1 =========================== 1 ===========================
2 Livepatch module ELF format !! 2 Livepatch module Elf format
3 =========================== 3 ===========================
4 4
5 This document outlines the ELF format requirem !! 5 This document outlines the Elf format requirements that livepatch modules must follow.
6 6
7 7
8 .. Table of Contents 8 .. Table of Contents
9 9
10 .. contents:: :local: !! 10 1. Background and motivation
11 !! 11 2. Livepatch modinfo field
>> 12 3. Livepatch relocation sections
>> 13 3.1 Livepatch relocation section format
>> 14 4. Livepatch symbols
>> 15 4.1 A livepatch module's symbol table
>> 16 4.2 Livepatch symbol format
>> 17 5. Symbol table and Elf section access
12 18
13 1. Background and motivation 19 1. Background and motivation
14 ============================ 20 ============================
15 21
16 Formerly, livepatch required separate architec 22 Formerly, livepatch required separate architecture-specific code to write
17 relocations. However, arch-specific code to wr 23 relocations. However, arch-specific code to write relocations already
18 exists in the module loader, so this former ap 24 exists in the module loader, so this former approach produced redundant
19 code. So, instead of duplicating code and re-i 25 code. So, instead of duplicating code and re-implementing what the module
20 loader can already do, livepatch leverages exi 26 loader can already do, livepatch leverages existing code in the module
21 loader to perform the all the arch-specific re 27 loader to perform the all the arch-specific relocation work. Specifically,
22 livepatch reuses the apply_relocate_add() func 28 livepatch reuses the apply_relocate_add() function in the module loader to
23 write relocations. The patch module ELF format !! 29 write relocations. The patch module Elf format described in this document
24 enables livepatch to be able to do this. The h 30 enables livepatch to be able to do this. The hope is that this will make
25 livepatch more easily portable to other archit 31 livepatch more easily portable to other architectures and reduce the amount
26 of arch-specific code required to port livepat 32 of arch-specific code required to port livepatch to a particular
27 architecture. 33 architecture.
28 34
29 Since apply_relocate_add() requires access to 35 Since apply_relocate_add() requires access to a module's section header
30 table, symbol table, and relocation section in !! 36 table, symbol table, and relocation section indices, Elf information is
31 preserved for livepatch modules (see section 5 37 preserved for livepatch modules (see section 5). Livepatch manages its own
32 relocation sections and symbols, which are des 38 relocation sections and symbols, which are described in this document. The
33 ELF constants used to mark livepatch symbols a !! 39 Elf constants used to mark livepatch symbols and relocation sections were
34 selected from OS-specific ranges according to 40 selected from OS-specific ranges according to the definitions from glibc.
35 41
36 Why does livepatch need to write its own reloc 42 Why does livepatch need to write its own relocations?
37 ---------------------------------------------- 43 -----------------------------------------------------
38 A typical livepatch module contains patched ve 44 A typical livepatch module contains patched versions of functions that can
39 reference non-exported global symbols and non- 45 reference non-exported global symbols and non-included local symbols.
40 Relocations referencing these types of symbols 46 Relocations referencing these types of symbols cannot be left in as-is
41 since the kernel module loader cannot resolve 47 since the kernel module loader cannot resolve them and will therefore
42 reject the livepatch module. Furthermore, we c 48 reject the livepatch module. Furthermore, we cannot apply relocations that
43 affect modules not yet loaded at patch module 49 affect modules not yet loaded at patch module load time (e.g. a patch to a
44 driver that is not loaded). Formerly, livepatc 50 driver that is not loaded). Formerly, livepatch solved this problem by
45 embedding special "dynrela" (dynamic rela) sec 51 embedding special "dynrela" (dynamic rela) sections in the resulting patch
46 module ELF output. Using these dynrela section !! 52 module Elf output. Using these dynrela sections, livepatch could resolve
47 symbols while taking into account its scope an 53 symbols while taking into account its scope and what module the symbol
48 belongs to, and then manually apply the dynami 54 belongs to, and then manually apply the dynamic relocations. However this
49 approach required livepatch to supply arch-spe 55 approach required livepatch to supply arch-specific code in order to write
50 these relocations. In the new format, livepatc 56 these relocations. In the new format, livepatch manages its own SHT_RELA
51 relocation sections in place of dynrela sectio 57 relocation sections in place of dynrela sections, and the symbols that the
52 relas reference are special livepatch symbols 58 relas reference are special livepatch symbols (see section 2 and 3). The
53 arch-specific livepatch relocation code is rep 59 arch-specific livepatch relocation code is replaced by a call to
54 apply_relocate_add(). 60 apply_relocate_add().
55 61
56 2. Livepatch modinfo field 62 2. Livepatch modinfo field
57 ========================== 63 ==========================
58 64
59 Livepatch modules are required to have the "li 65 Livepatch modules are required to have the "livepatch" modinfo attribute.
60 See the sample livepatch module in samples/liv 66 See the sample livepatch module in samples/livepatch/ for how this is done.
61 67
62 Livepatch modules can be identified by users b 68 Livepatch modules can be identified by users by using the 'modinfo' command
63 and looking for the presence of the "livepatch 69 and looking for the presence of the "livepatch" field. This field is also
64 used by the kernel module loader to identify l 70 used by the kernel module loader to identify livepatch modules.
65 71
66 Example: 72 Example:
67 -------- 73 --------
68 74
69 **Modinfo output:** 75 **Modinfo output:**
70 76
71 :: 77 ::
72 78
73 % modinfo livepatch-meminfo.ko 79 % modinfo livepatch-meminfo.ko
74 filename: livepatch-memi 80 filename: livepatch-meminfo.ko
75 livepatch: Y 81 livepatch: Y
76 license: GPL 82 license: GPL
77 depends: 83 depends:
78 vermagic: 4.3.0+ SMP mod 84 vermagic: 4.3.0+ SMP mod_unload
79 85
80 3. Livepatch relocation sections 86 3. Livepatch relocation sections
81 ================================ 87 ================================
82 88
83 A livepatch module manages its own ELF relocat !! 89 A livepatch module manages its own Elf relocation sections to apply
84 relocations to modules as well as to the kerne 90 relocations to modules as well as to the kernel (vmlinux) at the
85 appropriate time. For example, if a patch modu 91 appropriate time. For example, if a patch module patches a driver that is
86 not currently loaded, livepatch will apply the 92 not currently loaded, livepatch will apply the corresponding livepatch
87 relocation section(s) to the driver once it lo 93 relocation section(s) to the driver once it loads.
88 94
89 Each "object" (e.g. vmlinux, or a module) with 95 Each "object" (e.g. vmlinux, or a module) within a patch module may have
90 multiple livepatch relocation sections associa 96 multiple livepatch relocation sections associated with it (e.g. patches to
91 multiple functions within the same object). Th 97 multiple functions within the same object). There is a 1-1 correspondence
92 between a livepatch relocation section and the 98 between a livepatch relocation section and the target section (usually the
93 text section of a function) to which the reloc 99 text section of a function) to which the relocation(s) apply. It is
94 also possible for a livepatch module to have n 100 also possible for a livepatch module to have no livepatch relocation
95 sections, as in the case of the sample livepat 101 sections, as in the case of the sample livepatch module (see
96 samples/livepatch). 102 samples/livepatch).
97 103
98 Since ELF information is preserved for livepat !! 104 Since Elf information is preserved for livepatch modules (see Section 5), a
99 livepatch relocation section can be applied si 105 livepatch relocation section can be applied simply by passing in the
100 appropriate section index to apply_relocate_ad 106 appropriate section index to apply_relocate_add(), which then uses it to
101 access the relocation section and apply the re 107 access the relocation section and apply the relocations.
102 108
103 Every symbol referenced by a rela in a livepat 109 Every symbol referenced by a rela in a livepatch relocation section is a
104 livepatch symbol. These must be resolved befor 110 livepatch symbol. These must be resolved before livepatch can call
105 apply_relocate_add(). See Section 3 for more i 111 apply_relocate_add(). See Section 3 for more information.
106 112
107 3.1 Livepatch relocation section format 113 3.1 Livepatch relocation section format
108 ======================================= 114 =======================================
109 115
110 Livepatch relocation sections must be marked w 116 Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH
111 section flag. See include/uapi/linux/elf.h for 117 section flag. See include/uapi/linux/elf.h for the definition. The module
112 loader recognizes this flag and will avoid app 118 loader recognizes this flag and will avoid applying those relocation sections
113 at patch module load time. These sections must 119 at patch module load time. These sections must also be marked with SHF_ALLOC,
114 so that the module loader doesn't discard them 120 so that the module loader doesn't discard them on module load (i.e. they will
115 be copied into memory along with the other SHF 121 be copied into memory along with the other SHF_ALLOC sections).
116 122
117 The name of a livepatch relocation section mus 123 The name of a livepatch relocation section must conform to the following
118 format:: 124 format::
119 125
120 .klp.rela.objname.section_name 126 .klp.rela.objname.section_name
121 ^ ^^ ^ ^ ^ 127 ^ ^^ ^ ^ ^
122 |________||_____| |__________| 128 |________||_____| |__________|
123 [A] [B] [C] 129 [A] [B] [C]
124 130
125 [A] 131 [A]
126 The relocation section name is prefixed with 132 The relocation section name is prefixed with the string ".klp.rela."
127 133
128 [B] 134 [B]
129 The name of the object (i.e. "vmlinux" or na 135 The name of the object (i.e. "vmlinux" or name of module) to
130 which the relocation section belongs follows 136 which the relocation section belongs follows immediately after the prefix.
131 137
132 [C] 138 [C]
133 The actual name of the section to which this 139 The actual name of the section to which this relocation section applies.
134 140
135 Examples: 141 Examples:
136 --------- 142 ---------
137 143
138 **Livepatch relocation section names:** 144 **Livepatch relocation section names:**
139 145
140 :: 146 ::
141 147
142 .klp.rela.ext4.text.ext4_attr_store 148 .klp.rela.ext4.text.ext4_attr_store
143 .klp.rela.vmlinux.text.cmdline_proc_show 149 .klp.rela.vmlinux.text.cmdline_proc_show
144 150
145 **`readelf --sections` output for a patch 151 **`readelf --sections` output for a patch
146 module that patches vmlinux and modules 9p, bt 152 module that patches vmlinux and modules 9p, btrfs, ext4:**
147 153
148 :: 154 ::
149 155
150 Section Headers: 156 Section Headers:
151 [Nr] Name Type 157 [Nr] Name Type Address Off Size ES Flg Lk Inf Al
152 [ snip ] 158 [ snip ]
153 [29] .klp.rela.9p.text.caches.show RELA 159 [29] .klp.rela.9p.text.caches.show RELA 0000000000000000 002d58 0000c0 18 AIo 64 9 8
154 [30] .klp.rela.btrfs.text.btrfs.feature.attr 160 [30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA 0000000000000000 002e18 000060 18 AIo 64 11 8
155 [ snip ] 161 [ snip ]
156 [34] .klp.rela.ext4.text.ext4.attr.store REL 162 [34] .klp.rela.ext4.text.ext4.attr.store RELA 0000000000000000 002fd8 0000d8 18 AIo 64 13 8
157 [35] .klp.rela.ext4.text.ext4.attr.show RELA 163 [35] .klp.rela.ext4.text.ext4.attr.show RELA 0000000000000000 0030b0 000150 18 AIo 64 15 8
158 [36] .klp.rela.vmlinux.text.cmdline.proc.sho 164 [36] .klp.rela.vmlinux.text.cmdline.proc.show RELA 0000000000000000 003200 000018 18 AIo 64 17 8
159 [37] .klp.rela.vmlinux.text.meminfo.proc.sho 165 [37] .klp.rela.vmlinux.text.meminfo.proc.show RELA 0000000000000000 003218 0000f0 18 AIo 64 19 8
160 [ snip ] 166 [ snip ] ^ ^
161 167 | |
162 168 [*] [*]
163 169
164 [*] 170 [*]
165 Livepatch relocation sections are SHT_RELA s 171 Livepatch relocation sections are SHT_RELA sections but with a few special
166 characteristics. Notice that they are marked 172 characteristics. Notice that they are marked SHF_ALLOC ("A") so that they will
167 not be discarded when the module is loaded i 173 not be discarded when the module is loaded into memory, as well as with the
168 SHF_RELA_LIVEPATCH flag ("o" - for OS-specif 174 SHF_RELA_LIVEPATCH flag ("o" - for OS-specific).
169 175
170 **`readelf --relocs` output for a patch module 176 **`readelf --relocs` output for a patch module:**
171 177
172 :: 178 ::
173 179
174 Relocation section '.klp.rela.btrfs.text.btr 180 Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
175 Offset Info Type 181 Offset Info Type Symbol's Value Symbol's Name + Addend
176 000000000000001f 0000005e00000002 R_X86_64_ 182 000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
177 0000000000000028 0000003d0000000b R_X86_64_ 183 0000000000000028 0000003d0000000b R_X86_64_32S 0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0
178 0000000000000036 0000003b00000002 R_X86_64_ 184 0000000000000036 0000003b00000002 R_X86_64_PC32 0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4
179 000000000000004c 0000004900000002 R_X86_64_ 185 000000000000004c 0000004900000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4
180 [ snip ] 186 [ snip ] ^
181 187 |
182 188 [*]
183 189
184 [*] 190 [*]
185 Every symbol referenced by a relocation is a 191 Every symbol referenced by a relocation is a livepatch symbol.
186 192
187 4. Livepatch symbols 193 4. Livepatch symbols
188 ==================== 194 ====================
189 195
190 Livepatch symbols are symbols referred to by l 196 Livepatch symbols are symbols referred to by livepatch relocation sections.
191 These are symbols accessed from new versions o 197 These are symbols accessed from new versions of functions for patched
192 objects, whose addresses cannot be resolved by 198 objects, whose addresses cannot be resolved by the module loader (because
193 they are local or unexported global syms). Sin 199 they are local or unexported global syms). Since the module loader only
194 resolves exported syms, and not every symbol r 200 resolves exported syms, and not every symbol referenced by the new patched
195 functions is exported, livepatch symbols were 201 functions is exported, livepatch symbols were introduced. They are used
196 also in cases where we cannot immediately know 202 also in cases where we cannot immediately know the address of a symbol when
197 a patch module loads. For example, this is the 203 a patch module loads. For example, this is the case when livepatch patches
198 a module that is not loaded yet. In this case, 204 a module that is not loaded yet. In this case, the relevant livepatch
199 symbols are resolved simply when the target mo 205 symbols are resolved simply when the target module loads. In any case, for
200 any livepatch relocation section, all livepatc 206 any livepatch relocation section, all livepatch symbols referenced by that
201 section must be resolved before livepatch can 207 section must be resolved before livepatch can call apply_relocate_add() for
202 that reloc section. 208 that reloc section.
203 209
204 Livepatch symbols must be marked with SHN_LIVE 210 Livepatch symbols must be marked with SHN_LIVEPATCH so that the module
205 loader can identify and ignore them. Livepatch 211 loader can identify and ignore them. Livepatch modules keep these symbols
206 in their symbol tables, and the symbol table i 212 in their symbol tables, and the symbol table is made accessible through
207 module->symtab. 213 module->symtab.
208 214
209 4.1 A livepatch module's symbol table 215 4.1 A livepatch module's symbol table
210 ===================================== 216 =====================================
211 Normally, a stripped down copy of a module's s 217 Normally, a stripped down copy of a module's symbol table (containing only
212 "core" symbols) is made available through modu 218 "core" symbols) is made available through module->symtab (See layout_symtab()
213 in kernel/module/kallsyms.c). For livepatch mo !! 219 in kernel/module.c). For livepatch modules, the symbol table copied into memory
214 into memory on module load must be exactly the !! 220 on module load must be exactly the same as the symbol table produced when the
215 when the patch module was compiled. This is be !! 221 patch module was compiled. This is because the relocations in each livepatch
216 livepatch relocation section refer to their re !! 222 relocation section refer to their respective symbols with their symbol indices,
217 indices, and the original symbol indices (and !! 223 and the original symbol indices (and thus the symtab ordering) must be
218 preserved in order for apply_relocate_add() to 224 preserved in order for apply_relocate_add() to find the right symbol.
219 225
220 For example, take this particular rela from a 226 For example, take this particular rela from a livepatch module:::
221 227
222 Relocation section '.klp.rela.btrfs.text.btr 228 Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
223 Offset Info Type 229 Offset Info Type Symbol's Value Symbol's Name + Addend
224 000000000000001f 0000005e00000002 R_X86_64_ 230 000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
225 231
226 This rela refers to the symbol '.klp.sym.vml 232 This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded
227 in 'Info'. Here its symbol index is 0x5e, wh 233 in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the
228 symbol index 94. 234 symbol index 94.
229 And in this patch module's corresponding sym 235 And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol:
230 [ snip ] 236 [ snip ]
231 94: 0000000000000000 0 NOTYPE GLOBAL DE 237 94: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0
232 [ snip ] 238 [ snip ]
233 239
234 4.2 Livepatch symbol format 240 4.2 Livepatch symbol format
235 =========================== 241 ===========================
236 242
237 Livepatch symbols must have their section inde 243 Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so
238 that the module loader can identify them and n 244 that the module loader can identify them and not attempt to resolve them.
239 See include/uapi/linux/elf.h for the actual de 245 See include/uapi/linux/elf.h for the actual definitions.
240 246
241 Livepatch symbol names must conform to the fol 247 Livepatch symbol names must conform to the following format::
242 248
243 .klp.sym.objname.symbol_name,sympos 249 .klp.sym.objname.symbol_name,sympos
244 ^ ^^ ^ ^ ^ ^ 250 ^ ^^ ^ ^ ^ ^
245 |_______||_____| |_________| | 251 |_______||_____| |_________| |
246 [A] [B] [C] [D] 252 [A] [B] [C] [D]
247 253
248 [A] 254 [A]
249 The symbol name is prefixed with the string 255 The symbol name is prefixed with the string ".klp.sym."
250 256
251 [B] 257 [B]
252 The name of the object (i.e. "vmlinux" or na 258 The name of the object (i.e. "vmlinux" or name of module) to
253 which the symbol belongs follows immediately 259 which the symbol belongs follows immediately after the prefix.
254 260
255 [C] 261 [C]
256 The actual name of the symbol. 262 The actual name of the symbol.
257 263
258 [D] 264 [D]
259 The position of the symbol in the object (as 265 The position of the symbol in the object (as according to kallsyms)
260 This is used to differentiate duplicate symb 266 This is used to differentiate duplicate symbols within the same
261 object. The symbol position is expressed num 267 object. The symbol position is expressed numerically (0, 1, 2...).
262 The symbol position of a unique symbol is 0. 268 The symbol position of a unique symbol is 0.
263 269
264 Examples: 270 Examples:
265 --------- 271 ---------
266 272
267 **Livepatch symbol names:** 273 **Livepatch symbol names:**
268 274
269 :: 275 ::
270 276
271 .klp.sym.vmlinux.snprintf,0 277 .klp.sym.vmlinux.snprintf,0
272 .klp.sym.vmlinux.printk,0 278 .klp.sym.vmlinux.printk,0
273 .klp.sym.btrfs.btrfs_ktype,0 279 .klp.sym.btrfs.btrfs_ktype,0
274 280
275 **`readelf --symbols` output for a patch modul 281 **`readelf --symbols` output for a patch module:**
276 282
277 :: 283 ::
278 284
279 Symbol table '.symtab' contains 127 entries: 285 Symbol table '.symtab' contains 127 entries:
280 Num: Value Size Type Bind 286 Num: Value Size Type Bind Vis Ndx Name
281 [ snip ] 287 [ snip ]
282 73: 0000000000000000 0 NOTYPE GLOBA 288 73: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0
283 74: 0000000000000000 0 NOTYPE GLOBA 289 74: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0
284 75: 0000000000000000 0 NOTYPE GLOBA 290 75: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0
285 76: 0000000000000000 0 NOTYPE GLOBA 291 76: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0
286 [ snip ] 292 [ snip ] ^
287 293 |
288 294 [*]
289 295
290 [*] 296 [*]
291 Note that the 'Ndx' (Section index) for thes 297 Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
292 "OS" means OS-specific. 298 "OS" means OS-specific.
293 299
294 5. Symbol table and ELF section access !! 300 5. Symbol table and Elf section access
295 ====================================== 301 ======================================
296 A livepatch module's symbol table is accessibl 302 A livepatch module's symbol table is accessible through module->symtab.
297 303
298 Since apply_relocate_add() requires access to 304 Since apply_relocate_add() requires access to a module's section headers,
299 symbol table, and relocation section indices, !! 305 symbol table, and relocation section indices, Elf information is preserved for
300 livepatch modules and is made accessible by th 306 livepatch modules and is made accessible by the module loader through
301 module->klp_info, which is a :c:type:`klp_modi !! 307 module->klp_info, which is a klp_modinfo struct. When a livepatch module loads,
302 loads, this struct is filled in by the module !! 308 this struct is filled in by the module loader. Its fields are documented below::
>> 309
>> 310 struct klp_modinfo {
>> 311 Elf_Ehdr hdr; /* Elf header */
>> 312 Elf_Shdr *sechdrs; /* Section header table */
>> 313 char *secstrings; /* String table for the section headers */
>> 314 unsigned int symndx; /* The symbol table section index */
>> 315 };
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