Re: [PATCH v2 1/4] perf build: enable -fno-strict-aliasing

[Ian Rogers]

On Fri, Jun 13, 2025 at 9:43 PM Eric Biggers <ebiggers@xxxxxxxxxx> wrote:
>
> From: Eric Biggers <ebiggers@xxxxxxxxxx>
>
> 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.

Wow, good catch! I wonder if -fsanitize=type could be used to capture
when perf's code is broken like this? Perhaps we should just remove
linux/unaligned.h in tools because of this, the alternative of using
memcpy doesn't look particularly burdensome. Given the memcpys are of
a known/fixed size I'd expect the compiler to be able to optimize
things just as well. Perhaps we should rewrite unaligned.h in tools
but perhaps the kernel too. Something like:

#define __get_unaligned_t(type, ptr) ({
                 \
        const struct { type x; } __packed * __get_pptr =
(typeof(__get_pptr))(ptr);     \
        __get_pptr->x;
                 \
})

becomes:

#define __get_unaligned_t(type, ptr) ({
                 \
        type __get_val; memcpy(&__get_val, ptr, sizeof(__get_val));     \
        __get_val;
             \
})

Thanks,
Ian

> Signed-off-by: Eric Biggers <ebiggers@xxxxxxxxxx>
> ---
>  tools/perf/Makefile.config | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
> index d1ea7bf449647..1691b47c4694c 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
> +
>  # Enabled Wthread-safety analysis for clang builds.
>  ifeq ($(CC_NO_CLANG), 0)
>    CFLAGS += -Wthread-safety
>  endif
>
> --
> 2.49.0
>