sk_buff.qlen can be accessed concurrently as noticed by KCSAN, BUG: KCSAN: data-race in __skb_try_recv_from_queue / unix_dgram_sendmsg read to 0xffff8a1b1d8a81c0 of 4 bytes by task 5371 on cpu 96: unix_dgram_sendmsg+0x9a9/0xb70 include/linux/skbuff.h:1821 net/unix/af_unix.c:1761 ____sys_sendmsg+0x33e/0x370 ___sys_sendmsg+0xa6/0xf0 __sys_sendmsg+0x69/0xf0 __x64_sys_sendmsg+0x51/0x70 do_syscall_64+0x91/0xb47 entry_SYSCALL_64_after_hwframe+0x49/0xbe write to 0xffff8a1b1d8a81c0 of 4 bytes by task 1 on cpu 99: __skb_try_recv_from_queue+0x327/0x410 include/linux/skbuff.h:2029 __skb_try_recv_datagram+0xbe/0x220 unix_dgram_recvmsg+0xee/0x850 ____sys_recvmsg+0x1fb/0x210 ___sys_recvmsg+0xa2/0xf0 __sys_recvmsg+0x66/0xf0 __x64_sys_recvmsg+0x51/0x70 do_syscall_64+0x91/0xb47 entry_SYSCALL_64_after_hwframe+0x49/0xbe Since only the read is operating as lockless, it could introduce a logic bug in unix_recvq_full() due to the load tearing. Fix it by adding a lockless variant of skb_queue_len() and unix_recvq_full() where READ_ONCE() is on the read while WRITE_ONCE() is on the write similar to the commit d7d16a89350a ("net: add skb_queue_empty_lockless()").
Unified · Split
| @@ -1821,6 +1821,18 @@ static inline __u32 skb_queue_len(const struct sk_buff_head *list_) · 1820 unmodified lines | ||
| 1821 | 1821 | return list_->qlen; |
| 1822 | 1822 | } |
| 1823 | 1823 | |
| 1824 | +/** | |
| 1825 | + * skb_queue_len_lockless - get queue length | |
| 1826 | + * @list_: list to measure | |
| 1827 | + * | |
| 1828 | + * Return the length of an &sk_buff queue. | |
| 1829 | + * This variant can be used in lockless contexts. | |
| 1830 | + */ | |
| 1831 | +static inline __u32 skb_queue_len_lockless(const struct sk_buff_head *list_) | |
| 1832 | +{ | |
| 1833 | + return READ_ONCE(list_->qlen); | |
| 1834 | +} | |
| 1835 | + | |
| 1824 | 1836 | /** |
| 1825 | 1837 | * __skb_queue_head_init - initialize non-spinlock portions of sk_buff_head |
| 1826 | 1838 | * @list: queue to initialize |
| @@ -2026,7 +2038,7 @@ static inline void __skb_unlink(struct sk_buff *skb, struct sk_buff_head *list) · 199 unmodified lines | ||
| 2026 | 2038 | { |
| 2027 | 2039 | struct sk_buff *next, *prev; |
| 2028 | 2040 | |
| 2029 | − list->qlen--; | |
| 2041 | + WRITE_ONCE(list->qlen, list->qlen - 1); | |
| 2030 | 2042 | next = skb->next; |
| 2031 | 2043 | prev = skb->prev; |
| 2032 | 2044 | skb->next = skb->prev = NULL; |
| @@ -189,11 +189,17 @@ static inline int unix_may_send(struct sock *sk, struct sock *osk) · 188 unmodified lines | ||
| 189 | 189 | return unix_peer(osk) == NULL || unix_our_peer(sk, osk); |
| 190 | 190 | } |
| 191 | 191 | |
| 192 | −static inline int unix_recvq_full(struct sock const *sk) | |
| 192 | +static inline int unix_recvq_full(const struct sock *sk) | |
| 193 | 193 | { |
| 194 | 194 | return skb_queue_len(&sk->sk_receive_queue) > sk->sk_max_ack_backlog; |
| 195 | 195 | } |
| 196 | 196 | |
| 197 | +static inline int unix_recvq_full_lockless(const struct sock *sk) | |
| 198 | +{ | |
| 199 | + return skb_queue_len_lockless(&sk->sk_receive_queue) > | |
| 200 | + READ_ONCE(sk->sk_max_ack_backlog); | |
| 201 | +} | |
| 202 | + | |
| 197 | 203 | struct sock *unix_peer_get(struct sock *s) |
| 198 | 204 | { |
| 199 | 205 | struct sock *peer; |
| @@ -1758,7 +1764,8 @@ restart_locked: · 1558 unmodified lines | ||
| 1758 | 1764 | * - unix_peer(sk) == sk by time of get but disconnected before lock |
| 1759 | 1765 | */ |
| 1760 | 1766 | if (other != sk && |
| 1761 | − unlikely(unix_peer(other) != sk && unix_recvq_full(other))) { | |
| 1767 | + unlikely(unix_peer(other) != sk && | |
| 1768 | + unix_recvq_full_lockless(other))) { | |
| 1762 | 1769 | if (timeo) { |
| 1763 | 1770 | timeo = unix_wait_for_peer(other, timeo); |
| 1764 | 1771 | |