136 lines
4.5 KiB
Plaintext
136 lines
4.5 KiB
Plaintext
Introduction:
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-------------
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Relative relocations are the bulk of dynamic relocations (the .rel.dyn
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or .rela.dyn sections) in libchrome.<version>.so. The ELF standard
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representation of them is wasteful.
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Packing uses a combination of run length encoding, delta encoding, and LEB128
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encoding to store them more efficiently. Packed relocations are placed in
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a new .android.rel.dyn or .android.rela.dyn section. Packing reduces
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the footprint of libchrome.<version>.so in the filesystem, in APK downloads,
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and in memory when loaded on the device.
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A packed libchrome.<version>.so is designed so that it can be loaded directly
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on Android, but requires the explicit support of a crazy linker that has been
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extended to understand packed relocations. Packed relocations are currently
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only supported on ARM.
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A packed libchrome.<version>.so cannot currently be used with the standard
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Android runtime linker.
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See src/*.h for design and implementation notes.
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Notes:
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------
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Packing does not adjust debug data. An unstripped libchrome.<version>.so
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can be packed and will run, but may no longer be useful for debugging.
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Unpacking on the device requires the explicit support of an extended crazy
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linker. Adds the following new .dynamic tags, used by the crazy linker to
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find the packed .android.rel.dyn or .android.rela.dyn section data:
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DT_ANDROID_REL_OFFSET = DT_LOOS (Operating System specific: 0x6000000d)
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- The offset of packed relocation data in libchrome.<version>.so
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DT_ANDROID_REL_SIZE = DT_LOOS + 1 (Operating System Specific: 0x6000000e)
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- The size of packed relocation data in bytes
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32 bit ARM libraries use relocations without addends. 64 bit ARM libraries
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use relocations with addends. The packing strategy necessarily differs for
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the two relocation types.
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Where libchrome.<version>.so contains relocations without addends, the format
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of .android.rel.dyn data is:
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"APR1" identifier
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N: the number of count-delta pairs in the encoding
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A: the initial offset
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N * C,D: N count-delta pairs
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Where libchrome.<version>.so contains relocations with addends, the format
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of .android.rela.dyn data is:
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"APA1" identifier
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N: the number of addr-addend delta pairs in the encoding
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N * A,V: N addr-addend delta pairs
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All numbers in the encoding stream are stored as LEB128 values. For details
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see http://en.wikipedia.org/wiki/LEB128.
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The streaming unpacking algorithm for 32 bit ARM is:
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skip over "APR1"
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pairs, addr = next leb128 value, next leb128 value
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emit R_ARM_RELATIVE relocation with r_offset = addr
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while pairs:
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count, delta = next leb128 value, next leb128 value
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while count:
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addr += delta
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emit R_ARM_RELATIVE relocation with r_offset = addr
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count--
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pairs--
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The streaming unpacking algorithm for 64 bit ARM is:
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skip over "APA1"
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pairs = next signed leb128 value
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addr, addend = 0, 0
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while pairs:
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addr += next signed leb128 value
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addend += next signed leb128 value
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emit R_AARCH64_RELATIVE relocation with r_offset = addr, r_addend = addend
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pairs--
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Usage instructions:
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-------------------
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To pack relocations, add an empty .android.rel.dyn or .android.rela.dyn and
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then run the tool:
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echo -n 'NULL' >/tmp/small
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if file libchrome.<version>.so | grep -q 'ELF 32'; then
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arm-linux-androideabi-objcopy
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--add-section .android.rel.dyn=/tmp/small
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libchrome.<version>.so libchrome.<version>.so.packed
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else
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aarch64-linux-android-objcopy
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--add-section .android.rela.dyn=/tmp/small
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libchrome.<version>.so libchrome.<version>.so.packed
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fi
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rm /tmp/small
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relocation_packer libchrome.<version>.so.packed
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To unpack and restore the shared library to its original state:
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cp libchrome.<version>.so.packed unpackable
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relocation_packer -u unpackable
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if file libchrome.<version>.so | grep -q 'ELF 32'; then
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arm-linux-androideabi-objcopy \
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--remove-section=.android.rel.dyn unpackable libchrome.<version>.so
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else
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aarch64-linux-android-objcopy \
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--remove-section=.android.rela.dyn unpackable libchrome.<version>.so
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endif
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rm unpackable
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Bugs & TODOs:
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-------------
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Requires two free slots in the .dynamic section. Uses these to add data that
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tells the crazy linker where to find the packed relocation data. Fails
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if insufficient free slots exist (use gold --spare-dynamic-slots to increase
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the allocation).
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Requires libelf 0.158 or later. Earlier libelf releases may be buggy in
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ways that prevent the packer from working correctly.
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Testing:
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--------
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Unittests run under gtest, on the host system.
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