Re: [PATCH] perf: use __builtin_preserve_field_info for GCC compatibility

[Namhyung Kim <namhyung@kernel.org>]

Hello,

On Wed, Aug 06, 2025 at 01:03:01AM +0100, Sam James wrote:
> When exploring building bpf_skel with GCC's BPF support, there was a
> buid failure because of bpf_core_field_exists vs the mem_hops bitfield:
> ```
>  In file included from util/bpf_skel/sample_filter.bpf.c:6:
> util/bpf_skel/sample_filter.bpf.c: In function 'perf_get_sample':
> tools/perf/libbpf/include/bpf/bpf_core_read.h:169:42: error: cannot take address of bit-field 'mem_hops'
>   169 | #define ___bpf_field_ref1(field)        (&(field))
>       |                                          ^
> tools/perf/libbpf/include/bpf/bpf_helpers.h:222:29: note: in expansion of macro '___bpf_field_ref1'
>   222 | #define ___bpf_concat(a, b) a ## b
>       |                             ^
> tools/perf/libbpf/include/bpf/bpf_helpers.h:225:29: note: in expansion of macro '___bpf_concat'
>   225 | #define ___bpf_apply(fn, n) ___bpf_concat(fn, n)
>       |                             ^~~~~~~~~~~~~
> tools/perf/libbpf/include/bpf/bpf_core_read.h:173:9: note: in expansion of macro '___bpf_apply'
>   173 |         ___bpf_apply(___bpf_field_ref, ___bpf_narg(args))(args)
>       |         ^~~~~~~~~~~~
> tools/perf/libbpf/include/bpf/bpf_core_read.h:188:39: note: in expansion of macro '___bpf_field_ref'
>   188 |         __builtin_preserve_field_info(___bpf_field_ref(field), BPF_FIELD_EXISTS)
>       |                                       ^~~~~~~~~~~~~~~~
> util/bpf_skel/sample_filter.bpf.c:167:29: note: in expansion of macro 'bpf_core_field_exists'
>   167 |                         if (bpf_core_field_exists(data->mem_hops))
>       |                             ^~~~~~~~~~~~~~~~~~~~~
> cc1: error: argument is not a field access
> ```
> 
> ___bpf_field_ref1 was adapted for GCC in 12bbcf8e840f40b82b02981e96e0a5fbb0703ea9
> but the trick added for compatibility in 3a8b8fc3174891c4c12f5766d82184a82d4b2e3e
> isn't compatible with that as an address is used as an argument.
> 
> Workaround this by calling __builtin_preserve_field_info directly as the
> bpf_core_field_exists macro does, but without the ___bpf_field_ref use.

IIUC GCC doesn't support bpf_core_fields_exists() for bitfield members,
right?  Is it gonna change in the future?

> 
> Link: https://gcc.gnu.org/PR121420
> Co-authored-by: Andrew Pinski <quic_apinski@quicinc.com>
> Signed-off-by: Sam James <sam@gentoo.org>
> ---
>  tools/perf/util/bpf_skel/sample_filter.bpf.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/perf/util/bpf_skel/sample_filter.bpf.c b/tools/perf/util/bpf_skel/sample_filter.bpf.c
> index b195e6efeb8be..e5666d4c17228 100644
> --- a/tools/perf/util/bpf_skel/sample_filter.bpf.c
> +++ b/tools/perf/util/bpf_skel/sample_filter.bpf.c
> @@ -164,7 +164,7 @@ static inline __u64 perf_get_sample(struct bpf_perf_event_data_kern *kctx,
>  		if (entry->part == 8) {
>  			union perf_mem_data_src___new *data = (void *)&kctx->data->data_src;
>  
> -			if (bpf_core_field_exists(data->mem_hops))
> +			if (__builtin_preserve_field_info(data->mem_hops, BPF_FIELD_EXISTS))

I believe those two are equivalent (maybe worth a comment?).  But it'd
be great if BPF/clang folks can review if it's ok.

Anyway, I can build it with clang.

Tested-by: Namhyung Kim <namhyung@kernel.org>

Thanks,
Namhyung


>  				return data->mem_hops;
>  
>  			return 0;
> -- 
> 2.50.1
>