linux-kernel - Re: [PATCH vfs.fixes] eventpoll: Prevent hang in epoll

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Message-ID: <a5t4wx72g3mnyz6h7ko5joairx6zjycdt5jkfdfgmvbrfwhlus@jtsewowpwsxd>
Date: Tue, 29 Apr 2025 20:19:03 +0200
From: Jan Kara <jack@...e.cz>
To: Joe Damato <jdamato@...tly.com>
Cc: linux-fsdevel@...r.kernel.org, Christian Brauner <brauner@...nel.org>, 
	Mike Pagano <mpagano@...too.org>, Carlos Llamas <cmllamas@...gle.com>, 
	Alexander Viro <viro@...iv.linux.org.uk>, Jan Kara <jack@...e.cz>, 
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH vfs.fixes] eventpoll: Prevent hang in epoll_wait

On Tue 29-04-25 15:34:19, Joe Damato wrote:
> In commit 0a65bc27bd64 ("eventpoll: Set epoll timeout if it's in the
> future"), a bug was introduced causing the loop in ep_poll to hang under
> certain circumstances.
> 
> When the timeout is non-NULL and ep_schedule_timeout returns false, the
> flag timed_out was not set to true. This causes a hang.
> 
> Adjust the logic and set timed_out, if needed, fixing the original code.
> 
> Reported-by: Christian Brauner <brauner@...nel.org>
> Closes: https://lore.kernel.org/linux-fsdevel/20250426-haben-redeverbot-0b58878ac722@brauner/
> Reported-by: Mike Pagano <mpagano@...too.org>
> Closes: https://bugs.gentoo.org/954806
> Reported-by: Carlos Llamas <cmllamas@...gle.com>
> Closes: https://lore.kernel.org/linux-fsdevel/aBAB_4gQ6O_haAjp@google.com/
> Fixes: 0a65bc27bd64 ("eventpoll: Set epoll timeout if it's in the future")
> Tested-by: Carlos Llamas <cmllamas@...gle.com>
> Signed-off-by: Joe Damato <jdamato@...tly.com>

Looks good. Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  fs/eventpoll.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/eventpoll.c b/fs/eventpoll.c
> index 4bc264b854c4..1a5d1147f082 100644
> --- a/fs/eventpoll.c
> +++ b/fs/eventpoll.c
> @@ -2111,7 +2111,9 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events,
>  
>  		write_unlock_irq(&ep->lock);
>  
> -		if (!eavail && ep_schedule_timeout(to))
> +		if (!ep_schedule_timeout(to))
> +			timed_out = 1;
> +		else if (!eavail)
>  			timed_out = !schedule_hrtimeout_range(to, slack,
>  							      HRTIMER_MODE_ABS);
>  		__set_current_state(TASK_RUNNING);
> 
> base-commit: f520bed25d17bb31c2d2d72b0a785b593a4e3179
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR