Add a section to Documentation/livepatch/module-elf-format.rst describing how klp-convert works for fixing relocations. Signed-off-by: Lukas Hruska <lhruska@xxxxxxx> --- Documentation/livepatch/module-elf-format.rst | 67 +++++++++++++++++++ 1 file changed, 67 insertions(+) diff --git a/Documentation/livepatch/module-elf-format.rst b/Documentation/livepatch/module-elf-format.rst index a03ed02ec57e..2aa9b11cd806 100644 --- a/Documentation/livepatch/module-elf-format.rst +++ b/Documentation/livepatch/module-elf-format.rst @@ -300,3 +300,70 @@ symbol table, and relocation section indices, ELF information is preserved for livepatch modules and is made accessible by the module loader through module->klp_info, which is a :c:type:`klp_modinfo` struct. When a livepatch module loads, this struct is filled in by the module loader. + +6. klp-convert tool +=================== +The livepatch relocation sections might be created using +scripts/livepatch/klp-convert. It is called automatically during +the build as part of a module post processing. + +The tool is not able to find the symbols and all the metadata +automatically. Instead, all needed information must already be +part of rela entry for the given symbol. Such a rela can +be created easily by using KLP_RELOC_SYMBOL() macro after +the symbol declaration. + +KLP_RELOC_SYMBOL causes that the relocation entries for +the given symbol will be created in the following format:: + + .klp.sym.rela.lp_object.sym_object.sym_name,sympos + ^ ^ ^ ^ ^ ^ ^ ^ ^ + |___________| |_______| |________| |______| | + [A] [B] [C] [D] [E] + +[A] + The symbol name is prefixed with the string ".klp.sym.rela." + +[B] + The name of the object (i.e. "vmlinux" or name of module) which + is livepatched. + +[C] + The name of the object (i.e. "vmlinux" or name of module) to + which the symbol belongs follows immediately after the prefix. + +[D] + The actual name of the symbol. + +[E] + The position of the symbol in the object (as according to kallsyms) + This is used to differentiate duplicate symbols within the same + object. The symbol position is expressed numerically (0, 1, 2...). + The symbol position of a unique symbol is 0. + +Example: +-------- +**Livepatch source code:** + +:: + + extern char *saved_command_line \ + KLP_RELOC_SYMBOL(vmlinux, vmlinux, saved_command_line, 0); + +**`readelf -r -W` output of compiled module:** + +:: + + Relocation section '.rela.text' at offset 0x32e60 contains 10 entries: + Offset Info Type Symbol's Value Symbol's Name + Addend + ... + 0000000000000068 0000003c00000002 R_X86_64_PC32 0000000000000000 .klp.sym.rela.vmlinux.vmlinux.saved_command_line,0 - 4 + ... + +**`readelf -r -W` output of transformed module by klp-convert:** + +:: + + Relocation section '.klp.rela.vmlinux.text' at offset 0x5cb60 contains 1 entry: + Offset Info Type Symbol's Value Symbol's Name + Addend + 0000000000000068 0000003c00000002 R_X86_64_PC32 0000000000000000 .klp.sym.vmlinux.saved_command_line,0 - 4 -- 2.42.0
