On Tue, Dec 22, 2020 at 07:09:33PM -0800, sdf@google.com wrote:

On 12/22, Martin KaFai Lau wrote:

quoted

On Thu, Dec 17, 2020 at 09:23:23AM -0800, Stanislav Fomichev wrote:

quoted

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 <redacted>
---
 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?

I was asking because, while 128 byte may work best for TCP_ZEROCOPY_RECEIVCE,
it is many unnecessary byte-zeroings for most options though.
Hence, I am interested to see if there is a practical bpf
use case for TCP_ZEROCOPY_RECEIVE.