linux-kernel - Re: [PATCH] selftests/bpf: Fix the address is NULL

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Date:   Fri, 16 Jun 2023 09:33:57 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Yonghong Song <yhs@...a.com>
Cc:     wuyonggang001@...suo.com, andrii@...nel.org, shuah@...nel.org,
        bpf@...r.kernel.org, linux-kselftest@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] selftests/bpf: Fix the address is NULL

On Wed, Jun 14, 2023 at 7:09 AM Yonghong Song <yhs@...a.com> wrote:
>
>
>
> On 6/14/23 12:42 AM, wuyonggang001@...suo.com wrote:
> > Fix the following coccicheck error:
> >
> > tools/testing/selftests/bpf/progs/test_ksyms_weak.c:53:6-20: ERROR: test
> > of a variable/field address
>
> I didn't see clang/gcc compiler warns about this. Maybe need some
> additional flags beyond what current selftest/bpf already has
> in order to trigger this warning?
> If you feel this warning has some merit, could you propose
> it to gcc/llvm community?

bpf_link_fops2 is a weak symbol, this check is totally valid and reasonable.

There are two problems here, though:

a) coccicheck shouldn't warn about "test of a variable/field address"
for weak symbols, because they can be NULL.

b) this patch is not even fixing that warning, it does a no-op change
from implicit non-NULL check to explicit non-NULL check. And the
former is actually the preferred style.

So this patch is doubly wrong.

>
> >
> > Signed-off-by: Yonggang Wu <wuyonggang001@...suo.com>
> > ---
> >   tools/testing/selftests/bpf/progs/test_ksyms_weak.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/tools/testing/selftests/bpf/progs/test_ksyms_weak.c
> > b/tools/testing/selftests/bpf/progs/test_ksyms_weak.c
> > index d00268c91e19..768a4d6ee6f5 100644
> > --- a/tools/testing/selftests/bpf/progs/test_ksyms_weak.c
> > +++ b/tools/testing/selftests/bpf/progs/test_ksyms_weak.c
> > @@ -50,7 +50,7 @@ int pass_handler(const void *ctx)
> >       /* tests non-existent symbols. */
> >       out__non_existent_typed = (__u64)&bpf_link_fops2;
> >
> > -    if (&bpf_link_fops2) /* can't happen */
> > +    if (&bpf_link_fops2 != NULL) /* can't happen */
> >           out__non_existent_typed =
> > (__u64)bpf_per_cpu_ptr(&bpf_link_fops2, 0);
> >
> >       if (!bpf_ksym_exists(bpf_task_acquire))
> >