Re: [PATCH v7 02/10] livepatch: Add klp-convert tool

Marcos Paulo de Souza
2023-03-14T18:26:32Z

On Mon, Mar 06, 2023 at 09:08:16AM -0500, Joe Lawrence wrote:
> Livepatches may use symbols which are not contained in its own scope,
> and, because of that, may end up compiled with relocations that will
> only be resolved during module load. Yet, when the referenced symbols
> are not exported, solving this relocation requires information on the
> object that holds the symbol (either vmlinux or modules) and its
> position inside the object, as an object may contain multiple symbols
> with the same name. Providing such information must be done accordingly
> to what is specified in Documentation/livepatch/module-elf-format.txt.
>
> Currently, there is no trivial way to embed the required information as
> requested in the final livepatch elf object. klp-convert solves this
> problem in two different forms: (i) by relying on symbols.klp, which is
> built during kernel compilation, to automatically infer the relocation
> targeted symbol, and, when such inference is not possible (ii) by using
> annotations in the elf object to convert the relocation accordingly to
> the specification, enabling it to be handled by the livepatch loader.
>
> Given the above, create scripts/livepatch to hold tools developed for
> livepatches and add source files for klp-convert there.
>
> The core file of klp-convert is scripts/livepatch/klp-convert.c, which
> implements the heuristics used to solve the relocations and the
> conversion of unresolved symbols into the expected format, as defined in
> [1].
>
> klp-convert receives as arguments the symbols.klp file, an input
> livepatch module to be converted and the output name for the converted
> livepatch. When it starts running, klp-convert parses symbols.klp and
> builds two internal lists of symbols, one containing the exported and
> another containing the non-exported symbols. Then, by parsing the rela
> sections in the elf object, klp-convert identifies which symbols must be
> converted, which are those unresolved and that do not have a
> corresponding exported symbol, and attempts to convert them accordingly
> to the specification.
>
> By using symbols.klp, klp-convert identifies which symbols have names
> that only appear in a single kernel object, thus being capable of
> resolving these cases without the intervention of the developer. When
> various homonymous symbols exist through kernel objects, it is not
> possible to infer the right one, thus klp-convert falls back into using
> developer annotations. If these were not provided, then the tool will
> print a list with all acceptable targets for the symbol being processed.
>
> Annotations in the context of klp-convert are accessible as struct
> klp_module_reloc entries in sections named .klp.module_relocs.<objname>.
> These entries are pairs of symbol references and positions which are to
> be resolved against definitions in <objname>.
>
> Define the structure klp_module_reloc in include/linux/uapi/livepatch.h
> allowing developers to annotate the livepatch source code with it.
>
> klp-convert relies on libelf and on a list implementation. Add files
> scripts/livepatch/elf.c and scripts/livepatch/elf.h, which are a libelf
> interfacing layer and scripts/livepatch/list.h, which is a list
> implementation.
>
> Update Makefiles to correctly support the compilation of the new tool,
> update MAINTAINERS file and add a .gitignore file.
>
> [1] - Documentation/livepatch/module-elf-format.txt

LGTM:

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

I only have two remarks:

>
> Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
> Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
> Signed-off-by: Joao Moreira <jmoreira@suse.de>
> Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>

...

> +#if 0
> + /*
> + * klp-relocations forbidden in sections that otherwise would
> + * match in allowed_prefixes[]
> + */
> + static const char * const not_allowed[] = {
> + ".rela.data.rel.ro",
> + ".rela.data.rel.ro.local",
> + ".rela.data..ro_after_init",
> + NULL
> + };
> +#endif
> +
> + /* klp-relocations allowed in sections only for vmlinux */
> + static const char * const allowed_vmlinux[] = {
> + ".rela__jump_table",
> + NULL
> + };
> +
> + /* klp-relocations allowed in sections with prefixes */
> + static const char * const allowed_prefixes[] = {
> + ".rela.data",
> + ".rela.rodata", // supported ???
> + ".rela.sdata",
> + ".rela.text",
> + ".rela.toc",
> + NULL
> + };
> +
> + const char * const *name;
> +
> +#if 0
> + for (name = not_allowed; *name; name++)
> + if (strcmp(sec->name, *name) == 0)
> + return false;
> +#endif
> +

Have you needed to enable the not_allowed checks when creating your livepatches?
Otherwise I believe that this can be removed and added again in the future is
needed.

> +int main(int argc, const char **argv)
> +{
> + const char *klp_in_module, *klp_out_module, *symbols_list;

...

> +
> +/* Functions kept commented since they might be useful for future debugging */
> +
> +/* Dumps sympos list (useful for debugging purposes)
> + * static void dump_sympos(void)
> + * {
> + * struct sympos *sp;
> + *
> + * fprintf(stderr, "BEGIN OF SYMPOS DUMP\n");
> + * list_for_each_entry(sp, &usr_symbols, list) {
> + * fprintf(stderr, "%s %s %d\n", sp->symbol_name, sp->object_name,
> + * sp->pos);
> + * }
> + * fprintf(stderr, "END OF SYMPOS DUMP\n");
> + * }
> + *
> + *
> + * / Dump symbols list for debugging purposes /
> + * static void dump_symbols(void)
> + * {
> + * struct symbol_entry *entry;
> + *
> + * fprintf(stderr, "BEGIN OF SYMBOLS DUMP\n");
> + * list_for_each_entry(entry, &symbols, list)
> + * printf("%s %s\n", entry->object_name, entry->symbol_name);
> + * fprintf(stderr, "END OF SYMBOLS DUMP\n");
> + * }

Same here. Have you used these functions recently when debugging klp-convert?
Othewise it can be removed as well.