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

Joe Lawrence
2023-03-17T20:07:47Z

On 3/14/23 14:26, Marcos Paulo de Souza wrote:
> 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.
>

Good question.

I left the disabled blocks in the code as a bookmark for an outstanding
question: should klp-convert avoid converting relocations in any
read-only-ish section?

This becomes interesting in the late module loading case -- some arches
(ppc64le IIRC) do not like the module loader tweaking these relocations
post module load. [1]

In "[PATCH v7 09/10] livepatch/selftests: add data relocations test"
test_klp_convert_data.c, you'll see a few "// .rela.data.rel.ro,
.rela.rodata supported ??" comments. Those would generate relocations
in such sections.

Do they *need* to be supported? AFAIK kpatch-build hasn't needed to
create any of those. That said, it's not too difficult for this
patchset's self-tests to generate these. klp-convert could easily
detect this scenario. The livepatch author could be advised to remove
const or __ro_after_init annotation to move the relocation out of the
read-only-ish section.

[1] https://github.com/joe-lawrence/klp-convert-tree/issues/5

>> +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.
>

I was tinkering with an alternate sympos annotation (I'll describe it in
a separate reply) and think that these debug routines could be activated
with --debug cmdline flag(s). They can be handy for debug/development,
so better to make them always active rather than #if 0'd out.

--
Joe