Re: [PATCH bpf-next] bpf: avoid uninitialized value in BPF_CORE_READ_BITFIELD

[Yonghong Song <yonghong.song@linux.dev>]

On 5/7/24 4:39 AM, Jose E. Marchesi wrote:
> GCC warns that `val' may be used uninitialized in the
> BPF_CORE_READ_BITFIELD macro, defined in bpf_core_read.h as:
>
> 	[...]
> 	unsigned long long val;						      \
> 	[...]								      \
> 	switch (__CORE_RELO(s, field, BYTE_SIZE)) {			      \
> 	case 1: val = *(const unsigned char *)p; break;			      \
> 	case 2: val = *(const unsigned short *)p; break;		      \
> 	case 4: val = *(const unsigned int *)p; break;			      \
> 	case 8: val = *(const unsigned long long *)p; break;		      \
>          }       							      \
> 	[...]
> 	val;								      \
> 	}								      \
>
> This patch initializes `val' to zero in order to avoid the warning,
> and random values to be used in case __builtin_preserve_field_info
> returns unexpected values for BPF_FIELD_BYTE_SIZE.

In clang, __builtin_preserve_field_info either returns correct value
or caused compilation error. Do you mean for gcc __builtin_preserve_field_info
might return an unexpected value here?

BTW, your change makes sense to silent this warning. So Ack below.

>
> Tested in bpf-next master.
> No regressions.
>
> Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
> Cc: david.faust@oracle.com
> Cc: cupertino.miranda@oracle.com
> Cc: Eduard Zingerman <eddyz87@gmail.com>
> Cc: Yonghong Song <yonghong.song@linux.dev>

Acked-by: Yonghong Song <yonghong.song@linux.dev>

> ---
>   tools/lib/bpf/bpf_core_read.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> index b5c7ce5c243a..88d129b5f0a1 100644
> --- a/tools/lib/bpf/bpf_core_read.h
> +++ b/tools/lib/bpf/bpf_core_read.h
> @@ -89,7 +89,7 @@ enum bpf_enum_value_kind {
>    */
>   #define BPF_CORE_READ_BITFIELD(s, field) ({				      \
>   	const void *p = (const void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \
> -	unsigned long long val;						      \
> +	unsigned long long val = 0;					      \
>   									      \
>   	/* This is a so-called barrier_var() operation that makes specified   \
>   	 * variable "a black box" for optimizing compiler.		      \