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Message-ID: <1d2e0a47-a486-0991-91e2-fed54163898e@gmail.com>
Date: Mon, 8 Nov 2021 15:10:24 -0800
From: Eric Dumazet <eric.dumazet@...il.com>
To: Kuniyuki Iwashima <kuniyu@...zon.co.jp>,
"David S. Miller" <davem@...emloft.net>,
Jakub Kicinski <kuba@...nel.org>
Cc: Kuniyuki Iwashima <kuni1840@...il.com>,
Benjamin Herrenschmidt <benh@...zon.com>,
netdev@...r.kernel.org
Subject: Re: [PATCH net-next 13/13] af_unix: Relax race in unix_autobind().
On 11/6/21 2:17 AM, Kuniyuki Iwashima wrote:
> When we bind an AF_UNIX socket without a name specified, the kernel selects
> an available one from 0x00000 to 0xFFFFF. unix_autobind() starts searching
> from a number in the 'static' variable and increments it after acquiring
> two locks.
>
> If multiple processes try autobind, they obtain the same lock and check if
> a socket in the hash list has the same name. If not, one process uses it,
> and all except one end up retrying the _next_ number (actually not, it may
> be incremented by the other processes). The more we autobind sockets in
> parallel, the longer the latency gets. We can avoid such a race by
> searching for a name from a random number.
>
> These show latency in unix_autobind() while 64 CPUs are simultaneously
> autobind-ing 1024 sockets for each.
>
> Without this patch:
>
> usec : count distribution
> 0 : 1176 |*** |
> 2 : 3655 |*********** |
> 4 : 4094 |************* |
> 6 : 3831 |************ |
> 8 : 3829 |************ |
> 10 : 3844 |************ |
> 12 : 3638 |*********** |
> 14 : 2992 |********* |
> 16 : 2485 |******* |
> 18 : 2230 |******* |
> 20 : 2095 |****** |
> 22 : 1853 |***** |
> 24 : 1827 |***** |
> 26 : 1677 |***** |
> 28 : 1473 |**** |
> 30 : 1573 |***** |
> 32 : 1417 |**** |
> 34 : 1385 |**** |
> 36 : 1345 |**** |
> 38 : 1344 |**** |
> 40 : 1200 |*** |
>
> With this patch:
>
> usec : count distribution
> 0 : 1855 |****** |
> 2 : 6464 |********************* |
> 4 : 9936 |******************************** |
> 6 : 12107 |****************************************|
> 8 : 10441 |********************************** |
> 10 : 7264 |*********************** |
> 12 : 4254 |************** |
> 14 : 2538 |******** |
> 16 : 1596 |***** |
> 18 : 1088 |*** |
> 20 : 800 |** |
> 22 : 670 |** |
> 24 : 601 |* |
> 26 : 562 |* |
> 28 : 525 |* |
> 30 : 446 |* |
> 32 : 378 |* |
> 34 : 337 |* |
> 36 : 317 |* |
> 38 : 314 |* |
> 40 : 298 | |
>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.co.jp>
> ---
> net/unix/af_unix.c | 21 +++++++++++----------
> 1 file changed, 11 insertions(+), 10 deletions(-)
>
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 643f0358bf7a..55d570b23475 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -1075,8 +1075,7 @@ static int unix_autobind(struct sock *sk)
> unsigned int new_hash, old_hash = sk->sk_hash;
> struct unix_sock *u = unix_sk(sk);
> struct unix_address *addr;
> - unsigned int retries = 0;
> - static u32 ordernum = 1;
> + u32 initnum, ordernum;
> int err;
>
> err = mutex_lock_interruptible(&u->bindlock);
> @@ -1091,31 +1090,33 @@ static int unix_autobind(struct sock *sk)
> if (!addr)
> goto out;
>
> + addr->len = offsetof(struct sockaddr_un, sun_path) + 6;
> addr->name->sun_family = AF_UNIX;
> refcount_set(&addr->refcnt, 1);
>
> + initnum = ordernum = prandom_u32();
> retry:
> - addr->len = sprintf(addr->name->sun_path + 1, "%05x", ordernum) +
> - offsetof(struct sockaddr_un, sun_path) + 1;
> + ordernum = (ordernum + 1) & 0xFFFFF;
> + sprintf(addr->name->sun_path + 1, "%05x", ordernum);
>
> new_hash = unix_abstract_hash(addr->name, addr->len, sk->sk_type);
> unix_table_double_lock(old_hash, new_hash);
> - ordernum = (ordernum+1)&0xFFFFF;
>
> if (__unix_find_socket_byname(sock_net(sk), addr->name, addr->len, new_hash)) {
> unix_table_double_unlock(old_hash, new_hash);
>
> - /*
> - * __unix_find_socket_byname() may take long time if many names
> + /* __unix_find_socket_byname() may take long time if many names
> * are already in use.
> */
> cond_resched();
> - /* Give up if all names seems to be in use. */
> - if (retries++ == 0xFFFFF) {
> +
> + if (ordernum == initnum) {
Infinite loop alert, in the likely case initnum >= 2^16
> + /* Give up if all names seems to be in use. */
> err = -ENOSPC;
> - kfree(addr);
> + unix_release_addr(addr);
> goto out;
> }
> +
> goto retry;
> }
>
>
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