OLD | NEW |
---|---|
(Empty) | |
1 // Copyright 2014 The Chromium Authors. All rights reserved. | |
2 // Use of this source code is governed by a BSD-style license that can be | |
3 // found in the LICENSE file. | |
4 | |
5 // Run-length encode and decode R_ARM_RELATIVE relocations. | |
6 // | |
7 // R_ARM_RELATIVE relocations are the bulk of dynamic relocations (the | |
8 // .rel.dyn section) in libchrome<version>.so, and the ELF standard | |
9 // representation of them is wasteful. | |
10 // | |
11 // A relocation is 8 bytes (16 bytes on 64 bit plaforms), split into offset | |
12 // and info fields. Offsets strictly increase, and each is commonly a | |
13 // few bytes different from its predecessor. There are long runs where | |
14 // the difference does not change. The info field is always 0x17. Example, | |
15 // from 'readelf -x4 libchrome.<version>.so': | |
16 // | |
17 // offset info offset info | |
18 // 808fef01 17000000 848fef01 17000000 ................ | |
19 // 888fef01 17000000 8c8fef01 17000000 ................ | |
20 // 908fef01 17000000 948fef01 17000000 ................ | |
21 // | |
22 // Run length encoding packs this data more efficiently, by representing it | |
23 // as a delta and a count of entries each differing from its predecessor | |
24 // by this delta. The above can be represented as a start address followed | |
25 // by an encoded count of 6 and offset difference of 4: | |
26 // | |
27 // start count diff | |
28 // 808fef01 00000006 00000004 | |
29 // | |
30 // Because R_ARM_RELATIVE relocation offsets strictly increase, the complete | |
31 // set of R_ARM_RELATIVE relocations in libchrome.<version>.so can be | |
32 // represented by a single start address followed by one or more difference | |
33 // and count encoded word pairs: | |
34 // | |
35 // start run1 count run1 diff run2 count run2 diff | |
36 // 808fef01 00000006 00000004 00000010 00000008 ... | |
37 // | |
38 // Decoding regenerates R_ARM_RELATIVE relocations beginning at address | |
39 // 'start' and for each encoded run, incrementing the address by 'difference' | |
40 // for 'count' iterations and emitting a new R_ARM_RELATIVE relocation. | |
41 // | |
42 // Once encoded, data is prefixed by a single word count of packed delta and | |
43 // count pairs. A final run-length encoded R_ARM_RELATIVE relocations vector | |
44 // might therefore look something like: | |
45 // | |
46 // pairs start run 1 run 2 ... run 15 | |
47 // 0000000f 808fef01 00000006 00000004 00000010 00000008 ... | |
48 // Interpreted as: | |
49 // pairs=15 start=.. count=6,delta=4 count=16,delta=8 | |
rmcilroy
2014/06/02 15:16:35
It seems like this should be applicable to all Elf
simonb (inactive)
2014/06/04 16:40:35
Noted.
| |
50 | |
51 #ifndef RELOCATION_PACKER_RLE_H | |
52 #define RELOCATION_PACKER_RLE_H | |
53 | |
54 #include <stdint.h> | |
55 #include <string.h> | |
56 #include <vector> | |
57 | |
58 #include "elf.h" | |
59 | |
60 namespace relocation_packer { | |
61 | |
62 // A RelocationRunLengthCodec packs vectors of R_ARM_RELATIVE relocations | |
63 // into more compact forms, and unpacks them to reproduce the pre-packed data. | |
64 class RelocationRunLengthCodec { | |
65 public: | |
66 // Encode R_ARM_RELATIVE relocations into a more compact form. | |
67 // |relocations| is a vector of R_ARM_RELATIVE relocation structs. | |
68 // |packed| is the vector of packed words into which relocations are packed. | |
69 static void Encode(const std::vector<Elf32_Rel>& relocations, | |
70 std::vector<Elf32_Word>* packed); | |
71 | |
72 // Decode R_ARM_RELATIVE relocations from their more compact form. | |
73 // |packed| is the vector of packed relocations. | |
74 // |relocations| is a vector of unpacked R_ARM_RELATIVE relocation structs. | |
75 static void Decode(const std::vector<Elf32_Word>& packed, | |
76 std::vector<Elf32_Rel>* relocations); | |
77 }; | |
78 | |
79 } // namespace relocation_packer | |
80 | |
81 #endif // RELOCATION_PACKER_PACKER_H | |
OLD | NEW |