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

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

On 5/7/24 11:20 AM, Jose E. Marchesi wrote:
>> 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.

I thought -Wmaybe-unitialized also available to clang but just checked
that clang does not have this. So your above change looks good to me.

>
> 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;