bpf: Drop duplicate check_app_limited in tcp_bpf_push

Commit c5c37af6ec ("tcp: Convert do_tcp_sendpages() to use
MSG_SPLICE_PAGES") moved tcp_rate_check_app_limited() inside
do_tcp_sendpages(), turning it into a wrapper around tcp_sendmsg_locked().
Later, commit ebf2e8860e ("tcp_bpf: Inline do_tcp_sendpages as it's now
a wrapper around tcp_sendmsg") inlined the wrapper in tcp_bpf_push() with
direct tcp_sendmsg_locked() calls, which perform the check on every path
that queues data, but kept the outer tcp_rate_check_app_limited() that
was previously needed to cover do_tcp_sendpages(). The outer call is now
redundant.

The site changed here, tcp_bpf_push(), holds the socket lock and invokes
tcp_sendmsg_locked() on every iteration. The early-return paths in
tcp_sendmsg_locked() that skip tcp_rate_check_app_limited() - the
MSG_ZEROCOPY allocation failure and MSG_FASTOPEN branches - return without
queueing any MSG_SPLICE_PAGES data, so there is no functional consequence
from omitting the outer check.

A potential benefit of this change is that it facilitates future reuse of
tcp_bpf_push() for sockmap support in protocols beyond TCP, such as MPTCP.
Since tcp_rate_check_app_limited() is TCP-specific while sendmsg_locked()
is a generic interface in struct proto_ops, this change allows us to switch
to different protocols via sk->sk_socket->ops->sendmsg_locked() without
carrying protocol-specific assumptions.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Link: https://patch.msgid.link/1f7dc605b16fc0590f7cbf5a27d57271926c01ce.1790074764.git.tanggeliang@kylinos.cn

net/ipv4/tcp_bpf.c

@@ -108,7 +108,6 @@ static int tcp_bpf_push(struct sock *sk, struct sk_msg *msg, u32 apply_bytes,
 		off  = sge->offset;
 		page = sg_page(sge);
 
-		tcp_rate_check_app_limited(sk);
 retry:
 		msghdr.msg_flags = flags | MSG_SPLICE_PAGES;
 		has_tx_ulp = tls_sw_has_ctx_tx(sk);