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Subject: Re: [PATCH] tty: serial_core: Fix uart_state refcnt leak when the
 port startup
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Xiyu Yang <xiyuyang19@fudan.edu.cn>
Cc:     linux-serial@vger.kernel.org, linux-kernel@vger.kernel.org,
        yuanxzhang@fudan.edu.cn, kjlu@umn.edu,
        Xin Tan <tanxin.ctf@gmail.com>
References: <1592052738-95202-1-git-send-email-xiyuyang19@fudan.edu.cn>
 <20200624093407.GB1751086@kroah.com>
From:   Jiri Slaby <jirislaby@kernel.org>
Message-ID: <954840f0-2078-fe0f-1e52-d985a4997564@kernel.org>
Date:   Wed, 24 Jun 2020 11:42:59 +0200
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On 24. 06. 20, 11:34, Greg Kroah-Hartman wrote:
> On Sat, Jun 13, 2020 at 08:52:18PM +0800, Xiyu Yang wrote:
>> uart_port_startup() invokes uart_port_lock(), which returns a reference
>> of the uart_port object if increases the refcount of the uart_state
>> object successfully or returns NULL if fails.
>>
>> However, uart_port_startup() don't take the return value of
>> uart_port_lock() as the new uart_port object to "uport" and use the old
>> "uport" instead to balance refcount in uart_port_unlock(), which may
>> cause a redundant decrement of refcount occurred when the new "uport"
>> equals to NULL and then cause a potential memory leak.
>>
>> Fix this issue by update the "uport" object to the return value of
>> uart_port_lock() when invoking uart_port_lock().
>>
>> Signed-off-by: Xiyu Yang <xiyuyang19@fudan.edu.cn>
>> Signed-off-by: Xin Tan <tanxin.ctf@gmail.com>
>> ---
>>  drivers/tty/serial/serial_core.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
>> index 57840cf90388..968fd619aec0 100644
>> --- a/drivers/tty/serial/serial_core.c
>> +++ b/drivers/tty/serial/serial_core.c
>> @@ -205,7 +205,7 @@ static int uart_port_startup(struct tty_struct *tty, struct uart_state *state,
>>  	if (!page)
>>  		return -ENOMEM;
>>  
>> -	uart_port_lock(state, flags);
>> +	uport = uart_port_lock(state, flags);
> 
> How is this a different pointer than you originally had?

Was this patch sent twice? As I had very same questions on the other
one, but never received a feedback:
https://lore.kernel.org/linux-serial/bf6c1e7b-3dc6-aba6-955a-fee351a6d800@suse.com/


Oh, wait: this is uart_port_startup, I commented on the uart_shutdown
one. But whatever, I would scratch both of them.

> And if it is a different pointer, shouldn't you be calling this function
> and using the pointer much earlier in the function instead of just here?
> 
> Can you trigger a problem that this patch solves?  If so, how?

thanks,
-- 
js
suse labs