netdev - [PATCH net-next] net: fix __sock_gen

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Date:   Fri, 18 Nov 2022 04:38:43 +0000
From:   Eric Dumazet <edumazet@...gle.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com,
        Eric Dumazet <edumazet@...gle.com>,
        coverity-bot <keescook+coverity-bot@...omium.org>
Subject: [PATCH net-next] net: fix __sock_gen_cookie()

I was mistaken how atomic64_try_cmpxchg(&sk_cookie, &res, new)
is working.

I was assuming @res would contain the final sk_cookie value,
regardless of the success of our cmpxchg()

We could do something like:

if (atomic64_try_cmpxchg(&sk_cookie, &res, new)
	res = new;

But we can avoid a conditional and read sk_cookie again.

atomic64_cmpxchg(&sk_cookie, res, new);
res = atomic64_read(&sk_cookie);

Reported-by: coverity-bot <keescook+coverity-bot@...omium.org>
Addresses-Coverity-ID: 1527347 ("Error handling issues")
Fixes: 4ebf802cf1c6 ("net: __sock_gen_cookie() cleanup")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
---
 net/core/sock_diag.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/net/core/sock_diag.c b/net/core/sock_diag.c
index b11593cae5a09b15a10d6ba35bccc22263cb8fc8..b1e29e18d1d60cb5c87c884652f547c083ba81cd 100644
--- a/net/core/sock_diag.c
+++ b/net/core/sock_diag.c
@@ -30,7 +30,10 @@ u64 __sock_gen_cookie(struct sock *sk)
 	if (!res) {
 		u64 new = gen_cookie_next(&sock_cookie);
 
-		atomic64_try_cmpxchg(&sk->sk_cookie, &res, new);
+		atomic64_cmpxchg(&sk->sk_cookie, res, new);
+
+		/* Another thread might have changed sk_cookie before us. */
+		res = atomic64_read(&sk->sk_cookie);
 	}
 	return res;
 }
-- 
2.38.1.584.g0f3c55d4c2-goog