1===========================
2Livepatch module Elf format
3===========================
4
5This document outlines the Elf format requirements that livepatch modules must follow.
6
7
8.. Table of Contents
9
10   1. Background and motivation
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. Architecture-specific sections
18   6. Symbol table and Elf section access
19
201. Background and motivation
21============================
22
23Formerly, livepatch required separate architecture-specific code to write
24relocations. However, arch-specific code to write relocations already
25exists in the module loader, so this former approach produced redundant
26code. So, instead of duplicating code and re-implementing what the module
27loader can already do, livepatch leverages existing code in the module
28loader to perform the all the arch-specific relocation work. Specifically,
29livepatch reuses the apply_relocate_add() function in the module loader to
30write relocations. The patch module Elf format described in this document
31enables livepatch to be able to do this. The hope is that this will make
32livepatch more easily portable to other architectures and reduce the amount
33of arch-specific code required to port livepatch to a particular
34architecture.
35
36Since apply_relocate_add() requires access to a module's section header
37table, symbol table, and relocation section indices, Elf information is
38preserved for livepatch modules (see section 5). Livepatch manages its own
39relocation sections and symbols, which are described in this document. The
40Elf constants used to mark livepatch symbols and relocation sections were
41selected from OS-specific ranges according to the definitions from glibc.
42
43Why does livepatch need to write its own relocations?
44-----------------------------------------------------
45A typical livepatch module contains patched versions of functions that can
46reference non-exported global symbols and non-included local symbols.
47Relocations referencing these types of symbols cannot be left in as-is
48since the kernel module loader cannot resolve them and will therefore
49reject the livepatch module. Furthermore, we cannot apply relocations that
50affect modules not yet loaded at patch module load time (e.g. a patch to a
51driver that is not loaded). Formerly, livepatch solved this problem by
52embedding special "dynrela" (dynamic rela) sections in the resulting patch
53module Elf output. Using these dynrela sections, livepatch could resolve
54symbols while taking into account its scope and what module the symbol
55belongs to, and then manually apply the dynamic relocations. However this
56approach required livepatch to supply arch-specific code in order to write
57these relocations. In the new format, livepatch manages its own SHT_RELA
58relocation sections in place of dynrela sections, and the symbols that the
59relas reference are special livepatch symbols (see section 2 and 3). The
60arch-specific livepatch relocation code is replaced by a call to
61apply_relocate_add().
62
632. Livepatch modinfo field
64==========================
65
66Livepatch modules are required to have the "livepatch" modinfo attribute.
67See the sample livepatch module in samples/livepatch/ for how this is done.
68
69Livepatch modules can be identified by users by using the 'modinfo' command
70and looking for the presence of the "livepatch" field. This field is also
71used by the kernel module loader to identify livepatch modules.
72
73Example:
74--------
75
76**Modinfo output:**
77
78::
79
80	% modinfo livepatch-meminfo.ko
81	filename:		livepatch-meminfo.ko
82	livepatch:		Y
83	license:		GPL
84	depends:
85	vermagic:		4.3.0+ SMP mod_unload
86
873. Livepatch relocation sections
88================================
89
90A livepatch module manages its own Elf relocation sections to apply
91relocations to modules as well as to the kernel (vmlinux) at the
92appropriate time. For example, if a patch module patches a driver that is
93not currently loaded, livepatch will apply the corresponding livepatch
94relocation section(s) to the driver once it loads.
95
96Each "object" (e.g. vmlinux, or a module) within a patch module may have
97multiple livepatch relocation sections associated with it (e.g. patches to
98multiple functions within the same object). There is a 1-1 correspondence
99between a livepatch relocation section and the target section (usually the
100text section of a function) to which the relocation(s) apply. It is
101also possible for a livepatch module to have no livepatch relocation
102sections, as in the case of the sample livepatch module (see
103samples/livepatch).
104
105Since Elf information is preserved for livepatch modules (see Section 5), a
106livepatch relocation section can be applied simply by passing in the
107appropriate section index to apply_relocate_add(), which then uses it to
108access the relocation section and apply the relocations.
109
110Every symbol referenced by a rela in a livepatch relocation section is a
111livepatch symbol. These must be resolved before livepatch can call
112apply_relocate_add(). See Section 3 for more information.
113
1143.1 Livepatch relocation section format
115=======================================
116
117Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH
118section flag. See include/uapi/linux/elf.h for the definition. The module
119loader recognizes this flag and will avoid applying those relocation sections
120at patch module load time. These sections must also be marked with SHF_ALLOC,
121so that the module loader doesn't discard them on module load (i.e. they will
122be copied into memory along with the other SHF_ALLOC sections).
123
124The name of a livepatch relocation section must conform to the following
125format::
126
127  .klp.rela.objname.section_name
128  ^        ^^     ^ ^          ^
129  |________||_____| |__________|
130     [A]      [B]        [C]
131
132[A]
133  The relocation section name is prefixed with the string ".klp.rela."
134
135[B]
136  The name of the object (i.e. "vmlinux" or name of module) to
137  which the relocation section belongs follows immediately after the prefix.
138
139[C]
140  The actual name of the section to which this relocation section applies.
141
142Examples:
143---------
144
145**Livepatch relocation section names:**
146
147::
148
149  .klp.rela.ext4.text.ext4_attr_store
150  .klp.rela.vmlinux.text.cmdline_proc_show
151
152**`readelf --sections` output for a patch
153module that patches vmlinux and modules 9p, btrfs, ext4:**
154
155::
156
157  Section Headers:
158  [Nr] Name                          Type                    Address          Off    Size   ES Flg Lk Inf Al
159  [ snip ]
160  [29] .klp.rela.9p.text.caches.show RELA                    0000000000000000 002d58 0000c0 18 AIo 64   9  8
161  [30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA     0000000000000000 002e18 000060 18 AIo 64  11  8
162  [ snip ]
163  [34] .klp.rela.ext4.text.ext4.attr.store RELA              0000000000000000 002fd8 0000d8 18 AIo 64  13  8
164  [35] .klp.rela.ext4.text.ext4.attr.show RELA               0000000000000000 0030b0 000150 18 AIo 64  15  8
165  [36] .klp.rela.vmlinux.text.cmdline.proc.show RELA         0000000000000000 003200 000018 18 AIo 64  17  8
166  [37] .klp.rela.vmlinux.text.meminfo.proc.show RELA         0000000000000000 003218 0000f0 18 AIo 64  19  8
167  [ snip ]                                       ^                                             ^
168                                                 |                                             |
169                                                [*]                                           [*]
170
171[*]
172  Livepatch relocation sections are SHT_RELA sections but with a few special
173  characteristics. Notice that they are marked SHF_ALLOC ("A") so that they will
174  not be discarded when the module is loaded into memory, as well as with the
175  SHF_RELA_LIVEPATCH flag ("o" - for OS-specific).
176
177**`readelf --relocs` output for a patch module:**
178
179::
180
181  Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
182      Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
183  000000000000001f  0000005e00000002 R_X86_64_PC32          0000000000000000 .klp.sym.vmlinux.printk,0 - 4
184  0000000000000028  0000003d0000000b R_X86_64_32S           0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0
185  0000000000000036  0000003b00000002 R_X86_64_PC32          0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4
186  000000000000004c  0000004900000002 R_X86_64_PC32          0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4
187  [ snip ]                                                                   ^
188                                                                             |
189                                                                            [*]
190
191[*]
192  Every symbol referenced by a relocation is a livepatch symbol.
193
1944. Livepatch symbols
195====================
196
197Livepatch symbols are symbols referred to by livepatch relocation sections.
198These are symbols accessed from new versions of functions for patched
199objects, whose addresses cannot be resolved by the module loader (because
200they are local or unexported global syms). Since the module loader only
201resolves exported syms, and not every symbol referenced by the new patched
202functions is exported, livepatch symbols were introduced. They are used
203also in cases where we cannot immediately know the address of a symbol when
204a patch module loads. For example, this is the case when livepatch patches
205a module that is not loaded yet. In this case, the relevant livepatch
206symbols are resolved simply when the target module loads. In any case, for
207any livepatch relocation section, all livepatch symbols referenced by that
208section must be resolved before livepatch can call apply_relocate_add() for
209that reloc section.
210
211Livepatch symbols must be marked with SHN_LIVEPATCH so that the module
212loader can identify and ignore them. Livepatch modules keep these symbols
213in their symbol tables, and the symbol table is made accessible through
214module->symtab.
215
2164.1 A livepatch module's symbol table
217=====================================
218Normally, a stripped down copy of a module's symbol table (containing only
219"core" symbols) is made available through module->symtab (See layout_symtab()
220in kernel/module.c). For livepatch modules, the symbol table copied into memory
221on module load must be exactly the same as the symbol table produced when the
222patch module was compiled. This is because the relocations in each livepatch
223relocation section refer to their respective symbols with their symbol indices,
224and the original symbol indices (and thus the symtab ordering) must be
225preserved in order for apply_relocate_add() to find the right symbol.
226
227For example, take this particular rela from a livepatch module:::
228
229  Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries:
230      Offset             Info             Type               Symbol's Value  Symbol's Name + Addend
231  000000000000001f  0000005e00000002 R_X86_64_PC32          0000000000000000 .klp.sym.vmlinux.printk,0 - 4
232
233  This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded
234  in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the
235  symbol index 94.
236  And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol:
237  [ snip ]
238  94: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0
239  [ snip ]
240
2414.2 Livepatch symbol format
242===========================
243
244Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so
245that the module loader can identify them and not attempt to resolve them.
246See include/uapi/linux/elf.h for the actual definitions.
247
248Livepatch symbol names must conform to the following format::
249
250  .klp.sym.objname.symbol_name,sympos
251  ^       ^^     ^ ^         ^ ^
252  |_______||_____| |_________| |
253     [A]     [B]       [C]    [D]
254
255[A]
256  The symbol name is prefixed with the string ".klp.sym."
257
258[B]
259  The name of the object (i.e. "vmlinux" or name of module) to
260  which the symbol belongs follows immediately after the prefix.
261
262[C]
263  The actual name of the symbol.
264
265[D]
266  The position of the symbol in the object (as according to kallsyms)
267  This is used to differentiate duplicate symbols within the same
268  object. The symbol position is expressed numerically (0, 1, 2...).
269  The symbol position of a unique symbol is 0.
270
271Examples:
272---------
273
274**Livepatch symbol names:**
275
276::
277
278	.klp.sym.vmlinux.snprintf,0
279	.klp.sym.vmlinux.printk,0
280	.klp.sym.btrfs.btrfs_ktype,0
281
282**`readelf --symbols` output for a patch module:**
283
284::
285
286  Symbol table '.symtab' contains 127 entries:
287     Num:    Value          Size Type    Bind   Vis     Ndx         Name
288     [ snip ]
289      73: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0
290      74: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0
291      75: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0
292      76: 0000000000000000     0 NOTYPE  GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0
293    [ snip ]                                               ^
294                                                           |
295                                                          [*]
296
297[*]
298  Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20).
299  "OS" means OS-specific.
300
3015. Architecture-specific sections
302=================================
303Architectures may override arch_klp_init_object_loaded() to perform
304additional arch-specific tasks when a target module loads, such as applying
305arch-specific sections. On x86 for example, we must apply per-object
306.altinstructions and .parainstructions sections when a target module loads.
307These sections must be prefixed with ".klp.arch.$objname." so that they can
308be easily identified when iterating through a patch module's Elf sections
309(See arch/x86/kernel/livepatch.c for a complete example).
310
3116. Symbol table and Elf section access
312======================================
313A livepatch module's symbol table is accessible through module->symtab.
314
315Since apply_relocate_add() requires access to a module's section headers,
316symbol table, and relocation section indices, Elf information is preserved for
317livepatch modules and is made accessible by the module loader through
318module->klp_info, which is a klp_modinfo struct. When a livepatch module loads,
319this struct is filled in by the module loader. Its fields are documented below::
320
321	struct klp_modinfo {
322		Elf_Ehdr hdr; /* Elf header */
323		Elf_Shdr *sechdrs; /* Section header table */
324		char *secstrings; /* String table for the section headers */
325		unsigned int symndx; /* The symbol table section index */
326	};
327