Re: [PATCH bpf-next 0/1] use preserve_static_offset in bpf uapi headers

[Eduard Zingerman <eddyz87@gmail.com>]

On Thu, 2023-12-07 at 18:28 -0800, Yonghong Song wrote:
[...]
> All context types are defined in include/linux/bpf_types.h.
> The context type bpf_nf_ctx is missing.

convert_ctx_access() is not applied for bpf_nf_ctx. Searching through
kernel code shows that BPF programs access this structure directly
(net/netfilter/nf_bpf_link.c):

    static unsigned int nf_hook_run_bpf(void *bpf_prog, struct sk_buff *skb,
                        const struct nf_hook_state *s)
    {
        const struct bpf_prog *prog = bpf_prog;
        struct bpf_nf_ctx ctx = {
            .state = s,
            .skb = skb,
        };

        return bpf_prog_run(prog, &ctx);
    }

I added __bpf_ctx only for types that are subject to convert_ctx_access()
transformation. On the other hand, applying it to each context type
should not hurt either. Which way would you prefer?

[...]

> > How to add the same definitions in vmlinux.h is an open question,
> > and most likely requires bpftool modification:
> > - Hard code generation of __bpf_ctx based on type names?
> > - Mark context types with some special
> >    __attribute__((btf_decl_tag("preserve_static_offset")))
> >    and convert it to __attribute__((preserve_static_offset))?
> 
> The number of context types is limited, I would just go through
> the first approach with hard coding the list of ctx types and
> mark them with preserve_static_offset attribute in vmlinux.h.

Tbh, I'm with Alan here, generic approach seems a tad nicer.
Lets collect some more votes :)