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

Rong Xu
2024-10-29T05:51:35Z

I think the objtool works on individual object files (or vmlinux.o).
The -Wl,--gc-sections flag, on the other hand, is a linker flag that
acts on the final link -- it's applied after objtool invocations.
Therefore, even with -Wl,--gc-sections, we'll still encounter those
spurious warnings from objtool.

-Rong

On Mon, Oct 28, 2024 at 5:19 PM H. Peter Anvin <hpa@zytor.com> wrote:
>
> On 10/28/24 17:16, 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>
> >
>
> Does this happen even with -Wl,--gc-sections?
>
> -hpa
>