[RFC 27/31] objtool: Fix weak symbol detection

Josh Poimboeuf
2024-09-03T04:00:43Z

find_symbol_hole_containing() fails to find a symbol hole (aka stripped
weak symbol) if its section has no symbols before the hole. This breaks
weak symbol detection if -ffunction-sections is enabled.

Fix it by allowing the interval tree to contain section symbols, which
are always at offset zero for a given section.

Fixes a bunch of (-ffunction-sections) warnings like:

vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x10: unreachable instruction
vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x14: unreachable instruction
vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x19: unreachable instruction
vmlinux.o: warning: objtool: .text.__x64_sys_io_setup+0x20: unreachable instruction

Fixes: 4adb23686795 ("objtool: Ignore extra-symbol code")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
---
tools/objtool/elf.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 471df0336aa7..3109277804cc 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -110,7 +110,7 @@ struct symbol_hole {
};

/*
- * Find !section symbol where @offset is after it.
+ * Find the last symbol before @offset.
*/
static int symbol_hole_by_offset(const void *key, const struct rb_node *node)
{
@@ -121,8 +121,7 @@ static int symbol_hole_by_offset(const void *key, const struct rb_node *node)
return -1;

if (sh->offset >= s->offset + s->len) {
- if (s->type != STT_SECTION)
- sh->sym = s;
+ sh->sym = s;
return 1;
}

@@ -416,7 +415,8 @@ static void elf_add_symbol(struct elf *elf, struct symbol *sym)
sym->len = sym->sym.st_size;

__sym_for_each(s, &sym->sec->symbol_tree, sym->offset, sym->offset) {
- if (s->offset == sym->offset && s->type == sym->type)
+ if (s->type == sym->type && s->offset == sym->offset &&
+ s->len == sym->len)
s->alias = sym;
}

@@ -433,9 +433,13 @@ static void elf_add_symbol(struct elf *elf, struct symbol *sym)
/*
* Don't store empty STT_NOTYPE symbols in the rbtree. They
* can exist within a function, confusing the sorting.
+ *
+ * TODO: is this still true?
*/
- if (!sym->len)
+#if 0
+ if (sym->type == STT_NOTYPE && !sym->len)
__sym_remove(sym, &sym->sec->symbol_tree);
+#endif
}

static void read_symbols(struct elf *elf)
--
2.45.2