Add a host tool to pack R_ARM_RELATIVE relocations in libchrome.<ver>.so.

tools/relocation_packer/src/relocation_packer_elf_file.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// ELF shared object file updates handler.
+//
+// Provides functions to remove R_ARM_RELATIVE relocations from the .rel.dyn
+// section and pack them in .android.rel.dyn, and unpack to return the file
+// to its pre-packed state.
+//
+// Files to be packed or unpacked must include an existing .android.rel.dyn
+// section.  A standard libchrome.<version>.so will not contain this section,
+// so the following can be used to add one:
+//
+//   echo -n 'NULL' >/tmp/small
+//   arm-linux-gnueabi-objcopy
+//       --add-section .android.rel.dyn=/tmp/small
+//       libchrome.<version>.so libchrome.<version>.so.packed
+//   rm /tmp/small
+//
+// To use, open the file and pass the file descriptor to the Load() function,
+// pack or unpack as desired, and then call Flush().  Example:
+//
+//   int fd = open(..., O_RDWR);
+//
+//   ElfFile elf_file;
+//   if (!elf_file.Load(fd))
+//     abort();
+//
+//   bool status;
+//   if (is_packing)
+//     status = elf_file.PackRelocations();
+//   else
+//     status = elf_file.UnpackRelocations();
+//
+//   if (status)
+//     elf_file.Flush();
+//
+// SetPadding() causes PackRelocations() to pad .rel.dyn with R_ARM_NONE
+// entries rather than cutting a hole out of the shared object file.  This
+// keeps all load addresses and offsets constant, and enables easier
+// debugging and testing.
+//
+// A packed shared object file has all of its R_ARM_RELATIVE relocations
+// removed from .rel.dyn, and replaced as packed data in .android.rel.dyn.
+// The resulting file is shorter than its non-packed original.
+//
+// Unpacking a packed file restores the file to its non-packed state, by
+// expanding the packed data in android.rel.dyn, combining the R_ARM_RELATIVE
+// relocations with the data already in .rel.dyn, and then writing back the
+// now expanded .rel.dyn section.
+
+#ifndef RELOCATION_PACKER_ELF_FILE_H
+#define RELOCATION_PACKER_ELF_FILE_H
+
+#include <string.h>
+
+#include "elf.h"
+#include "libelf.h"
+#include "relocation_packer_packer.h"
+
+namespace relocation_packer {
+
+// An ElfFile reads shared objects, and shuttles R_ARM_RELATIVE relocations
+// between .rel.dyn and .android.rel.dyn sections.
+class ElfFile {

[rmcilroy, 2014/06/02 15:16:35]

How about "class ArmElfFile : public ElfFile" (with the platform independent
parts in the ElfFile superclass)? This would make porting easier.

[simonb (inactive), 2014/06/04 16:40:35]

There's no current plan to port to other architectures; there is a possibility
of extending this in future to arm64.

[rmcilroy, 2014/06/07 11:49:06]

Arm64 is a different architecture.  I'm fine with this being left until / if
this is ported to Arm64 though.

+ public:
+  // Stub identifier written to 'null out' packed data, "NULL".
+  static const Elf32_Word kStubIdentifier = 0x4c4c554eu;
+
+  // Additional dynamic tags used to indicate the offset and size of the
+  // .android.rel.dyn section.
+  static const Elf32_Sword DT_ANDROID_ARM_REL_OFFSET = DT_LOPROC;
+  static const Elf32_Sword DT_ANDROID_ARM_REL_SIZE = DT_LOPROC + 1;

[rmcilroy, 2014/06/02 15:16:35]

Do these (and kStubIdentifier above) need to be in the header?  They only seem
to be used in the relocation_packer_elf_file.cc file.

[simonb (inactive), 2014/06/04 16:40:35]

Done.

+
+  ElfFile() { ::memset(this, 0, sizeof(*this)); }
+  ~ElfFile() {}
+
+  // Load a new ElfFile from a filedescriptor.  If flushing, the file must
+  // be open for read/write.
+  // |fd| is an open file descriptor for the shared object.
+  bool Load(int fd);

[rmcilroy, 2014/06/02 15:16:35]

nit - mention what is returned on success or failure (for all functions here).

[simonb (inactive), 2014/06/04 16:40:35]

Done.

+
+  // Set padding mode.  When padding, PackRelocations() will not shrink
+  // the .rel.dyn section, but instead replace R_ARM_RELATIVE with
+  // R_ARM_NONE entries.
+  // |flag| is true to pad .rel.dyn, false to shrink it.
+  inline void SetPadding(bool flag) { is_padding_rel_dyn_ = flag; }
+
+  // Transfer R_ARM_RELATIVE relocations from .rel.dyn to a packed
+  // representation in .android.rel.dyn.
+  bool PackRelocations();
+
+  // Transfer R_ARM_RELATIVE relocations from a packed representation in
+  // .android.rel.dyn to .rel.dyn.
+  bool UnpackRelocations();
+
+  // Write ELF file changes.
+  void Flush();

[rmcilroy, 2014/06/02 15:16:35]

Is there a reason to have Load and Flush as separate public functions? Why not
just have the constructor take the fd, and have
PackRelocations/UnpackRelocations explicitly Open / Flush the file themselves?

[simonb (inactive), 2014/06/04 16:40:35]

Done.

+
+ private:
+  // If set, pad rather than shrink .rel.dyn.  Primarily for debugging,
+  // allows packing to be checked without affecting load addresses.
+  bool is_padding_rel_dyn_;
+
+  // File descriptor opened on the shared object.
+  int fd_;
+
+  // Libelf handle, assigned by Load().
+  Elf* elf_;
+
+  // Sections that we manipulate, assigned by Load().
+  Elf_Scn* rel_dyn_section_;
+  Elf_Scn* dynamic_section_;
+  Elf_Scn* android_rel_dyn_section_;
+};
+
+}  // namespace relocation_packer
+
+#endif  // RELOCATION_PACKER_ELF_FILE_H