Re: [PATCH 2/6] objtool: Fix unreachable instruction warnings for weak funcitons

[Peter Zijlstra <peterz@infradead.org>]

On Sun, Jul 28, 2024 at 01:29:55PM -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>
> Signed-off-by: Rong Xu <xur@google.com>
> Suggested-by: Sriraman Tallam <tmsriram@google.com>
> Suggested-by: Krzysztof Pszeniczny <kpszeniczny@google.com>
> ---
>  tools/objtool/elf.c | 13 ++++++++-----
>  1 file changed, 8 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index 3d27983dc908..fa88bb254ccc 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -224,12 +224,15 @@ int find_symbol_hole_containing(const struct section *sec, unsigned long offset)
>  	if (n)
>  		return 0; /* not a hole */
>  
> -	/* didn't find a symbol for which @offset is after it */
> -	if (!hole.sym)
> -		return 0; /* not a hole */
> +	/*
> +	 * @offset >= sym->offset + sym->len, find symbol after it.
> +	 * Use the first node in rb_tree when hole.sym is NULL.
> +	 */

	/*
	 * If we are not right of any symbol, the next symbol must be
	 * the first symbol. Either way, the next symbol -- if there is
	 * one -- provides the rightmost boundary of the hole.
	 */
	if (!hole.sym)
		n = rb_first_cached(&sec->symbol_tree);
	else
		n = rb_next(&hole.sym->node);


That tells us more of why, rather than of what. Hmm?

> +	if (hole.sym)
> +		n = rb_next(&hole.sym->node);
> +	else
> +		n = rb_first_cached(&sec->symbol_tree);
>  
> -	/* @offset >= sym->offset + sym->len, find symbol after it */
> -	n = rb_next(&hole.sym->node);
>  	if (!n)
>  		return -1; /* until end of address space */
>  
> -- 
> 2.46.0.rc1.232.g9752f9e123-goog
>