| 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. |
| |
| Packing uses run length encoding to store them more efficiently. Packed |
| relocations are placed in a new .android.rel.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: |
| |
| DT_ANDROID_ARM_REL_OFFSET = DT_LOPROC (Processor specific: 0x70000000) |
| - The offset of .android.rel.dyn 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 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 |
| |
| 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: |
| |
| 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--; |
| |
| |
| Usage instructions: |
| ------------------- |
| |
| To pack relocations, add an empty .android.rel.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 |
| 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 |
| 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 |
| 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. |
