Re: [PATCH v2 2/2] x86/cfi,bpf: Fix BPF JIT call

Sami Tolvanen
2023-12-04T18:59:45Z

On Mon, Dec 4, 2023 at 10:34 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> So afaict this is used through bpf_for_each_map_elem(), where the
> argument still is properly callback_fn. However, in the desriptor
> bpf_for_each_map_elem_proto the argument gets described as:
> ARG_PTR_TO_FUNC, which in turn has a comment like:
>
> ARG_PTR_TO_FUNC, /* pointer to a bpf program function */
>
> Which to me sounds like there is definite type punning involved. The
> call in bpf_for_each_array_elem() is a regular C indirect call, which
> gets adorned with the kCFI magic.
>
> But I doubt the BPF function that gets used gets the correct matching
> bits on.
>
> TL;DR, I think this is a pre-existing problem with kCFI + eBPF and not
> caused by my patches.

It is a pre-existing problem, I ran into the same failures when I
looked into this briefly last year:

https://github.com/ClangBuiltLinux/linux/issues/1727

In addition to bpf_for_each_array_elem, a few other callers also use
the same function pointer type that doesn't match cfi_bpf_hash.

Sami