1 Introduction: 2 ------------- 3 4 R_ARM_RELATIVE relocations are the bulk of dynamic relocations (the .rel.dyn 5 section) in libchrome.<version>.so. The ELF standard representation of them 6 is wasteful. 7 8 Packing uses run length encoding to store them more efficiently. Packed 9 relocations are placed in a new .android.rel.dyn section. Packing reduces 10 the footprint of libchrome.<version>.so in the filesystem, in APK downloads, 11 and in memory when loaded on the device. 12 13 A packed libchrome.<version>.so is designed so that it can be loaded directly 14 on Android, but requires the explicit support of a crazy linker that has been 15 extended to understand packed relocations. Packed relocations are currently 16 only supported on ARM. 17 18 A packed libchrome.<version>.so cannot currently be used with the standard 19 Android runtime linker. 20 21 See src/*.h for design and implementation notes. 22 23 24 Notes: 25 ------ 26 27 Packing does not adjust debug data. An unstripped libchrome.<version>.so 28 can be packed and will run, but may no longer be useful for debugging. 29 30 Unpacking on the device requires the explicit support of an extended crazy 31 linker. Adds the following new .dynamic tags, used by the crazy linker to 32 find the packed .android.rel.dyn section data: 33 34 DT_ANDROID_ARM_REL_OFFSET = DT_LOPROC (Processor specific: 0x70000000) 35 - The offset of .android.rel.dyn data in libchrome.<version>.so 36 DT_ANDROID_ARM_REL_SIZE = DT_LOPROC + 1 (Processor Specific: 0x70000001) 37 - The size of .android.rel.dyn data in bytes 38 39 The format of .android.rel.dyn data is: 40 41 "APR1" identifier 42 N: the number of count-delta pairs in the encoding 43 A: the initial offset 44 N * C,D: N count-delta pairs 45 46 All numbers in the encoding stream are stored as LEB128 values. For details 47 see http://en.wikipedia.org/wiki/LEB128. 48 49 The streaming unpacking algorithm is: 50 51 skip over "APR1" 52 pairs, addr = next leb128 value, next leb128 value 53 emit R_ARM_RELATIVE relocation with r_offset = addr 54 while pairs: 55 count, delta = next leb128 value, next leb128 value 56 while count: 57 addr += delta 58 emit R_ARM_RELATIVE relocation with r_offset = addr 59 count-- 60 pairs--; 61 62 63 Usage instructions: 64 ------------------- 65 66 To pack relocations, add an empty .android.rel.dyn and then run the tool: 67 68 echo -n 'NULL' >/tmp/small 69 arm-linux-gnueabi-objcopy \ 70 --add-section .android.rel.dyn=/tmp/small \ 71 libchrome.<version>.so libchrome.<version>.so.packed 72 rm /tmp/small 73 relocation_packer libchrome.<version>.so.packed 74 75 To unpack and restore the shared library to its original state: 76 77 cp libchrome.<version>.so.packed unpackable 78 relocation_packer -u unpackable 79 arm-linux-gnueabi-objcopy \ 80 --remove-section=.android.rel.dyn unpackable libchrome.<version>.so 81 rm unpackable 82 83 84 Bugs & TODOs: 85 ------------- 86 87 Currently only supports arm32. Support for arm64 requires some extension 88 and modification. 89 90 Requires two free slots in the .dynamic section. Uses these to add data that 91 tells the crazy linker where to find the packed .android.rel.dyn data. Fails 92 if insufficient free slots exist (use gold --spare-dynamic-slots to increase 93 the allocation). 94 95 Requires libelf 0.158 or later. Earlier libelf releases may be buggy in 96 ways that prevent the packer from working correctly. 97 98 99 Testing: 100 -------- 101 102 Unittests run under gtest, on the host system. 103
