Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

On Wed, Sep 29, 2021 at 5:38 AM Lorenz Bauer [off-list ref] wrote:

On Wed, 29 Sept 2021 at 13:10, Toke Høiland-Jørgensen [off-list ref] wrote:

quoted

Lorenz Bauer [off-list ref] writes:

quoted

On Thu, 16 Sept 2021 at 18:47, Jakub Kicinski [off-list ref] wrote:

quoted

Won't applications end up building something like skb_header_pointer()
based on bpf_xdp_adjust_data(), anyway? In which case why don't we
provide them what they need?

say:

void *xdp_mb_pointer(struct xdp_buff *xdp_md, u32 flags,
                     u32 offset, u32 len, void *stack_buf)

flags and offset can be squashed into one u64 as needed. Helper returns
pointer to packet data, either real one or stack_buf. Verifier has to
be taught that the return value is NULL or a pointer which is safe with
offsets up to @len.

If the reason for access is write we'd also need:

void *xdp_mb_pointer_flush(struct xdp_buff *xdp_md, u32 flags,
                           u32 offset, u32 len, void *stack_buf)

I'm missing something. Why do we need a separate flush() helper?
Can't we do:
char buf[64], *p;
p = xdp_mb_pointer(ctx, flags, off, len, buf);
read/write p[]
if (p == buf)
    xdp_store_bytes(ctx, off, buf, len, flags);