Re: [PATCH v1 5/5] documentation: Update on livepatch elf format

[Marcos Paulo de Souza <mpdesouza@suse.com>]

On Mon, 2023-11-06 at 17:25 +0100, Lukas Hruska wrote:
> Add a section to Documentation/livepatch/module-elf-format.rst
> describing how klp-convert works for fixing relocations.
> 
> Signed-off-by: Lukas Hruska <lhruska@suse.cz>

Reviewed-by: Marcos Paulo de Souza <mpdesouza@suse.com>

> ---
>  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