Re: [PATCH v6 2/7] objtool: Fix unreachable instruction warnings for weak functions

Kees Cook
2024-10-29T00:16:47Z

On Fri, Oct 25, 2024 at 10:14:04PM -0700, Rong Xu wrote:
> In the presence of both weak and strong function definitions, the
> linker drops the weak symbol in favor of a strong symbol, but
> leaves the code in place. Code in ignore_unreachable_insn() has
> some heuristics to suppress the warning, but it does not work when
> -ffunction-sections is enabled.
>
> Suppose function foo has both strong and weak definitions.
> Case 1: The strong definition has an annotated section name,
> like .init.text. Only the weak definition will be placed into
> .text.foo. But since the section has no symbols, there will be no
> "hole" in the section.
>
> Case 2: Both sections are without an annotated section name.
> Both will be placed into .text.foo section, but there will be only one
> symbol (the strong one). If the weak code is before the strong code,
> there is no "hole" as it fails to find the right-most symbol before
> the offset.
>
> The fix is to use the first node to compute the hole if hole.sym
> is empty. If there is no symbol in the section, the first node
> will be NULL, in which case, -1 is returned to skip the whole
> section.
>
> Co-developed-by: Han Shen <shenhan@google.com>
> Signed-off-by: Han Shen <shenhan@google.com>

This seems logically correct to me, but I'd love to see review from Josh
and/or Peter Z on this change too.

Reviewed-by: Kees Cook <kees@kernel.org>

--
Kees Cook