 @@ public:
   void stats(const char *when) const;
 
 private:
-  /**
-   * Returns a pointer to the Elf Symbol in the Dynamic Symbol table
-   * corresponding to the given symbol name (with a pre-computed hash).
-   */
-  const Elf::Sym *GetSymbol(const char *symbol, unsigned long hash) const;
-
-  /**
-   * Returns the address corresponding to the given symbol name (with a
-   * pre-computed hash).
-   */
-  void *GetSymbolPtr(const char *symbol, unsigned long hash) const;
-
   /**
    * Scan dependent libraries to find the address corresponding to the
    * given symbol name. This is used to find symbols that are undefined
@@ -77,7 +66,7 @@ private:
    * Private constructor
    */
   CustomElf(Mappable *mappable, const char *path)
-  : LibHandle(path)
+  : BaseElf(path)
   , mappable(mappable)
   , init(0)
   , fini(0)
@@ -85,24 +74,6 @@ private:
   , has_text_relocs(false)
   { }
 
-  /**
-   * Returns a pointer relative to the base address where the library is
-   * loaded.
-   */
-  void *GetPtr(const Elf::Addr offset) const
-  {
-    return base + offset;
-  }
-
-  /**
-   * Like the above, but returns a typed (const) pointer
-   */
-  template <typename T>
-  const T *GetPtr(const Elf::Addr offset) const
-  {
-    return reinterpret_cast<const T *>(base + offset);
-  }
-
   /**
    * Loads an Elf segment defined by the given PT_LOAD header.
    * Returns whether this succeeded or failed.
@@ -167,19 +138,6 @@ private:
   /* Appropriated Mappable */
   mozilla::RefPtr<Mappable> mappable;
 
-  /* Base address where the library is loaded */
-  MappedPtr base;
-
-  /* String table */
-  Elf::Strtab strtab;
-
-  /* Symbol table */
-  UnsizedArray<Elf::Sym> symtab;
-
-  /* Buckets and chains for the System V symbol hash table */
-  Array<Elf::Word> buckets;
-  UnsizedArray<Elf::Word> chains;
-
   /* List of dependent libraries */
   std::vector<mozilla::RefPtr<LibHandle> > dependencies;
 

mozglue/linker/ElfLoader.cpp

@@ -11,6 +11,7 @@
 #include <algorithm>
 #include <fcntl.h>
 #include "ElfLoader.h"
+#include "BaseElf.h"
 #include "CustomElf.h"
 #include "Mappable.h"
 #include "Logging.h"
@@ -38,6 +39,10 @@ extern "C" MOZ_EXPORT const void *
 __gnu_Unwind_Find_exidx(void *pc, int *pcount) __attribute__((weak));
 #endif
 
+/* Pointer to the PT_DYNAMIC section of the executable or library
+ * containing this code. */
+extern "C" Elf::Dyn _DYNAMIC[];
+
 using namespace mozilla;
 
 /**
@@ -331,6 +336,10 @@ ElfLoader::Load(const char *path, int flags, LibHandle *parent)
   /* Ensure logging is initialized or refresh if environment changed. */
   Logging::Init();
 
+  /* Ensure self_elf initialization. */
+  if (!self_elf)
+    Init();
+
   RefPtr<LibHandle> handle;
 
   /* Handle dlopen(nullptr) directly. */
@@ -466,6 +475,54 @@ ElfLoader::Forget(LibHandle *handle)
   }
 }
 
+void
+ElfLoader::Init()
+{
+  Dl_info info;
+  /* On Android < 4.1 can't reenter dl* functions. So when the library
+   * containing this code is dlopen()ed, it can't call dladdr from a
+   * static initializer. */
+  if (dladdr(_DYNAMIC, &info) != 0) {
+    /* Ideally, we wouldn't be initializing self_elf this way, but until
+     * SystemElf actually inherits from BaseElf, we'll just do it this
+     * (gross) way. */
+    UniquePtr<BaseElf> elf = mozilla::MakeUnique<BaseElf>(info.dli_fname);
+    elf->base.Assign(info.dli_fbase, -1);
+    size_t symnum = 0;
+    for (const Elf::Dyn *dyn = _DYNAMIC; dyn->d_tag; dyn++) {
+      switch (dyn->d_tag) {
+        case DT_HASH:
+          {
+            DEBUG_LOG("%s 0x%08" PRIxAddr, "DT_HASH", dyn->d_un.d_val);
+            const Elf::Word *hash_table_header = \
+              elf->GetPtr<Elf::Word>(dyn->d_un.d_ptr);
+            symnum = hash_table_header[1];
+            elf->buckets.Init(&hash_table_header[2], hash_table_header[0]);
+            elf->chains.Init(&*elf->buckets.end());
+          }
+          break;
+        case DT_STRTAB:
+          DEBUG_LOG("%s 0x%08" PRIxAddr, "DT_STRTAB", dyn->d_un.d_val);
+          elf->strtab.Init(elf->GetPtr(dyn->d_un.d_ptr));
+          break;
+        case DT_SYMTAB:
+          DEBUG_LOG("%s 0x%08" PRIxAddr, "DT_SYMTAB", dyn->d_un.d_val);
+          elf->symtab.Init(elf->GetPtr(dyn->d_un.d_ptr));
+          break;
+      }
+    }
+    if (!elf->buckets || !symnum) {
+      ERROR("%s: Missing or broken DT_HASH", info.dli_fname);
+    } else if (!elf->strtab) {
+      ERROR("%s: Missing DT_STRTAB", info.dli_fname);
+    } else if (!elf->symtab) {
+      ERROR("%s: Missing DT_SYMTAB", info.dli_fname);
+    } else {
+      self_elf = Move(elf);
+    }
+  }
+}
+
 ElfLoader::~ElfLoader()
 {
   LibHandleList list;
@@ -507,6 +564,10 @@ ElfLoader::~ElfLoader()
       }
     }
   }
+  /* Avoid self_elf->base destructor unmapping something that doesn't actually
+   * belong to it. */
+  if (self_elf)
+    self_elf->base.release();
 }
 
 void

mozglue/linker/ElfLoader.h

@@ -9,6 +9,7 @@
 #include <dlfcn.h>
 #include <signal.h>
 #include "mozilla/RefPtr.h"
+#include "mozilla/UniquePtr.h"
 #include "Zip.h"
 #include "Elfxx.h"
 #include "Mappable.h"
@@ -63,6 +64,9 @@ IsSignalHandlingBroken();
 
 }
 
+/* Forward declaration because BaseElf.h includes ElfLoader.h */
+class BaseElf;
+
 /**
  * Specialize RefCounted template for LibHandle. We may get references to
  * LibHandles during the execution of their destructor, so we need
@@ -439,6 +443,13 @@ protected:
 private:
   ~ElfLoader();
 
+  /* Initialization code that can't run during static initialization. */
+  void Init();
+
+  /* System loader handle for the library/program containing our code. This
+   * is used to resolve wrapped functions. */
+  mozilla::UniquePtr<BaseElf> self_elf;
+
   /* Bookkeeping */
   typedef std::vector<LibHandle *> LibHandleList;
   LibHandleList handles;
@@ -557,6 +568,12 @@ private:
     } r_state;
   };
 
+  /* Memory representation of ELF Auxiliary Vectors */
+  struct AuxVector {
+    Elf::Addr type;
+    Elf::Addr value;
+  };
+
   /* Helper class used to integrate libraries loaded by this linker in
    * r_debug */
   class DebuggerHelper
@@ -564,6 +581,8 @@ private:
   public:
     DebuggerHelper();
 
+    void Init(AuxVector *auvx);
+
     operator bool()
     {
       return dbg;

mozglue/linker/Utils.h

@@ -290,6 +290,10 @@ public:
       static_cast<T *>(this)->munmap(get(), GetLength());
   }
 
+  void release()
+  {
+    MemoryRange::Assign(MAP_FAILED, 0);
+  }
 };
 
 struct MappedPtr: public GenericMappedPtr<MappedPtr>

mozglue/linker/moz.build

@@ -5,6 +5,7 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 SOURCES += [
+    'BaseElf.cpp',
     'CustomElf.cpp',
     'ElfLoader.cpp',
     'Mappable.cpp',