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SubjectRe: [PATCH v3 0/9] klp-convert livepatch build tooling
[...]

> Result: a small tweak to sympos_sanity_check() to relax its symbol
> uniqueness verification: allow for duplicate <object, name, position>
> instances. Now it will only complain when a supplied symbol references
> the same <object, name> but a different <position>.
>
> diff --git a/scripts/livepatch/klp-convert.c b/scripts/livepatch/klp-convert.c
> index 82c27d219372..713835dfc9ec 100644
> --- a/scripts/livepatch/klp-convert.c
> +++ b/scripts/livepatch/klp-convert.c
> @@ -194,7 +194,10 @@ static void clear_sympos_annontations(struct elf *klp_elf)
> }
> }
>
> -/* Checks if two or more elements in usr_symbols have the same name */
> +/*
> + * Checks if two or more elements in usr_symbols have the same
> + * object and name, but different symbol position
> + */
> static bool sympos_sanity_check(void)
> {
> bool sane = true;
> @@ -203,7 +206,9 @@ static bool sympos_sanity_check(void)
> list_for_each_entry(sp, &usr_symbols, list) {
> aux = list_next_entry(sp, list);
> list_for_each_entry_from(aux, &usr_symbols, list) {
> - if (strcmp(sp->symbol_name, aux->symbol_name) == 0) {
> + if (sp->pos != aux->pos &&
> + strcmp(sp->object_name, aux->object_name) == 0 &&
> + strcmp(sp->symbol_name, aux->symbol_name) == 0)
> WARN("Conflicting KLP_SYMPOS definition: %s.%s,%d vs. %s.%s,%d.",
> sp->object_name, sp->symbol_name, sp->pos,
> aux->object_name, aux->symbol_name, aux->pos);

Looks good and I'd definitely include it in v4.

> Unique sympos
> -------------
>
> But even with the above workaround, specifying unique symbol positions
> will (and should) result in a klp-convert complaint.
>
> When modifying the test module with unique symbol position annotation
> values (1 and 2 respectively):
>
> test_klp_convert_multi_objs_a.c:
>
> extern void *state_show;
> ...
> KLP_MODULE_RELOC(vmlinux) vmlinux_relocs_b[] = {
> KLP_SYMPOS(state_show, 1)
> };
>
> test_klp_convert_multi_objs_b.c:
>
> extern void *state_show;
> ...
> KLP_MODULE_RELOC(vmlinux) vmlinux_relocs_b[] = {
> KLP_SYMPOS(state_show, 2)
> };
>
>
> Each object file's symbol table contains a "state_show" instance:
>
> % eu-readelf --symbols lib/livepatch/test_klp_convert_multi_objs_a.o | grep '\<state_show\>'
> 30: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UNDEF state_show
>
> % eu-readelf --symbols lib/livepatch/test_klp_convert_multi_objs_b.o | grep '\<state_show\>'
> 20: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UNDEF state_show
>
> and the intermediate test_klp_convert_multi_objs.klp.o file contains a
> combined .klp.module_relocs.vmlinux relocation section with two
> KLP_SYMPOS structures, each with a unique <sympos> value:
>
> % objcopy -O binary --only-section=.klp.module_relocs.vmlinux \
> lib/livepatch/test_klp_convert_multi_objs.klp.o >(hexdump)
>
> 0000000 0000 0000 0000 0000 0001 0000 0000 0000
> 0000010 0000 0000 0002 0000
>
> but the symbol tables were merged, sorted and non-unique symbols
> removed, leaving a *single* unresolved "state_show" instance:
>
> % eu-readelf --symbols lib/livepatch/test_klp_convert_multi_objs.klp.o | grep '\<state_show\>'
> 53: 0000000000000000 0 NOTYPE GLOBAL DEFAULT UNDEF state_show
>
> which means that even though we have separate relocations for each
> "state_show" instance:
>
> Relocation section [ 6] '.rela.text.unlikely' for section [ 5] '.text.unlikely' at offset 0x44510 contains 11 entries:
> Offset Type Value Addend Name
> 0x0000000000000003 X86_64_32S 000000000000000000 +0 state_show
> ...
> 0x0000000000000034 X86_64_32S 000000000000000000 +0 state_show
> ...
>
> they share the same rela->sym and there is no way to distinguish which
> one correlates to which KLP_SYMPOS structure.
>
>
> Future Work
> -----------
>
> I don't see an easy way to support multiple homonym <object, name>
> symbols with unique <position> values in the current livepatch module
> Elf format. The only solutions that come to mind right now include
> renaming homonym symbols somehow to retain the relocation->symbol
> relationship when separate object files are combined. Perhaps an
> intermediate linker step could make annotated symbols unique in some way
> to achieve this. /thinking out loud

I'd set this aside for now and we can return to it later. I think it could
be quite rare in practice.

I was thinking about renaming the symbol too. We can extend the symbol
naming convention we have now and deal with it in klp_resolve_symbols(),
but maybe Josh will come up with something clever and cleaner.

Miroslav

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