Re: [PATCH bpf-next] bpf: avoid uninitialized warnings in verifier_global_subprogs.c

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

> On 5/7/24 7:05 AM, Jose E. Marchesi wrote:
>> The BPF selftest verifier_global_subprogs.c contains code that
>> purposedly performs out of bounds access to memory, to check whether
>> the kernel verifier is able to catch them.  For example:
>>
>>    __noinline int global_unsupp(const int *mem)
>>    {
>> 	if (!mem)
>> 		return 0;
>> 	return mem[100]; /* BOOM */
>>    }
>>
>> With -O1 and higher and no inlining, GCC notices this fact and emits a
>> "maybe uninitialized" warning.  This is by design.  Note that the
>> emission of these warnings is highly dependent on the precise
>> optimizations that are performed.
>
> Interesting. The error message is 'maybe uninitialized' but not
> an error to complain out-of-bound access. But anyway, since gcc
> produces a warning, your patch silences it and LGTM.
>
> Acked-by: Yonghong Song <yonghong.song@linux.dev>

Please hold on.  The right warning to inhibit is -Wmaybe-uninitialized,
which is GCC specific.

So it must be:

  #if !defined(__clang__)
  #pragma GCC diagnostic ignored "-Wmaybe-uninitialized"
  #endif

Unless you disagree I am testing this and will send a V2 with your
Acked-by.

Sorry about this.  I hate to be erratic, but so many small patches
today.

>>
>> This patch adds a compiler pragma to verifier_global_subprogs.c to
>> ignore these warnings.
>>
>> 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: Yonghong Song <yonghong.song@linux.dev>
>> Cc: Eduard Zingerman <eddyz87@gmail.com>
>> ---
>>   tools/testing/selftests/bpf/progs/verifier_global_subprogs.c | 5 +++++
>>   1 file changed, 5 insertions(+)
>>
>> diff --git
>> a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
>> b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
>> index baff5ffe9405..d05dc218b7e9 100644
>> --- a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
>> +++ b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
>> @@ -8,6 +8,11 @@
>>   #include "xdp_metadata.h"
>>   #include "bpf_kfuncs.h"
>>   +/* The compiler may be able to detect the access to uninitialized
>> +   memory in the routines performing out of bound memory accesses and
>> +   emit warnings about it.  This is the case of GCC. */
>> +#pragma GCC diagnostic ignored "-Wuninitialized"
>> +
>>   int arr[1];
>>   int unkn_idx;
>>   const volatile bool call_dead_subprog = false;