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.. contents:: :local:
11
12
131. Background and motivation
14============================
15
16Formerly, livepatch required separate architecture-specific code to write
17relocations. However, arch-specific code to write relocations already
18exists in the module loader, so this former approach produced redundant
19code. So, instead of duplicating code and re-implementing what the module
20loader can already do, livepatch leverages existing code in the module
21loader to perform the all the arch-specific relocation work. Specifically,
22livepatch reuses the apply_relocate_add() function in the module loader to
23write relocations. The patch module Elf format described in this document
24enables livepatch to be able to do this. The hope is that this will make
25livepatch more easily portable to other architectures and reduce the amount
26of arch-specific code required to port livepatch to a particular
27architecture.
28
29Since apply_relocate_add() requires access to a module's section header
30table, symbol table, and relocation section indices, Elf information is
31preserved for livepatch modules (see section 5). Livepatch manages its own
32relocation sections and symbols, which are described in this document. The
33Elf constants used to mark livepatch symbols and relocation sections were
34selected from OS-specific ranges according to the definitions from glibc.
35
36Why does livepatch need to write its own relocations?
37-----------------------------------------------------
38A typical livepatch module contains patched versions of functions that can
39reference non-exported global symbols and non-included local symbols.
40Relocations referencing these types of symbols cannot be left in as-is
41since the kernel module loader cannot resolve them and will therefore
42reject the livepatch module. Furthermore, we cannot apply relocations that
43affect modules not yet loaded at patch module load time (e.g. a patch to a
44driver that is not loaded). Formerly, livepatch solved this problem by
45embedding special "dynrela" (dynamic rela) sections in the resulting patch
46module Elf output. Using these dynrela sections, livepatch could resolve
47symbols while taking into account its scope and what module the symbol
48belongs to, and then manually apply the dynamic relocations. However this
49approach required livepatch to supply arch-specific code in order to write
50these relocations. In the new format, livepatch manages its own SHT_RELA
51relocation sections in place of dynrela sections, and the symbols that the
52relas reference are special livepatch symbols (see section 2 and 3). The
53arch-specific livepatch relocation code is replaced by a call to
54apply_relocate_add().
55
562. Livepatch modinfo field
57==========================
58
59Livepatch modules are required to have the "livepatch" modinfo attribute.
60See the sample livepatch module in samples/livepatch/ for how this is done.
61
62Livepatch modules can be identified by users by using the 'modinfo' command
63and looking for the presence of the "livepatch" field. This field is also
64used by the kernel module loader to identify livepatch modules.
65
66Example:
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
803. Livepatch relocation sections
81================================
82
83A livepatch module manages its own Elf relocation sections to apply
84relocations to modules as well as to the kernel (vmlinux) at the
85appropriate time. For example, if a patch module patches a driver that is
86not currently loaded, livepatch will apply the corresponding livepatch
87relocation section(s) to the driver once it loads.
88
89Each "object" (e.g. vmlinux, or a module) within a patch module may have
90multiple livepatch relocation sections associated with it (e.g. patches to
91multiple functions within the same object). There is a 1-1 correspondence
92between a livepatch relocation section and the target section (usually the
93text section of a function) to which the relocation(s) apply. It is
94also possible for a livepatch module to have no livepatch relocation
95sections, as in the case of the sample livepatch module (see
96samples/livepatch).
97
98Since Elf information is preserved for livepatch modules (see Section 5), a
99livepatch relocation section can be applied simply by passing in the
100appropriate section index to apply_relocate_add(), which then uses it to
101access the relocation section and apply the relocations.
102
103Every symbol referenced by a rela in a livepatch relocation section is a
104livepatch symbol. These must be resolved before livepatch can call
105apply_relocate_add(). See Section 3 for more information.
106
1073.1 Livepatch relocation section format
108=======================================
109
110Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH
111section flag. See include/uapi/linux/elf.h for the definition. The module
112loader recognizes this flag and will avoid applying those relocation sections
113at patch module load time. These sections must also be marked with SHF_ALLOC,
114so that the module loader doesn't discard them on module load (i.e. they will
115be copied into memory along with the other SHF_ALLOC sections).
116
117The name of a livepatch relocation section must conform to the following
118format::
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
135Examples:
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
146module 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
1874. Livepatch symbols
188====================
189
190Livepatch symbols are symbols referred to by livepatch relocation sections.
191These are symbols accessed from new versions of functions for patched
192objects, whose addresses cannot be resolved by the module loader (because
193they are local or unexported global syms). Since the module loader only
194resolves exported syms, and not every symbol referenced by the new patched
195functions is exported, livepatch symbols were introduced. They are used
196also in cases where we cannot immediately know the address of a symbol when
197a patch module loads. For example, this is the case when livepatch patches
198a module that is not loaded yet. In this case, the relevant livepatch
199symbols are resolved simply when the target module loads. In any case, for
200any livepatch relocation section, all livepatch symbols referenced by that
201section must be resolved before livepatch can call apply_relocate_add() for
202that reloc section.
203
204Livepatch symbols must be marked with SHN_LIVEPATCH so that the module
205loader can identify and ignore them. Livepatch modules keep these symbols
206in their symbol tables, and the symbol table is made accessible through
207module->symtab.
208
2094.1 A livepatch module's symbol table
210=====================================
211Normally, a stripped down copy of a module's symbol table (containing only
212"core" symbols) is made available through module->symtab (See layout_symtab()
213in kernel/module.c). For livepatch modules, the symbol table copied into memory
214on module load must be exactly the same as the symbol table produced when the
215patch module was compiled. This is because the relocations in each livepatch
216relocation section refer to their respective symbols with their symbol indices,
217and the original symbol indices (and thus the symtab ordering) must be
218preserved in order for apply_relocate_add() to find the right symbol.
219
220For 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
2344.2 Livepatch symbol format
235===========================
236
237Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so
238that the module loader can identify them and not attempt to resolve them.
239See include/uapi/linux/elf.h for the actual definitions.
240
241Livepatch 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
264Examples:
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
2945. Symbol table and Elf section access
295======================================
296A livepatch module's symbol table is accessible through module->symtab.
297
298Since apply_relocate_add() requires access to a module's section headers,
299symbol table, and relocation section indices, Elf information is preserved for
300livepatch modules and is made accessible by the module loader through
301module->klp_info, which is a klp_modinfo struct. When a livepatch module loads,
302this struct is filled in by the module loader. Its fields are documented below::
303
304	struct klp_modinfo {
305		Elf_Ehdr hdr; /* Elf header */
306		Elf_Shdr *sechdrs; /* Section header table */
307		char *secstrings; /* String table for the section headers */
308		unsigned int symndx; /* The symbol table section index */
309	};
310