Re: Coverity: __sock_gen_cookie(): Error handling issues

[Eric Dumazet]

On Thu, Nov 17, 2022 at 2:22 PM coverity-bot <keescook@xxxxxxxxxxxx> wrote:
>
> Hello!
>
> This is an experimental semi-automated report about issues detected by
> Coverity from a scan of next-20221117 as part of the linux-next scan project:
> https://scan.coverity.com/projects/linux-next-weekly-scan
>
> You're getting this email because you were associated with the identified
> lines of code (noted below) that were touched by commits:
>
>   Wed Nov 16 12:42:01 2022 +0000
>     4ebf802cf1c6 ("net: __sock_gen_cookie() cleanup")
>
> Coverity reported the following:
>
> *** CID 1527347:  Error handling issues  (CHECKED_RETURN)
> net/core/sock_diag.c:33 in __sock_gen_cookie()
> 27     {
> 28      u64 res = atomic64_read(&sk->sk_cookie);
> 29
> 30      if (!res) {
> 31              u64 new = gen_cookie_next(&sock_cookie);
> 32
> vvv     CID 1527347:  Error handling issues  (CHECKED_RETURN)
> vvv     Calling "atomic64_try_cmpxchg" without checking return value (as is done elsewhere 8 out of 9 times).
> 33              atomic64_try_cmpxchg(&sk->sk_cookie, &res, new);


Hmmm. for some reason I thought @res was always updated...

A fix would be to read sk->sk_cookie, but I guess your tool will still
complain we do not care
of  atomic64_try_cmpxchg() return value ?

diff --git a/net/core/sock_diag.c b/net/core/sock_diag.c
index b11593cae5a09b15a10d6ba35bccc22263cb8fc8..58efb9c1c8dd4f8e5a3009a0176e1b96487daaff
100644
--- a/net/core/sock_diag.c
+++ b/net/core/sock_diag.c
@@ -31,6 +31,10 @@ u64 __sock_gen_cookie(struct sock *sk)
                u64 new = gen_cookie_next(&sock_cookie);

                atomic64_try_cmpxchg(&sk->sk_cookie, &res, new);
+               /* Another cpu/thread might have won the race,
+                * reload the final value.
+                */
+               res = atomic64_read(&sk->sk_cookie);
        }
        return res;
 }