linux-kernel - Re: [PATCH] TTY: n_gsm, fix false positive WARN

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Date:	Wed, 25 Nov 2015 14:32:51 +0800
From:	xinhui <xinhui@...ux.vnet.ibm.com>
To:	Jiri Slaby <jslaby@...e.cz>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:	dvyukov@...gle.com, linux-kernel@...r.kernel.org,
	Alan Cox <alan@...ux.intel.com>,
	stable <stable@...r.kernel.org>
Subject: Re: [PATCH] TTY: n_gsm, fix false positive WARN_ON

hi, Jiri
	This warning should blame on commit 5a640967 ("tty/n_gsm.c: fix a memory leak in gsmld_open()"). When gsm driver failed to activate one mux,there is memory leak. So I call this ->cleanup() to do the cleanup work. Seems I did not consider all cases.

I have one confusion. As there is field gsm->num to store the index of gsm_mux[]. so in gsm_cleanup_mux(), why we still use for-loop to find this mux?

In error handle path, for example, the call trace in this patch, as we failed to activate it and the
gsm->num is invalid(and the value is 0). we can just modify the codes like below:

if(gsm_mux[gsm->num] == gsm)
....other work
else
	return;

I think it would work, and the logic is correct. Or I just miss something important?

thanks
xinhui

On 2015/11/25 00:54, Jiri Slaby wrote:
> Dmitry reported, that the current cleanup code in n_gsm can trigger a
> warning:
> WARNING: CPU: 2 PID: 24238 at drivers/tty/n_gsm.c:2048 gsm_cleanup_mux+0x166/0x6b0()
> ...
> Call Trace:
> ...
>   [<ffffffff81247ab9>] warn_slowpath_null+0x29/0x30 kernel/panic.c:490
>   [<ffffffff828d0456>] gsm_cleanup_mux+0x166/0x6b0 drivers/tty/n_gsm.c:2048
>   [<ffffffff828d4d87>] gsmld_open+0x5b7/0x7a0 drivers/tty/n_gsm.c:2386
>   [<ffffffff828b9078>] tty_ldisc_open.isra.2+0x78/0xd0 drivers/tty/tty_ldisc.c:447
>   [<ffffffff828b973a>] tty_set_ldisc+0x1ca/0xa70 drivers/tty/tty_ldisc.c:567
>   [<     inline     >] tiocsetd drivers/tty/tty_io.c:2650
>   [<ffffffff828a14ea>] tty_ioctl+0xb2a/0x2140 drivers/tty/tty_io.c:2883
> ...
>
> But this is a legal path when open fails to find a space in the
> gsm_mux array and tries to clean up. So make it a standard test
> instead of a warning.
>
> Reported-by: "Dmitry Vyukov" <dvyukov@...gle.com>
> Cc: Alan Cox <alan@...ux.intel.com>
> Link: http://lkml.kernel.org/r/CACT4Y+bHQbAB68VFi7Romcs-Z9ZW3kQRvcq+BvHH1oa5NcAdLA@mail.gmail.com
> Fixes: e1eaea46bb40 ("tty: n_gsm line discipline")
> Cc: stable <stable@...r.kernel.org>
> Signed-off-by: Jiri Slaby <jslaby@...e.cz>
> ---
>   drivers/tty/n_gsm.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/n_gsm.c b/drivers/tty/n_gsm.c
> index c3fe026d3168..9aff37186246 100644
> --- a/drivers/tty/n_gsm.c
> +++ b/drivers/tty/n_gsm.c
> @@ -2045,7 +2045,9 @@ static void gsm_cleanup_mux(struct gsm_mux *gsm)
>   		}
>   	}
>   	spin_unlock(&gsm_mux_lock);
> -	WARN_ON(i == MAX_MUX);
> +	/* open failed before registering => nothing to do */
> +	if (i == MAX_MUX)
> +		return;
>
>   	/* In theory disconnecting DLCI 0 is sufficient but for some
>   	   modems this is apparently not the case. */
>

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