[PATCH bpf RESEND 2/4] bpf: Remove size check for sk in bpf_skb_is_valid_access for 32-bit architecture

[Alexei Starovoitov]

On Thu, Nov 3, 2022 at 4:23 AM Russell King (Oracle)
<linux at armlinux.org.uk> wrote:
>
> On Thu, Nov 03, 2022 at 05:21:16PM +0800, Yang Jihong wrote:
> > The error code -EACCES is returned when bpf prog is tested in 32-bit environment,
> > This is because bpf_object__relocate modifies the instruction to change memory
> > size to 4 bytes, as shown in the following messages:
> >
> > libbpf: prog 'kfunc_call_test1': relo #2: matching candidate #0 <byte_off> [18342] struct __sk_buff.sk (0:30:0 @ offset 168)
> > libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) off 168 -> 168
> > libbpf: prog 'kfunc_call_test1': relo #2: patched insn #1 (LDX/ST/STX) mem_sz 8 -> 4
> >
> > As a result, the bpf_skb_is_valid_access check fails. For 32-bit architecture,
> > unnecessary checks need to be deleted.
>
> Isn't the purpose of this check to ensure that the entire pointer is
> written, and BPF can't write half of it?
>
>
> >       case offsetof(struct __sk_buff, sk):
> > -             if (type == BPF_WRITE || size != sizeof(__u64))
> > -                     return false;
>
> Wouldn't "(size != sizeof(struct bpf_sock *) && size != sizeof(__u64))"
> be more appropriate here, so 32-bit can only write the 32-bit pointer
> or the full 64-bit value, and 64-bit can only write the 64-bit pointer?
> Or is there a reason not to? bpf folk?

You're correct. The patch is completely wrong.
The bug is elsewhere.