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

Alexei Starovoitov
2023-12-08T21:04:49Z

On Fri, Dec 8, 2023 at 12:56 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Dec 08, 2023 at 12:45:51PM -0800, Alexei Starovoitov wrote:
>
> > I mean we don't need to store a pointer to a func in stubs.
> > Can it be, roughly:
> >
> > extern void bpf_tcp_ca_cong_avoid(struct sock *sk, u32 ack, u32 acked);
> > KCFI_MACRO(hash_of_cong_avoid, bpf_tcp_ca_cong_avoid);
> > u32 __array_of_kcfi_hash[] = {hash_of_cong_avoid, hash_of_set_state,...};
> > .bpf_ops_stubs = __array_of_kcfi_hash,
>
> But then how do I index this array? The bpf_ops_stubs thing having the
> same layout at the target struct made it easy and we could use 'moff'
> for both.
>
> That also remains working if someone adds a member to the struct or
> moves some members around.

I was thinking to just moff/2 assuming u32 array will have the same order,
but I missed the fact that init() and release() in tcp_congestion_ops
come after some variables.
And you're right it's more fragile when things change in tcp_congestion_ops.
Storing u32 hash as (void *) into function pointers is just ugly.
Let's go with your initial approach.