[PATCH v2] objtool: Report section name in elf_init_reloc_text_sym() warning

[PATCH v2] objtool: Report section name in elf_init_reloc_text_sym() warning

[Kees Cook]

While tracking down issues with LKDTM's .rodata "function", I found
the warning from elf_init_reloc_text_sym() to be unhelpful because it
wasn't clear which calling path it was coming from. Report the sec->name
and rephrase the warning a bit. Additionally check for NULL sym->name,
which may happen.

Before:

vmlinux.o: warning: objtool: bad call to elf_init_reloc_text_sym() for data symbol .rodata

After:

vmlinux.o: warning: objtool: .cfi_sites: unexpected reference to non-executable symbol '.rodata'

Signed-off-by: Kees Cook <keescook@chromium.org>
---
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
 v1: https://lore.kernel.org/lkml/20240430235106.work.680-kees@kernel.org/
---
 tools/objtool/elf.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 3d27983dc908..b38cedd4fd55 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -891,8 +891,8 @@ struct reloc *elf_init_reloc_text_sym(struct elf *elf, struct section *sec,
 	int addend = insn_off;
 
 	if (!(insn_sec->sh.sh_flags & SHF_EXECINSTR)) {
-		WARN("bad call to %s() for data symbol %s",
-		     __func__, sym->name);
+		WARN("%s: unexpected reference to non-executable symbol '%s'",
+		     sec->name, sym->name ?: "<unknown>");
 		return NULL;
 	}
 
-- 
2.34.1

Re: [PATCH v2] objtool: Report section name in elf_init_reloc_text_sym() warning

[Josh Poimboeuf]

On Mon, May 06, 2024 at 08:55:54AM -0700, Kees Cook wrote:
> While tracking down issues with LKDTM's .rodata "function", I found
> the warning from elf_init_reloc_text_sym() to be unhelpful because it
> wasn't clear which calling path it was coming from. Report the sec->name
> and rephrase the warning a bit. Additionally check for NULL sym->name,
> which may happen.

Hm, as far as I know sym->name shouldn't ever be NULL.

Though, looking at this code again I'm realizing that 'sym' could be
NULL because it refers to the symbol for the function's *section* rather
than for the function itself.  And sometimes the toolchain doesn't
generate symbols for some sections, if they're not referenced anywhere.
(Note the code immediately below your patch actually makes a section
symbol if it doesn't exist.)

But also, this warning doesn't even need the section's symbol.  It can
just get the name from the section directly (insn_sec->name).  And it
would probably also be useful to print the offset (insn_off), like
".rodata+0x1234".

Though, even better, we have an offstr() helper which can pretty-print
the name of the function or variable, if one exists at that address.

tl;dr, does the following work?

		WARN("%s: unexpected reference to non-executable '%s'",
		     sec->name, offstr(insn_sec, insn_off));

-- 
Josh