linux-kernel - Re: [PATCH v3 1/4] perf build: enable -fno-strict-aliasing

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Message-ID: <aF7JC8zkG5-_-nY_@google.com>
Date: Fri, 27 Jun 2025 09:38:35 -0700
From: Namhyung Kim <namhyung@...nel.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Adrian Hunter <adrian.hunter@...el.com>,
	Liang Kan <kan.liang@...ux.intel.com>,
	Yuzhuo Jing <yuzhuo@...gle.com>
Subject: Re: [PATCH v3 1/4] perf build: enable -fno-strict-aliasing

On Wed, Jun 25, 2025 at 01:23:08PM -0700, Eric Biggers wrote:
> perf pulls in code from kernel headers that assumes it is being built
> with -fno-strict-aliasing, namely put_unaligned_*() from
> <linux/unaligned.h> which write the data using packed structs that lack
> the may_alias attribute.  Enable -fno-strict-aliasing to prevent
> miscompilations in sha1.c which would otherwise occur due to this issue.
> 
> Signed-off-by: Eric Biggers <ebiggers@...nel.org>
> ---
>  tools/perf/Makefile.config | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
> index 24736b0bbb302..70a3e771c7c08 100644
> --- a/tools/perf/Makefile.config
> +++ b/tools/perf/Makefile.config
> @@ -17,10 +17,14 @@ detected     = $(shell echo "$(1)=y"       >> $(OUTPUT).config-detected)
>  detected_var = $(shell echo "$(1)=$($(1))" >> $(OUTPUT).config-detected)
>  
>  CFLAGS := $(EXTRA_CFLAGS) $(filter-out -Wnested-externs,$(EXTRA_WARNINGS))
>  HOSTCFLAGS := $(filter-out -Wnested-externs,$(EXTRA_WARNINGS))
>  
> +# This is required because the kernel is built with this and some of the code
> +# borrowed from kernel headers depends on it, e.g. put_unaligned_*().
> +CFLAGS += -fno-strict-aliasing

This makes a build error with REFCNT_CHECKING=1.

  In file included from util/symbol.c:27:
  In function ‘dso__set_symbol_names_len’,
      inlined from ‘dso__sort_by_name’ at util/symbol.c:638:4:
  util/dso.h:654:46: error: ‘len’ may be used uninitialized [-Werror=maybe-uninitialized]
    654 |         RC_CHK_ACCESS(dso)->symbol_names_len = len;
        |                                              ^
  util/symbol.c: In function ‘dso__sort_by_name’:
  util/symbol.c:634:24: note: ‘len’ was declared here
    634 |                 size_t len;
        |                        ^~~

I'll simply add this to work around it:

diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c
index 8b30c6f16a9eeac1..73dab94fab74e829 100644
--- a/tools/perf/util/symbol.c
+++ b/tools/perf/util/symbol.c
@@ -631,7 +631,7 @@ void dso__sort_by_name(struct dso *dso)
 {
        mutex_lock(dso__lock(dso));
        if (!dso__sorted_by_name(dso)) {
-               size_t len;
+               size_t len = 0;
 
                dso__set_symbol_names(dso, symbols__sort_by_name(dso__symbols(dso), &len));
                if (dso__symbol_names(dso)) {

Thanks,
Namhyung

> +
>  # Enabled Wthread-safety analysis for clang builds.
>  ifeq ($(CC_NO_CLANG), 0)
>    CFLAGS += -Wthread-safety
>  endif
>  
> -- 
> 2.50.0
>