OLD | NEW |
---|---|
(Empty) | |
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 | |
rmcilroy
2014/06/02 15:16:35
Would it be safer to put this in a section called
simonb (inactive)
2014/06/04 16:40:35
The idea (or at least the hope) is that this will
rmcilroy
2014/06/07 11:49:06
Should be fine as is.
simonb (inactive)
2014/06/09 14:39:19
Okay.
| |
10 the footprint of libchrome.<version>.so in the filesystem, in APK downloads, | |
11 and in memory when loaded on the device. | |
rmcilroy
2014/06/02 15:16:35
"in memory when loaded on the device" - is this tr
simonb (inactive)
2014/06/04 16:40:35
This should be true. The current relocations are
| |
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. | |
rmcilroy
2014/06/02 15:16:35
nit - mention there is only Arm support currently
simonb (inactive)
2014/06/04 16:40:35
Done.
| |
16 | |
17 A packed libchrome.<version>.so cannot currently be used with the standard | |
18 Android runtime linker. | |
19 | |
20 See src/*.h for design and implementation notes. | |
21 | |
22 | |
23 Notes: | |
24 ------ | |
25 | |
26 Packing does not adjust debug data. An unstripped libchrome.<version>.so | |
27 can be packed and will run, but may no longer be useful for debugging. | |
28 | |
29 Requires the explicit support of an extended crazy linker. Adds the | |
rmcilroy
2014/06/02 15:16:35
nit - Unpacking requires the...
simonb (inactive)
2014/06/04 16:40:35
Done.
| |
30 following new .dynamic tags, used by the crazy linker to find the packed | |
31 .android.rel.dyn section data: | |
32 | |
33 DT_ANDROID_ARM_REL_OFFSET = DT_LOPROC (Processor specific: 0x70000000) | |
34 - The offset of .android.rel.dyn data in libchrome.<version>.so | |
35 DT_ANDROID_ARM_REL_SIZE = DT_LOPROC + 1 (Processor Specific: 0x70000001) | |
36 - The size of .android.rel.dyn data in bytes | |
37 | |
38 The format of .android.rel.dyn data is: | |
39 | |
40 "APR1" identifier | |
41 N: the number of count-delta pairs in the encoding | |
42 A: the initial offset | |
43 N * C,D: N count-delta pairs | |
44 | |
45 All numbers in the encoding stream are stored as LEB128 values. For details | |
46 see http://en.wikipedia.org/wiki/LEB128. | |
47 | |
48 The streaming unpacking algorithm is: | |
49 | |
50 skip over "APR1" | |
51 pairs, addr = next leb128 value, next leb128 value | |
52 emit R_ARM_RELATIVE relocation with r_offset = addr | |
53 while pairs: | |
54 count, delta = next leb128 value, next leb128 value | |
55 while count: | |
56 addr += delta | |
57 emit R_ARM_RELATIVE relocation with r_offset = addr | |
58 count-- | |
59 pairs--; | |
60 | |
61 | |
62 Usage instructions: | |
63 ------------------- | |
64 | |
65 To pack relocations, add an empty .android.rel.dyn and then run the tool: | |
66 | |
67 echo -n 'NULL' >/tmp/small | |
68 arm-linux-gnueabi-objcopy \ | |
69 --add-section .android.rel.dyn=/tmp/small \ | |
70 libchrome.<version>.so libchrome.<version>.so.packed | |
71 rm /tmp/small | |
72 pack-relocations libchrome.<version>.so.packed | |
rmcilroy
2014/06/02 15:16:35
Could the tool do the adding and removing of secti
simonb (inactive)
2014/06/04 16:40:35
It's icky work that it is better to avoid. Adding
| |
73 | |
74 To unpack and restore the shared library to its original state: | |
75 | |
76 cp libchrome.<version>.so.packed unpackable | |
77 pack-relocations -u unpackable | |
rmcilroy
2014/06/02 15:16:35
From the gyp file, looks like the executable is "r
simonb (inactive)
2014/06/04 16:40:35
Done. (Resulted from a last-minute change to the
rmcilroy
2014/06/07 11:49:06
relocation_packer (not relocations_packer) ;)
simonb (inactive)
2014/06/09 14:39:19
Done.
| |
78 arm-linux-gnueabi-objcopy \ | |
79 --remove-section=.android.rel.dyn unpackable libchrome.<version>.so | |
80 rm unpackable | |
81 | |
82 | |
83 Bugs & TODOs: | |
84 ------------- | |
85 | |
86 Currently only supports arm32. Support for arm64 requires some extension | |
87 and modification. | |
88 | |
89 Expects to find at least two free slots in the .dynamic section. Uses | |
90 these to add data that tells the crazy linker where to find the packed | |
91 .android.rel.dyn data. | |
rmcilroy
2014/06/02 15:16:35
What if there are not two free slots? Can we forc
simonb (inactive)
2014/06/04 16:40:35
We can, but... growing the table means resizing it
| |
92 | |
93 Requires libelf 0.158 or later. Earlier libelf releases may be buggy in | |
94 ways that prevent the packer from working correctly. | |
95 | |
96 | |
97 Testing: | |
98 -------- | |
99 | |
100 Unittests run under gtest, on the host system. | |
OLD | NEW |