Re: [PATCH 5/5] selftests/bpf: make cfi_stubs globals const

Alexei Starovoitov
2025-12-31T18:14:00Z

On Wed, Dec 31, 2025 at 10:09 AM Caleb Sander Mateos
<csander@purestorage.com> wrote:
>
> On Wed, Dec 31, 2025 at 10:04 AM <bot+bpf-ci@kernel.org> wrote:
> >
> > > diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
> > > index 90c4b1a51de6..5e460b1dbdb6 100644
> > > --- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
> > > +++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
> >
> > [ ... ]
> >
> > > @@ -1275,7 +1275,7 @@ bpf_testmod_ops__test_return_ref_kptr(int dummy, struct task_struct *task__ref,
> > > return NULL;
> > > }
> > >
> > > -static struct bpf_testmod_ops __bpf_testmod_ops = {
> > > +static const struct bpf_testmod_ops __bpf_testmod_ops = {
> > > .test_1 = bpf_testmod_test_1,
> > > .test_2 = bpf_testmod_test_2,
> >
> > Is it safe to make __bpf_testmod_ops const here? In bpf_testmod_init(),
> > this struct is modified at runtime:
> >
> > tramp = (void **)&__bpf_testmod_ops.tramp_1;
> > while (tramp <= (void **)&__bpf_testmod_ops.tramp_40)
> > *tramp++ = bpf_testmod_tramp;
> >
> > Writing to a const-qualified object is undefined behavior and may cause a
> > protection fault when the compiler places this in read-only memory. Would
> > the module fail to load on systems where .rodata is actually read-only?
>
> Yup, that's indeed the bug caught by KASAN. Missed this mutation at
> init time, I'll leave __bpf_testmod_ops as mutable.

No. You're missing the point. The whole patch set is no go.
The pointer to cfi stub can be updated just as well.