On Wed, Dec 31, 2025 at 10:13 AM Alexei Starovoitov <[email protected]> wrote: > > On Wed, Dec 31, 2025 at 10:09 AM Caleb Sander Mateos > <[email protected]> wrote: > > > > On Wed, Dec 31, 2025 at 10:04 AM <[email protected]> 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
