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

["Jose E. Marchesi" <jose.marchesi@oracle.com>]

> On Tue, May 7, 2024 at 4:40 AM Jose E. Marchesi
> <jose.marchesi@oracle.com> 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.
>>
>> 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>
>> ---
>>  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;                                           \
>
> let's add instead `default: val = 0; break;`
>
> as Yonghong mentioned, it's not expected to have invalid byte size
> value in the relocation

Ok.  I will send a V2 with that change.

> pw-bot: cr
>
>>                                                                               \
>>         /* This is a so-called barrier_var() operation that makes specified   \
>>          * variable "a black box" for optimizing compiler.                    \
>> --
>> 2.30.2
>>
>>