netdev - [PATCH bpf-next v3 5/5] tcp: simplify tcp_set_congestion

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Date:   Thu, 10 Sep 2020 15:35:36 -0400
From:   Neal Cardwell <ncardwell.kernel@...il.com>
To:     Alexei Starovoitov <ast@...nel.org>
Cc:     netdev@...r.kernel.org, Neal Cardwell <ncardwell@...gle.com>,
        Martin KaFai Lau <kafai@...com>,
        Lawrence Brakmo <brakmo@...com>,
        Eric Dumazet <edumazet@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>, Kevin Yang <yyd@...gle.com>
Subject: [PATCH bpf-next v3 5/5] tcp: simplify tcp_set_congestion_control() load=false case

From: Neal Cardwell <ncardwell@...gle.com>

Simplify tcp_set_congestion_control() by removing the initialization
code path for the !load case.

There are only two call sites for tcp_set_congestion_control(). The
EBPF call site is the only one that passes load=false; it also passes
cap_net_admin=true. Because of that, the exact same behavior can be
achieved by removing the special if (!load) branch of the logic. Both
before and after this commit, the EBPF case will call
bpf_try_module_get(), and if that succeeds then call
tcp_reinit_congestion_control() or if that fails then return EBUSY.

Note that this returns the logic to a structure very similar to the
structure before:
  commit 91b5b21c7c16 ("bpf: Add support for changing congestion control")
except that the CAP_NET_ADMIN status is passed in as a function
argument.

This clean-up was suggested by Martin KaFai Lau.

Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
Suggested-by: Martin KaFai Lau <kafai@...com>
Cc: Lawrence Brakmo <brakmo@...com>
Cc: Eric Dumazet <edumazet@...gle.com>
Cc: Yuchung Cheng <ycheng@...gle.com>
Cc: Kevin Yang <yyd@...gle.com>
---
 net/ipv4/tcp_cong.c | 15 ++++-----------
 1 file changed, 4 insertions(+), 11 deletions(-)

diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index a9b0fb52a1ec..db47ac24d057 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -362,21 +362,14 @@ int tcp_set_congestion_control(struct sock *sk, const char *name, bool load,
 		goto out;
 	}

-	if (!ca) {
+	if (!ca)
 		err = -ENOENT;
-	} else if (!load) {
-		if (bpf_try_module_get(ca, ca->owner)) {
-			tcp_reinit_congestion_control(sk, ca);
-		} else {
-			err = -EBUSY;
-		}
-	} else if (!((ca->flags & TCP_CONG_NON_RESTRICTED) || cap_net_admin)) {
+	else if (!((ca->flags & TCP_CONG_NON_RESTRICTED) || cap_net_admin))
 		err = -EPERM;
-	} else if (!bpf_try_module_get(ca, ca->owner)) {
+	else if (!bpf_try_module_get(ca, ca->owner))
 		err = -EBUSY;
-	} else {
+	else
 		tcp_reinit_congestion_control(sk, ca);
-	}
  out:
 	rcu_read_unlock();
 	return err;
-- 
2.28.0.618.gf4bc123cb7-goog