linux-kernel - Re: [PATCH 5/5] selftests/bpf: make cfi

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Message-ID: <CADUfDZq5Bf8mVD9o=VHsUqYgqyMJx82_fhy73ZzkvawQi2Ko2g@mail.gmail.com>
Date: Wed, 31 Dec 2025 19:28:29 -0500
From: Caleb Sander Mateos <csander@...estorage.com>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: bot+bpf-ci@...nel.org, Jiri Kosina <jikos@...nel.org>, 
	Benjamin Tissoires <bentiss@...nel.org>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, John Fastabend <john.fastabend@...il.com>, 
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	Martin KaFai Lau <martin.lau@...nel.org>, Chris Mason <clm@...a.com>, 
	Ihor Solodrai <ihor.solodrai@...ux.dev>
Subject: Re: [PATCH 5/5] selftests/bpf: make cfi_stubs globals const

On Wed, Dec 31, 2025 at 10:13 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Wed, Dec 31, 2025 at 10:09 AM Caleb Sander Mateos
> <csander@...estorage.com> wrote:
> >
> > On Wed, Dec 31, 2025 at 10:04 AM <bot+bpf-ci@...nel.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.

Do you mean the BPF core code would modify the struct pointed to by
cfi_stubs? Or some BPF struct_ops implementation (like this one in
bpf_testmod.c) would modify it? If you're talking about the BPF core
code, could you point out where this happens? I couldn't find it when
looking through the handful of uses of cfi_stubs (see patch 1/5). Or
are you talking about some hypothetical future code that would write
through the cfi_stubs pointer? If you're talking about a struct_ops
implementation, I certainly agree it could modify the struct pointed
to by cfi_stubs (before calling register_bpf_struct_ops()). But then
the struct_ops implementation doesn't have to declare the global
variable as const. A non-const pointer is allowed anywhere a const
pointer is expected.

Thanks,
Caleb