linux-kernel - Re: [PATCH 1/6] objtool: Support Clang AUTOFDO .cold functions

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Message-ID: <CAF1bQ=Qj3KDN4mygz_0kJJvhuQL=3Wh2Z_L20OadhtzaguZDYg@mail.gmail.com>
Date: Thu, 20 Nov 2025 14:18:21 -0800
From: Rong Xu <xur@...gle.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, 
	Peter Zijlstra <peterz@...radead.org>, Han Shen <shenhan@...gle.com>, 
	Sriraman Tallam <tmsriram@...gle.com>, Krzysztof Pszeniczny <kpszeniczny@...gle.com>
Subject: Re: [PATCH 1/6] objtool: Support Clang AUTOFDO .cold functions

Thanks Josh for the fix. I can confirm this fixes the objtool issue
related to split-function in AutoFDO and Propeller builds

Tested-by: xur@...gle.com
Reviewed-by: xur@...gle.com

-Rong

On Thu, Nov 20, 2025 at 12:52 PM Josh Poimboeuf <jpoimboe@...nel.org> wrote:
>
> AutoFDO enables -fsplit-machine-functions which can move the cold parts
> of a function to a <func>.cold symbol in a .text.split.<func> section.
>
> Unlike GCC, the Clang <func>.cold symbols are not marked STT_FUNC.  This
> confuses objtool in several ways, resulting in warnings like the
> following:
>
>   vmlinux.o: warning: objtool: apply_retpolines.cold+0xfc: unsupported instruction in callable function
>   vmlinux.o: warning: objtool: machine_check_poll.cold+0x2e: unsupported instruction in callable function
>   vmlinux.o: warning: objtool: free_deferred_objects.cold+0x1f: relocation to !ENDBR: free_deferred_objects.cold+0x26
>   vmlinux.o: warning: objtool: rpm_idle.cold+0xe0: relocation to !ENDBR: rpm_idle.cold+0xe7
>   vmlinux.o: warning: objtool: tcp_rcv_state_process.cold+0x1c: relocation to !ENDBR: tcp_rcv_state_process.cold+0x23
>
> Fix it by marking the .cold symbols as STT_FUNC.
>
> Fixes: 2fd65f7afd5a ("AutoFDO: Enable machine function split optimization for AutoFDO")
> Reported-by: Rong Xu <xur@...gle.com>
> Closes: https://lore.kernel.org/20251103215244.2080638-2-xur@google.com
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  tools/objtool/elf.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
>
> diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
> index 3f20b257ab25..7895f65aca2a 100644
> --- a/tools/objtool/elf.c
> +++ b/tools/objtool/elf.c
> @@ -502,8 +502,16 @@ static int elf_add_symbol(struct elf *elf, struct symbol *sym)
>         if (strstarts(sym->name, ".klp.sym"))
>                 sym->klp = 1;
>
> -       if (!sym->klp && is_func_sym(sym) && strstr(sym->name, ".cold"))
> +       if (!sym->klp && !is_sec_sym(sym) && strstr(sym->name, ".cold")) {
>                 sym->cold = 1;
> +
> +               /*
> +                * Clang doesn't mark cold subfunctions as STT_FUNC, which
> +                * breaks several objtool assumptions.  Fake it.
> +                */
> +               sym->type = STT_FUNC;
> +       }
> +
>         sym->pfunc = sym->cfunc = sym;
>
>         sym->demangled_name = demangle_name(sym);
> --
> 2.51.1
>