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1 ===========================
2 Livepatch module Elf format
3 ===========================
5 This document outlines the Elf format requirements that livepatch modules must follow.
7 -----------------
8 Table of Contents
9 -----------------
10 0. Background and motivation
11 1. Livepatch modinfo field
12 2. Livepatch relocation sections
13 2.1 What are livepatch relocation sections?
14 2.2 Livepatch relocation section format
15 2.2.1 Required flags
16 2.2.2 Required name format
17 2.2.3 Example livepatch relocation section names
18 2.2.4 Example `readelf --sections` output
19 2.2.5 Example `readelf --relocs` output
20 3. Livepatch symbols
21 3.1 What are livepatch symbols?
22 3.2 A livepatch module's symbol table
23 3.3 Livepatch symbol format
24 3.3.1 Required flags
25 3.3.2 Required name format
26 3.3.3 Example livepatch symbol names
27 3.3.4 Example `readelf --symbols` output
28 4. Symbol table and Elf section access
30 ----------------------------
31 0. Background and motivation
32 ----------------------------
34 Formerly, livepatch required separate architecture-specific code to write
35 relocations. However, arch-specific code to write relocations already
36 exists in the module loader, so this former approach produced redundant
37 code. So, instead of duplicating code and re-implementing what the module
38 loader can already do, livepatch leverages existing code in the module
39 loader to perform the all the arch-specific relocation work. Specifically,
40 livepatch reuses the apply_relocate_add() function in the module loader to
41 write relocations. The patch module Elf format described in this document
42 enables livepatch to be able to do this. The hope is that this will make
43 livepatch more easily portable to other architectures and reduce the amount
44 of arch-specific code required to port livepatch to a particular
45 architecture.
47 Since apply_relocate_add() requires access to a module's section header
48 table, symbol table, and relocation section indices, Elf information is
49 preserved for livepatch modules (see section 4). Livepatch manages its own
50 relocation sections and symbols, which are described in this document. The
51 Elf constants used to mark livepatch symbols and relocation sections were
52 selected from OS-specific ranges according to the definitions from glibc.
54 0.1 Why does livepatch need to write its own relocations?
55 ---------------------------------------------------------
56 A typical livepatch module contains patched versions of functions that can
57 reference non-exported global symbols and non-included local symbols.
58 Relocations referencing these types of symbols cannot be left in as-is
59 since the kernel module loader cannot resolve them and will therefore
60 reject the livepatch module. Furthermore, we cannot apply relocations that
61 affect modules not yet loaded at patch module load time (e.g. a patch to a
62 driver that is not loaded). Formerly, livepatch solved this problem by
63 embedding special "dynrela" (dynamic rela) sections in the resulting patch
64 module Elf output. Using these dynrela sections, livepatch could resolve
65 symbols while taking into account its scope and what module the symbol
66 belongs to, and then manually apply the dynamic relocations. However this
67 approach required livepatch to supply arch-specific code in order to write
68 these relocations. In the new format, livepatch manages its own SHT_RELA
69 relocation sections in place of dynrela sections, and the symbols that the
70 relas reference are special livepatch symbols (see section 2 and 3). The
71 arch-specific livepatch relocation code is replaced by a call to
72 apply_relocate_add().
74 ================================
75 PATCH MODULE FORMAT REQUIREMENTS
76 ================================
78 --------------------------
79 1. Livepatch modinfo field
80 --------------------------
82 Livepatch modules are required to have the "livepatch" modinfo attribute.
83 See the sample livepatch module in samples/livepatch/ for how this is done.
85 Livepatch modules can be identified by users by using the 'modinfo' command
86 and looking for the presence of the "livepatch" field. This field is also
87 used by the kernel module loader to identify livepatch modules.
89 Example modinfo output:
90 -----------------------
91 % modinfo livepatch-meminfo.ko
92 filename: livepatch-meminfo.ko
93 livepatch: Y
94 license: GPL
95 depends:
96 vermagic: 4.3.0+ SMP mod_unload
98 --------------------------------
99 2. Livepatch relocation sections
100 --------------------------------
102 -------------------------------------------
103 2.1 What are livepatch relocation sections?
104 -------------------------------------------
105 A livepatch module manages its own Elf relocation sections to apply
106 relocations to modules as well as to the kernel (vmlinux) at the
107 appropriate time. For example, if a patch module patches a driver that is
108 not currently loaded, livepatch will apply the corresponding livepatch
109 relocation section(s) to the driver once it loads.
111 Each "object" (e.g. vmlinux, or a module) within a patch module may have
112 multiple livepatch relocation sections associated with it (e.g. patches to
113 multiple functions within the same object). There is a 1-1 correspondence
114 between a livepatch relocation section and the target section (usually the
115 text section of a function) to which the relocation(s) apply. It is
116 also possible for a livepatch module to have no livepatch relocation
117 sections, as in the case of the sample livepatch module (see
118 samples/livepatch).
120 Since Elf information is preserved for livepatch modules (see Section 4), a
121 livepatch relocation section can be applied simply by passing in the
122 appropriate section index to apply_relocate_add(), which then uses it to
123 access the relocation section and apply the relocations.
125 Every symbol referenced by a rela in a livepatch relocation section is a
126 livepatch symbol. These must be resolved before livepatch can call
127 apply_relocate_add(). See Section 3 for more information.
129 ---------------------------------------
130 2.2 Livepatch relocation section format
131 ---------------------------------------
133 2.2.1 Required flags
134 --------------------
135 Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH
136 section flag. See include/uapi/linux/elf.h for the definition. The module
137 loader recognizes this flag and will avoid applying those relocation sections
138 at patch module load time. These sections must also be marked with SHF_ALLOC,
139 so that the module loader doesn't discard them on module load (i.e. they will
140 be copied into memory along with the other SHF_ALLOC sections).
142 2.2.2 Required name format
143 --------------------------
144 The name of a livepatch relocation section must conform to the following format:
146 .klp.rela.objname.section_name
147 ^ ^^ ^ ^ ^
148 |________||_____| |__________|
149 [A] [B] [C]
151 [A] The relocation section name is prefixed with the string ".klp.rela."
152 [B] The name of the object (i.e. "vmlinux" or name of module) to
153 which the relocation section belongs follows immediately after the prefix.
154 [C] The actual name of the section to which this relocation section applies.
156 2.2.3 Example livepatch relocation section names:
157 -------------------------------------------------
158 .klp.rela.ext4.text.ext4_attr_store
159 .klp.rela.vmlinux.text.cmdline_proc_show
161 2.2.4 Example `readelf --sections` output for a patch
162 module that patches vmlinux and modules 9p, btrfs, ext4:
163 --------------------------------------------------------
164 Section Headers:
165 [Nr] Name Type Address Off Size ES Flg Lk Inf Al
166 [ snip ]
167 [29] .klp.rela.9p.text.caches.show RELA 0000000000000000 002d58 0000c0 18 AIo 64 9 8
168 [30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA 0000000000000000 002e18 000060 18 AIo 64 11 8
169 [ snip ]
170 [34] .klp.rela.ext4.text.ext4.attr.store RELA 0000000000000000 002fd8 0000d8 18 AIo 64 13 8
171 [35] .klp.rela.ext4.text.ext4.attr.show RELA 0000000000000000 0030b0 000150 18 AIo 64 15 8
172 [36] .klp.rela.vmlinux.text.cmdline.proc.show RELA 0000000000000000 003200 000018 18 AIo 64 17 8
173 [37] .klp.rela.vmlinux.text.meminfo.proc.show RELA 0000000000000000 003218 0000f0 18 AIo 64 19 8
174 [ snip ] ^ ^
175 | |
176 [*] [*]
177 [*] Livepatch relocation sections are SHT_RELA sections but with a few special
178 characteristics. Notice that they are marked SHF_ALLOC ("A") so that they will
179 not be discarded when the module is loaded into memory, as well as with the
180 SHF_RELA_LIVEPATCH flag ("o" - for OS-specific).
182 2.2.5 Example `readelf --relocs` output for a patch module:
183 -----------------------------------------------------------
184 Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
185 Offset Info Type Symbol's Value Symbol's Name + Addend
186 000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
187 0000000000000028 0000003d0000000b R_X86_64_32S 0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0
188 0000000000000036 0000003b00000002 R_X86_64_PC32 0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4
189 000000000000004c 0000004900000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4
190 [ snip ] ^
191 |
192 [*]
193 [*] Every symbol referenced by a relocation is a livepatch symbol.
195 --------------------
196 3. Livepatch symbols
197 --------------------
199 -------------------------------
200 3.1 What are livepatch symbols?
201 -------------------------------
202 Livepatch symbols are symbols referred to by livepatch relocation sections.
203 These are symbols accessed from new versions of functions for patched
204 objects, whose addresses cannot be resolved by the module loader (because
205 they are local or unexported global syms). Since the module loader only
206 resolves exported syms, and not every symbol referenced by the new patched
207 functions is exported, livepatch symbols were introduced. They are used
208 also in cases where we cannot immediately know the address of a symbol when
209 a patch module loads. For example, this is the case when livepatch patches
210 a module that is not loaded yet. In this case, the relevant livepatch
211 symbols are resolved simply when the target module loads. In any case, for
212 any livepatch relocation section, all livepatch symbols referenced by that
213 section must be resolved before livepatch can call apply_relocate_add() for
214 that reloc section.
216 Livepatch symbols must be marked with SHN_LIVEPATCH so that the module
217 loader can identify and ignore them. Livepatch modules keep these symbols
218 in their symbol tables, and the symbol table is made accessible through
219 module->symtab.
221 -------------------------------------
222 3.2 A livepatch module's symbol table
223 -------------------------------------
224 Normally, a stripped down copy of a module's symbol table (containing only
225 "core" symbols) is made available through module->symtab (See layout_symtab()
226 in kernel/module.c). For livepatch modules, the symbol table copied into memory
227 on module load must be exactly the same as the symbol table produced when the
228 patch module was compiled. This is because the relocations in each livepatch
229 relocation section refer to their respective symbols with their symbol indices,
230 and the original symbol indices (and thus the symtab ordering) must be
231 preserved in order for apply_relocate_add() to find the right symbol.
233 For example, take this particular rela from a livepatch module:
234 Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
235 Offset Info Type Symbol's Value Symbol's Name + Addend
236 000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4
238 This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded
239 in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the
240 symbol index 94.
241 And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol:
242 [ snip ]
243 94: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0
244 [ snip ]
246 ---------------------------
247 3.3 Livepatch symbol format
248 ---------------------------
250 3.3.1 Required flags
251 --------------------
252 Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so
253 that the module loader can identify them and not attempt to resolve them.
254 See include/uapi/linux/elf.h for the actual definitions.
256 3.3.2 Required name format
257 --------------------------
258 Livepatch symbol names must conform to the following format:
260 .klp.sym.objname.symbol_name,sympos
261 ^ ^^ ^ ^ ^ ^
262 |_______||_____| |_________| |
263 [A] [B] [C] [D]
265 [A] The symbol name is prefixed with the string ".klp.sym."
266 [B] The name of the object (i.e. "vmlinux" or name of module) to
267 which the symbol belongs follows immediately after the prefix.
268 [C] The actual name of the symbol.
269 [D] The position of the symbol in the object (as according to kallsyms)
270 This is used to differentiate duplicate symbols within the same
271 object. The symbol position is expressed numerically (0, 1, 2...).
272 The symbol position of a unique symbol is 0.
274 3.3.3 Example livepatch symbol names:
275 -------------------------------------
276 .klp.sym.vmlinux.snprintf,0
277 .klp.sym.vmlinux.printk,0
278 .klp.sym.btrfs.btrfs_ktype,0
280 3.3.4 Example `readelf --symbols` output for a patch module:
281 ------------------------------------------------------------
282 Symbol table '.symtab' contains 127 entries:
283 Num: Value Size Type Bind Vis Ndx Name
284 [ snip ]
285 73: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0
286 74: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0
287 75: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0
288 76: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0
289 [ snip ] ^
290 |
291 [*]
292 [*] Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
293 "OS" means OS-specific.
295 --------------------------------------
296 4. Symbol table and Elf section access
297 --------------------------------------
298 A livepatch module's symbol table is accessible through module->symtab.
300 Since apply_relocate_add() requires access to a module's section headers,
301 symbol table, and relocation section indices, Elf information is preserved for
302 livepatch modules and is made accessible by the module loader through
303 module->klp_info, which is a klp_modinfo struct. When a livepatch module loads,
304 this struct is filled in by the module loader. Its fields are documented below:
306 struct klp_modinfo {
307 Elf_Ehdr hdr; /* Elf header */
308 Elf_Shdr *sechdrs; /* Section header table */
309 char *secstrings; /* String table for the section headers */
310 unsigned int symndx; /* The symbol table section index */
311 };