linux-kernel - Re: [PATCH 05/13] objtool: Increase per-function WARN

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Message-ID: <Z9foFvqpmo0nX1XP@google.com>
Date: Mon, 17 Mar 2025 09:15:02 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Nathan Chancellor <nathan@...nel.org>
Subject: Re: [PATCH 05/13] objtool: Increase per-function WARN_FUNC() rate
 limit

On Fri, Mar 14, 2025 at 12:29:03PM -0700, Josh Poimboeuf wrote:
> Increase the per-function WARN_FUNC() rate limit from 1 to 2.  If the
> number of warnings for a given function goes beyond 2, print "skipping
> duplicate warning(s)".  This helps root out additional warnings in a
> function that might be hiding behind the first one.
> 
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
> ---
>  tools/objtool/check.c                |  2 +-
>  tools/objtool/include/objtool/elf.h  |  2 +-
>  tools/objtool/include/objtool/warn.h | 14 +++++++++++---
>  3 files changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index 6b9ad3afe389..3ddaefe97f55 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -4528,7 +4528,7 @@ static int disas_warned_funcs(struct objtool_file *file)
>  	char *funcs = NULL, *tmp;
>  
>  	for_each_sym(file, sym) {
> -		if (sym->warned) {
> +		if (sym->warnings) {
>  			if (!funcs) {
>  				funcs = malloc(strlen(sym->name) + 1);
>  				strcpy(funcs, sym->name);
> diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
> index d7e815c2fd15..223ac1c24b90 100644
> --- a/tools/objtool/include/objtool/elf.h
> +++ b/tools/objtool/include/objtool/elf.h
> @@ -65,10 +65,10 @@ struct symbol {
>  	u8 return_thunk      : 1;
>  	u8 fentry            : 1;
>  	u8 profiling_func    : 1;
> -	u8 warned	     : 1;
>  	u8 embedded_insn     : 1;
>  	u8 local_label       : 1;
>  	u8 frame_pointer     : 1;
> +	u8 warnings	     : 2;
>  	struct list_head pv_target;
>  	struct reloc *relocs;
>  };
> diff --git a/tools/objtool/include/objtool/warn.h b/tools/objtool/include/objtool/warn.h
> index ac04d3fe4dd9..6180288927fd 100644
> --- a/tools/objtool/include/objtool/warn.h
> +++ b/tools/objtool/include/objtool/warn.h
> @@ -53,14 +53,22 @@ static inline char *offstr(struct section *sec, unsigned long offset)
>  	free(_str);					\
>  })
>  
> +#define WARN_LIMIT 2
> +
>  #define WARN_INSN(insn, format, ...)					\
>  ({									\
>  	struct instruction *_insn = (insn);				\
> -	if (!_insn->sym || !_insn->sym->warned)				\
> +	BUILD_BUG_ON(WARN_LIMIT > 2);					\

Shouldn't this be >3? Anyway, I think it would be clearer if the
coupling was more explicit, e.g:

diff --git i/tools/objtool/include/objtool/elf.h w/tools/objtool/include/objtool/elf.h
index 223ac1c24b90..a86e43d2056f 100644
--- i/tools/objtool/include/objtool/elf.h
+++ w/tools/objtool/include/objtool/elf.h
@@ -46,6 +46,8 @@ struct section {
        struct reloc *relocs;
 };

+#define STRUCT_SYMBOL_WARNING_BITS 2
+
 struct symbol {
        struct list_head list;
        struct rb_node node;
@@ -68,7 +70,7 @@ struct symbol {
        u8 embedded_insn     : 1;
        u8 local_label       : 1;
        u8 frame_pointer     : 1;
-       u8 warnings          : 2;
+       u8 warnings          : STRUCT_SYMBOL_WARNING_BITS;
        struct list_head pv_target;
        struct reloc *relocs;
 };
diff --git i/tools/objtool/include/objtool/warn.h w/tools/objtool/include/objtool/warn.h
index e72b9d630551..2fba6866fd2d 100644
--- i/tools/objtool/include/objtool/warn.h
+++ w/tools/objtool/include/objtool/warn.h
@@ -56,6 +56,7 @@ static inline char *offstr(struct section *sec, unsigned long offset)
 })

 #define WARN_LIMIT 2
+static_assert(WARN_LIMIT < (1 << STRUCT_SYMBOL_WARNING_BITS), "symbol.warnings too small");

 #define WARN_INSN(insn, format, ...)                                   \
 ({


Or just drop the bitfield (surely it can't be that important to save a
byte here?) and use sizeof, e.g:

BUILD_BUG_ON(ilog2(WARN_LIMIT) > sizeof(_insn->sym->warnings));