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()").
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| @@ -1821,6 +1821,18 @@ static inline __u32 skb_queue_len(const struct sk_buff_head *list_) · 1820 unmodified lines | |||
| 1821 | return list_->qlen; | 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 | * __skb_queue_head_init - initialize non-spinlock portions of sk_buff_head | 1837 | * __skb_queue_head_init - initialize non-spinlock portions of sk_buff_head |
| 1826 | * @list: queue to initialize | 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 | struct sk_buff *next, *prev; | 2039 | struct sk_buff *next, *prev; |
| 2028 | 2040 | ||
| 2029 | list->qlen--; | 2041 | WRITE_ONCE(list->qlen, list->qlen - 1); |
| 2030 | next = skb->next; | 2042 | next = skb->next; |
| 2031 | prev = skb->prev; | 2043 | prev = skb->prev; |
| 2032 | skb->next = skb->prev = NULL; | 2044 | skb->next = skb->prev = NULL; |