Re: [PATCH net] tcp: make probe0 timer handle expired user timeout

[Jakub Kicinski <kuba@kernel.org>]

On Mon, 13 Apr 2026 18:36:34 -0700 Altan Hacigumus wrote:
> tcp_clamp_probe0_to_user_timeout() computes remaining time in jiffies
> using subtraction with an unsigned lvalue.  If elapsed probing time
> already exceeds the configured TCP_USER_TIMEOUT, the subtraction
> underflows and yields a large value.
> 
> Handle this expiration case similarly to tcp_clamp_rto_to_user_timeout().
> 
> Fixes: 344db93ae3ee ("tcp: make TCP_USER_TIMEOUT accurate for zero window probes")
> Signed-off-by: Altan Hacigumus <ahacigu.linux@gmail.com>

Hi Eric, Neal, does this makes sense?

> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index 5a14a53a3c9e..4a43356a4e06 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -50,7 +50,8 @@ static u32 tcp_clamp_rto_to_user_timeout(const struct sock *sk)
>  u32 tcp_clamp_probe0_to_user_timeout(const struct sock *sk, u32 when)
>  {
>  	const struct inet_connection_sock *icsk = inet_csk(sk);
> -	u32 remaining, user_timeout;
> +	u32 user_timeout;
> +	s32 remaining;
>  	s32 elapsed;
>  
>  	user_timeout = READ_ONCE(icsk->icsk_user_timeout);
> @@ -61,6 +62,8 @@ u32 tcp_clamp_probe0_to_user_timeout(const struct sock *sk, u32 when)
>  	if (unlikely(elapsed < 0))
>  		elapsed = 0;
>  	remaining = msecs_to_jiffies(user_timeout) - elapsed;
> +	if (remaining <= 0)
> +		return 1;
>  	remaining = max_t(u32, remaining, TCP_TIMEOUT_MIN);
>  
>  	return min_t(u32, remaining, when);