Extend relocation packing to cover arm64.

tools/relocation_packer/README.TXT

 Introduction:
 -------------
 
-R_ARM_RELATIVE relocations are the bulk of dynamic relocations (the .rel.dyn
-section) in libchrome.<version>.so.  The ELF standard representation of them
-is wasteful.
+Relative relocations are the bulk of dynamic relocations (the .rel.dyn
+or .rela.dyn sections) in libchrome.<version>.so.  The ELF standard
+representation of them is wasteful.
 
-Packing uses run length encoding to store them more efficiently.  Packed
-relocations are placed in a new .android.rel.dyn section.  Packing reduces
+Packing uses combinations of run length encoding, delta encoding, and LEB128

[rmcilroy, 2014/07/28 10:08:44]

nit - s/combinations/a combination

[simonb (inactive), 2014/07/28 12:20:56]

Done.

+encoding to store them more efficiently.  Packed relocations are placed in
+a new .android.rel.dyn or .android.rela.dyn section.  Packing reduces
 the footprint of libchrome.<version>.so in the filesystem, in APK downloads,
 and in memory when loaded on the device.
 
 A packed libchrome.<version>.so is designed so that it can be loaded directly
 on Android, but requires the explicit support of a crazy linker that has been
 extended to understand packed relocations.  Packed relocations are currently
 only supported on ARM.
 
 A packed libchrome.<version>.so cannot currently be used with the standard
 Android runtime linker.
 
 See src/*.h for design and implementation notes.
 
 
 Notes:
 ------
 
 Packing does not adjust debug data.  An unstripped libchrome.<version>.so
 can be packed and will run, but may no longer be useful for debugging.
 
 Unpacking on the device requires the explicit support of an extended crazy
 linker.  Adds the following new .dynamic tags, used by the crazy linker to
-find the packed .android.rel.dyn section data:
+find the packed .android.rel.dyn or .android.rela.dyn section data:
 
   DT_ANDROID_ARM_REL_OFFSET = DT_LOPROC    (Processor specific: 0x70000000)

[rmcilroy, 2014/07/28 10:08:44]

/s/DT_ANDROID_ARM_REL_OFFSET/DT_ANDROID_REL_OFFSET (and for
DT_ANDROID_ARM_REL_SIZE)

[simonb (inactive), 2014/07/28 12:20:55]

Updated in patch set 4 (you're commenting here on a mildly out of date patch
set).

-    - The offset of .android.rel.dyn data in libchrome.<version>.so
+    - The offset of packed relocation data in libchrome.<version>.so
   DT_ANDROID_ARM_REL_SIZE = DT_LOPROC + 1  (Processor Specific: 0x70000001)
-    - The size of .android.rel.dyn data in bytes
+    - The size of packed relocation data in bytes
 
-The format of .android.rel.dyn data is:
+32 bit ARM libraries use relocations without addends.  64 bit ARM libraries
+use relocations with addends.  The packing strategy necessarily differs for
+the two relocation types.
+
+Where libchrome.<version>.so contains relocations without addends, the format
+of .android.rel.dyn data is:
 
   "APR1" identifier
   N: the number of count-delta pairs in the encoding
   A: the initial offset
   N * C,D: N count-delta pairs
 
+Where libchrome.<version>.so contains relocations with addends, the format
+of .android.rela.dyn data is:
+
+  "APA1" identifier
+  N: the number of addr-addend delta pairs in the encoding
+  N * A,V: N addr-addend delta pairs
+
 All numbers in the encoding stream are stored as LEB128 values.  For details
 see http://en.wikipedia.org/wiki/LEB128.
 
-The streaming unpacking algorithm is:
+The streaming unpacking algorithm for 32 bit ARM is:
 
   skip over "APR1"
   pairs, addr = next leb128 value, next leb128 value
   emit R_ARM_RELATIVE relocation with r_offset = addr
   while pairs:
     count, delta = next leb128 value, next leb128 value
     while count:
       addr += delta
       emit R_ARM_RELATIVE relocation with r_offset = addr
       count--
     pairs--;
 
+The streaming unpacking algorithm for 64 bit ARM is:
+
+  skip over "APA1"
+  pairs = next signed leb128 value
+  addr, addend = 0, 0
+  while pairs:
+    addr += next signed leb128 value
+    addend += next signed leb128 value
+    emit R_ARM_RELATIVE relocation with r_offset = addr, r_addend = addend

[rmcilroy, 2014/07/28 10:08:44]

R_AARCH64_RELATIVE

[simonb (inactive), 2014/07/28 12:20:55]

Done.  Good catch, thanks.

+    pairs--;
+
 
 Usage instructions:
 -------------------
 
-To pack relocations, add an empty .android.rel.dyn and then run the tool:
+To pack relocations, add an empty .android.rel.dyn or .android.rela.dyn and
+then run the tool:
 
     echo -n 'NULL' >/tmp/small
-    arm-linux-gnueabi-objcopy \
-        --add-section .android.rel.dyn=/tmp/small \
-        libchrome.<version>.so libchrome.<version>.so.packed
+    if file libchrome.<version>.so | grep -q 'ELF 32'; then
+      arm-linux-androideabi-objcopy
+          --add-section .android.rel.dyn=/tmp/small
+          libchrome.<version>.so libchrome.<version>.so.packed
+    else
+      aarch64-linux-android-objcopy
+          --add-section .android.rela.dyn=/tmp/small
+          libchrome.<version>.so libchrome.<version>.so.packed
+    fi
     rm /tmp/small
     relocation_packer libchrome.<version>.so.packed
 
 To unpack and restore the shared library to its original state:
 
     cp libchrome.<version>.so.packed unpackable
     relocation_packer -u unpackable
-    arm-linux-gnueabi-objcopy \
-        --remove-section=.android.rel.dyn unpackable libchrome.<version>.so
+    if file libchrome.<version>.so | grep -q 'ELF 32'; then
+      arm-linux-androideabi-objcopy \
+          --remove-section=.android.rel.dyn unpackable libchrome.<version>.so
+    else
+      aarch64-linux-android-objcopy \
+          --remove-section=.android.rela.dyn unpackable libchrome.<version>.so
+    endif
     rm unpackable
 
 
 Bugs & TODOs:
 -------------
 
-Currently only supports arm32.  Support for arm64 requires some extension
-and modification.
-
 Requires two free slots in the .dynamic section.  Uses these to add data that
-tells the crazy linker where to find the packed .android.rel.dyn data.  Fails
+tells the crazy linker where to find the packed relocation data.  Fails
 if insufficient free slots exist (use gold --spare-dynamic-slots to increase
 the allocation).
 
 Requires libelf 0.158 or later.  Earlier libelf releases may be buggy in
 ways that prevent the packer from working correctly.
 
 
 Testing:
 --------
 
 Unittests run under gtest, on the host system.