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

H. Peter Anvin
2024-10-29T00:20:03Z

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