# \[PATCH net v4 1/2\] tcp: clamp return value of tcp\_wmem\_schedule() to non-negative - From: Octavian Purdila \ - Date: 2026-10-26 03:22:20 - List: netdev - Page: https://ratatoskr.run/netdev/2026/10/17801896 - Thread: https://ratatoskr.run/netdev/2026/10/17801895/t.md ```text When sk_wmem_schedule(sk, copy) fails under TCP memory pressure and sk->sk_wmem_queued already reaches or exceeds sysctl_tcp_wmem[0], tcp_wmem_schedule() returns min(copy, sk->sk_forward_alloc). However, sk->sk_forward_alloc is a signed int that can transiently become negative when tcp_collapse() re-packs partially read or high payload-to-truesize SKBs in the receive queue and charges newly allocated SKBs via sk_mem_charge() before freeing the original SKB. If tcp_wmem_schedule() returns a negative value, tcp_sendmsg_locked() passes that negative copy length to skb_copy_to_page_nocache() -> copy_from_iter_full() -> check_copy_size(), where it is converted to an unsigned size_t > INT_MAX, triggering a WARN_ON_ONCE(bytes > INT_MAX) in production and returning -EFAULT instead of waiting for memory or returning -EAGAIN: WARNING: CPU: 1 PID: 939331 at include/linux/thread_info.h:249 tcp_sendmsg+0x10c1/0x1530 Call Trace: __sys_sendmsg+0x292/0x490 do_syscall_64+0x88/0x340 entry_SYSCALL_64_after_hwframe+0x76/0x7e Clamp the return value of tcp_wmem_schedule() to [0, copy] so callers treat negative sk_forward_alloc as 0 available bytes. Fixes: 849b425cd091 ("tcp: fix possible freeze in tx path under memory pressure") Assisted-by: LLM Reviewed-by: Kuniyuki Iwashima Reviewed-by: Eric Dumazet Signed-off-by: Octavian Purdila --- net/ipv4/tcp.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index 87ef6d5cbfeba..0e062fcd94a6c 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -1037,7 +1037,7 @@ int tcp_wmem_schedule(struct sock *sk, int copy) left = READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_wmem[0]) - sk->sk_wmem_queued; if (left > 0) sk_forced_mem_schedule(sk, min(left, copy)); - return min(copy, sk->sk_forward_alloc); + return clamp(sk->sk_forward_alloc, 0, copy); } void tcp_free_fastopen_req(struct tcp_sock *tp) -- 2.56.0.257.gf7497c7d27-goog ```