1=========================== 2Livepatch module Elf format 3=========================== 4 5This document outlines the Elf format requirements that livepatch modules must follow. 6 7----------------- 8Table of Contents 9----------------- 100. Background and motivation 111. Livepatch modinfo field 122. 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 203. 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 284. Architecture-specific sections 295. Symbol table and Elf section access 30 31---------------------------- 320. Background and motivation 33---------------------------- 34 35Formerly, livepatch required separate architecture-specific code to write 36relocations. However, arch-specific code to write relocations already 37exists in the module loader, so this former approach produced redundant 38code. So, instead of duplicating code and re-implementing what the module 39loader can already do, livepatch leverages existing code in the module 40loader to perform the all the arch-specific relocation work. Specifically, 41livepatch reuses the apply_relocate_add() function in the module loader to 42write relocations. The patch module Elf format described in this document 43enables livepatch to be able to do this. The hope is that this will make 44livepatch more easily portable to other architectures and reduce the amount 45of arch-specific code required to port livepatch to a particular 46architecture. 47 48Since apply_relocate_add() requires access to a module's section header 49table, symbol table, and relocation section indices, Elf information is 50preserved for livepatch modules (see section 5). Livepatch manages its own 51relocation sections and symbols, which are described in this document. The 52Elf constants used to mark livepatch symbols and relocation sections were 53selected from OS-specific ranges according to the definitions from glibc. 54 550.1 Why does livepatch need to write its own relocations? 56--------------------------------------------------------- 57A typical livepatch module contains patched versions of functions that can 58reference non-exported global symbols and non-included local symbols. 59Relocations referencing these types of symbols cannot be left in as-is 60since the kernel module loader cannot resolve them and will therefore 61reject the livepatch module. Furthermore, we cannot apply relocations that 62affect modules not yet loaded at patch module load time (e.g. a patch to a 63driver that is not loaded). Formerly, livepatch solved this problem by 64embedding special "dynrela" (dynamic rela) sections in the resulting patch 65module Elf output. Using these dynrela sections, livepatch could resolve 66symbols while taking into account its scope and what module the symbol 67belongs to, and then manually apply the dynamic relocations. However this 68approach required livepatch to supply arch-specific code in order to write 69these relocations. In the new format, livepatch manages its own SHT_RELA 70relocation sections in place of dynrela sections, and the symbols that the 71relas reference are special livepatch symbols (see section 2 and 3). The 72arch-specific livepatch relocation code is replaced by a call to 73apply_relocate_add(). 74 75================================ 76PATCH MODULE FORMAT REQUIREMENTS 77================================ 78 79-------------------------- 801. Livepatch modinfo field 81-------------------------- 82 83Livepatch modules are required to have the "livepatch" modinfo attribute. 84See the sample livepatch module in samples/livepatch/ for how this is done. 85 86Livepatch modules can be identified by users by using the 'modinfo' command 87and looking for the presence of the "livepatch" field. This field is also 88used by the kernel module loader to identify livepatch modules. 89 90Example modinfo output: 91----------------------- 92% modinfo livepatch-meminfo.ko 93filename: livepatch-meminfo.ko 94livepatch: Y 95license: GPL 96depends: 97vermagic: 4.3.0+ SMP mod_unload 98 99-------------------------------- 1002. Livepatch relocation sections 101-------------------------------- 102 103------------------------------------------- 1042.1 What are livepatch relocation sections? 105------------------------------------------- 106A livepatch module manages its own Elf relocation sections to apply 107relocations to modules as well as to the kernel (vmlinux) at the 108appropriate time. For example, if a patch module patches a driver that is 109not currently loaded, livepatch will apply the corresponding livepatch 110relocation section(s) to the driver once it loads. 111 112Each "object" (e.g. vmlinux, or a module) within a patch module may have 113multiple livepatch relocation sections associated with it (e.g. patches to 114multiple functions within the same object). There is a 1-1 correspondence 115between a livepatch relocation section and the target section (usually the 116text section of a function) to which the relocation(s) apply. It is 117also possible for a livepatch module to have no livepatch relocation 118sections, as in the case of the sample livepatch module (see 119samples/livepatch). 120 121Since Elf information is preserved for livepatch modules (see Section 5), a 122livepatch relocation section can be applied simply by passing in the 123appropriate section index to apply_relocate_add(), which then uses it to 124access the relocation section and apply the relocations. 125 126Every symbol referenced by a rela in a livepatch relocation section is a 127livepatch symbol. These must be resolved before livepatch can call 128apply_relocate_add(). See Section 3 for more information. 129 130--------------------------------------- 1312.2 Livepatch relocation section format 132--------------------------------------- 133 1342.2.1 Required flags 135-------------------- 136Livepatch relocation sections must be marked with the SHF_RELA_LIVEPATCH 137section flag. See include/uapi/linux/elf.h for the definition. The module 138loader recognizes this flag and will avoid applying those relocation sections 139at patch module load time. These sections must also be marked with SHF_ALLOC, 140so that the module loader doesn't discard them on module load (i.e. they will 141be copied into memory along with the other SHF_ALLOC sections). 142 1432.2.2 Required name format 144-------------------------- 145The name of a livepatch relocation section must conform to the following format: 146 147.klp.rela.objname.section_name 148^ ^^ ^ ^ ^ 149|________||_____| |__________| 150 [A] [B] [C] 151 152[A] The relocation section name is prefixed with the string ".klp.rela." 153[B] The name of the object (i.e. "vmlinux" or name of module) to 154 which the relocation section belongs follows immediately after the prefix. 155[C] The actual name of the section to which this relocation section applies. 156 1572.2.3 Example livepatch relocation section names: 158------------------------------------------------- 159.klp.rela.ext4.text.ext4_attr_store 160.klp.rela.vmlinux.text.cmdline_proc_show 161 1622.2.4 Example `readelf --sections` output for a patch 163module that patches vmlinux and modules 9p, btrfs, ext4: 164-------------------------------------------------------- 165 Section Headers: 166 [Nr] Name Type Address Off Size ES Flg Lk Inf Al 167 [ snip ] 168 [29] .klp.rela.9p.text.caches.show RELA 0000000000000000 002d58 0000c0 18 AIo 64 9 8 169 [30] .klp.rela.btrfs.text.btrfs.feature.attr.show RELA 0000000000000000 002e18 000060 18 AIo 64 11 8 170 [ snip ] 171 [34] .klp.rela.ext4.text.ext4.attr.store RELA 0000000000000000 002fd8 0000d8 18 AIo 64 13 8 172 [35] .klp.rela.ext4.text.ext4.attr.show RELA 0000000000000000 0030b0 000150 18 AIo 64 15 8 173 [36] .klp.rela.vmlinux.text.cmdline.proc.show RELA 0000000000000000 003200 000018 18 AIo 64 17 8 174 [37] .klp.rela.vmlinux.text.meminfo.proc.show RELA 0000000000000000 003218 0000f0 18 AIo 64 19 8 175 [ snip ] ^ ^ 176 | | 177 [*] [*] 178[*] Livepatch relocation sections are SHT_RELA sections but with a few special 179characteristics. Notice that they are marked SHF_ALLOC ("A") so that they will 180not be discarded when the module is loaded into memory, as well as with the 181SHF_RELA_LIVEPATCH flag ("o" - for OS-specific). 182 1832.2.5 Example `readelf --relocs` output for a patch module: 184----------------------------------------------------------- 185Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries: 186 Offset Info Type Symbol's Value Symbol's Name + Addend 187000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4 1880000000000000028 0000003d0000000b R_X86_64_32S 0000000000000000 .klp.sym.btrfs.btrfs_ktype,0 + 0 1890000000000000036 0000003b00000002 R_X86_64_PC32 0000000000000000 .klp.sym.btrfs.can_modify_feature.isra.3,0 - 4 190000000000000004c 0000004900000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.snprintf,0 - 4 191[ snip ] ^ 192 | 193 [*] 194[*] Every symbol referenced by a relocation is a livepatch symbol. 195 196-------------------- 1973. Livepatch symbols 198-------------------- 199 200------------------------------- 2013.1 What are livepatch symbols? 202------------------------------- 203Livepatch symbols are symbols referred to by livepatch relocation sections. 204These are symbols accessed from new versions of functions for patched 205objects, whose addresses cannot be resolved by the module loader (because 206they are local or unexported global syms). Since the module loader only 207resolves exported syms, and not every symbol referenced by the new patched 208functions is exported, livepatch symbols were introduced. They are used 209also in cases where we cannot immediately know the address of a symbol when 210a patch module loads. For example, this is the case when livepatch patches 211a module that is not loaded yet. In this case, the relevant livepatch 212symbols are resolved simply when the target module loads. In any case, for 213any livepatch relocation section, all livepatch symbols referenced by that 214section must be resolved before livepatch can call apply_relocate_add() for 215that reloc section. 216 217Livepatch symbols must be marked with SHN_LIVEPATCH so that the module 218loader can identify and ignore them. Livepatch modules keep these symbols 219in their symbol tables, and the symbol table is made accessible through 220module->symtab. 221 222------------------------------------- 2233.2 A livepatch module's symbol table 224------------------------------------- 225Normally, a stripped down copy of a module's symbol table (containing only 226"core" symbols) is made available through module->symtab (See layout_symtab() 227in kernel/module.c). For livepatch modules, the symbol table copied into memory 228on module load must be exactly the same as the symbol table produced when the 229patch module was compiled. This is because the relocations in each livepatch 230relocation section refer to their respective symbols with their symbol indices, 231and the original symbol indices (and thus the symtab ordering) must be 232preserved in order for apply_relocate_add() to find the right symbol. 233 234For example, take this particular rela from a livepatch module: 235Relocation section '.klp.rela.btrfs.text.btrfs_feature_attr_show' at offset 0x2ba0 contains 4 entries: 236 Offset Info Type Symbol's Value Symbol's Name + Addend 237000000000000001f 0000005e00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.printk,0 - 4 238 239This rela refers to the symbol '.klp.sym.vmlinux.printk,0', and the symbol index is encoded 240in 'Info'. Here its symbol index is 0x5e, which is 94 in decimal, which refers to the 241symbol index 94. 242And in this patch module's corresponding symbol table, symbol index 94 refers to that very symbol: 243[ snip ] 24494: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.printk,0 245[ snip ] 246 247--------------------------- 2483.3 Livepatch symbol format 249--------------------------- 250 2513.3.1 Required flags 252-------------------- 253Livepatch symbols must have their section index marked as SHN_LIVEPATCH, so 254that the module loader can identify them and not attempt to resolve them. 255See include/uapi/linux/elf.h for the actual definitions. 256 2573.3.2 Required name format 258-------------------------- 259Livepatch symbol names must conform to the following format: 260 261.klp.sym.objname.symbol_name,sympos 262^ ^^ ^ ^ ^ ^ 263|_______||_____| |_________| | 264 [A] [B] [C] [D] 265 266[A] The symbol name is prefixed with the string ".klp.sym." 267[B] The name of the object (i.e. "vmlinux" or name of module) to 268 which the symbol belongs follows immediately after the prefix. 269[C] The actual name of the symbol. 270[D] The position of the symbol in the object (as according to kallsyms) 271 This is used to differentiate duplicate symbols within the same 272 object. The symbol position is expressed numerically (0, 1, 2...). 273 The symbol position of a unique symbol is 0. 274 2753.3.3 Example livepatch symbol names: 276------------------------------------- 277.klp.sym.vmlinux.snprintf,0 278.klp.sym.vmlinux.printk,0 279.klp.sym.btrfs.btrfs_ktype,0 280 2813.3.4 Example `readelf --symbols` output for a patch module: 282------------------------------------------------------------ 283Symbol table '.symtab' contains 127 entries: 284 Num: Value Size Type Bind Vis Ndx Name 285 [ snip ] 286 73: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.snprintf,0 287 74: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.capable,0 288 75: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.find_next_bit,0 289 76: 0000000000000000 0 NOTYPE GLOBAL DEFAULT OS [0xff20] .klp.sym.vmlinux.si_swapinfo,0 290 [ snip ] ^ 291 | 292 [*] 293[*] Note that the 'Ndx' (Section index) for these symbols is SHN_LIVEPATCH (0xff20). 294 "OS" means OS-specific. 295 296--------------------------------- 2974. Architecture-specific sections 298--------------------------------- 299Architectures may override arch_klp_init_object_loaded() to perform 300additional arch-specific tasks when a target module loads, such as applying 301arch-specific sections. On x86 for example, we must apply per-object 302.altinstructions and .parainstructions sections when a target module loads. 303These sections must be prefixed with ".klp.arch.$objname." so that they can 304be easily identified when iterating through a patch module's Elf sections 305(See arch/x86/kernel/livepatch.c for a complete example). 306 307-------------------------------------- 3085. Symbol table and Elf section access 309-------------------------------------- 310A livepatch module's symbol table is accessible through module->symtab. 311 312Since apply_relocate_add() requires access to a module's section headers, 313symbol table, and relocation section indices, Elf information is preserved for 314livepatch modules and is made accessible by the module loader through 315module->klp_info, which is a klp_modinfo struct. When a livepatch module loads, 316this struct is filled in by the module loader. Its fields are documented below: 317 318struct klp_modinfo { 319 Elf_Ehdr hdr; /* Elf header */ 320 Elf_Shdr *sechdrs; /* Section header table */ 321 char *secstrings; /* String table for the section headers */ 322 unsigned int symndx; /* The symbol table section index */ 323}; 324
