On 12/22, Martin KaFai Lau wrote:
On Thu, Dec 17, 2020 at 09:23:23AM -0800, Stanislav Fomichev wrote:
> When we attach a bpf program to cgroup/getsockopt any other getsockopt()
> syscall starts incurring kzalloc/kfree cost. While, in general, it's
> not an issue, sometimes it is, like in the case of TCP_ZEROCOPY_RECEIVE.
> TCP_ZEROCOPY_RECEIVE (ab)uses getsockopt system call to implement
> fastpath for incoming TCP, we don't want to have extra allocations in
> there.
>
> Let add a small buffer on the stack and use it for small (majority)
> {s,g}etsockopt values. I've started with 128 bytes to cover
> the options we care about (TCP_ZEROCOPY_RECEIVE which is 32 bytes
> currently, with some planned extension to 64 + some headroom
> for the future).
>
> It seems natural to do the same for setsockopt, but it's a bit more
> involved when the BPF program modifies the data (where we have to
> kmalloc). The assumption is that for the majority of setsockopt
> calls (which are doing pure BPF options or apply policy) this
> will bring some benefit as well.
>
> Signed-off-by: Stanislav Fomichev <sdf@xxxxxxxxxx>
> ---
> include/linux/filter.h | 3 +++
> kernel/bpf/cgroup.c | 41 +++++++++++++++++++++++++++++++++++++++--
> 2 files changed, 42 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/filter.h b/include/linux/filter.h
> index 29c27656165b..362eb0d7af5d 100644
> --- a/include/linux/filter.h
> +++ b/include/linux/filter.h
> @@ -1281,6 +1281,8 @@ struct bpf_sysctl_kern {
> u64 tmp_reg;
> };
>
> +#define BPF_SOCKOPT_KERN_BUF_SIZE 128
Since these 128 bytes (which then needs to be zero-ed) is modeled after
the TCP_ZEROCOPY_RECEIVE use case, it will be useful to explain
a use case on how the bpf prog will interact with
getsockopt(TCP_ZEROCOPY_RECEIVE).
The only thing I would expect BPF program can do is to return EPERM
to cause application to fallback to non-zerocopy path (and, mostly,
bypass). I don't think BPF can meaningfully mangle the data in struct
tcp_zerocopy_receive.
Does it address your concern? Or do you want me to add a comment or
something?
