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

Josh Poimboeuf
2024-10-29T00:43:56Z

On Mon, Oct 28, 2024 at 05:16:44PM -0700, Kees Cook wrote:
> 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>

LGTM, thanks!

Acked-by: Josh Poimboeuf <jpoimboe@kernel.org>

--
Josh