[...]
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