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