linux-kernel - Re: [PATCH] line6: add midibuf init failure handling in line6_init

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Date: Sat, 18 May 2024 15:00:41 +0900
From: Yunseong Kim <yskelg@...il.com>
To: Takashi Iwai <tiwai@...e.de>
Cc: Jaroslav Kysela <perex@...ex.cz>, skhan@...uxfoundation.org,
 Bjorn Helgaas <helgaas@...nel.org>,
 linux-kernel-mentees@...ts.linuxfoundation.org,
 Austin Kim <austindh.kim@...il.com>, shjy180909@...il.com,
 linux-sound@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] line6: add midibuf init failure handling in
 line6_init_midi()



On 5/18/24 2:51 오전, Takashi Iwai wrote:
> On Fri, 17 May 2024 19:10:44 +0200,
> Yunseong Kim wrote:
>>
>> On 5/17/24 5:32 오후, Takashi Iwai wrote:
>>> The allocated object is already freed by snd_line6_midi_free() that is
>>> called via rawmidi private_free at its destruction.  So your change
>>> would lead to a double-free.
>>>
>>>
>>> thanks,
>>>
>>> Takashi
>>
>> Thanks for the review Takashi.
>>
>> I have one question:
>>
>> line6_midibuf_destroy() have no NULL check for the memory to free.
>>
>> If line6midi->midibuf_in is in this->buf = NULL from
>> line6_midibuf_init() with memory allocation failed,
>> won't the free(NULL) when accessed by snd_line6_midi_free() to
>> line6_midibuf_destroy()?
> 
> free(NULL) is fine, it's defined to be noop.
> Many codes just call free() unconditionally for that, as it reduces
> the code size.
> 
> 
> thanks,
> 
> Takashi

Thak you Takashi, I've been studying the kfree behavior in detail since
you mentioned it, so thanks for advicing the details.

Warm Regards,
Yunseong Kim

>>
>>>> ---
>>>>  sound/usb/line6/midi.c | 9 +++++++--
>>>>  1 file changed, 7 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/sound/usb/line6/midi.c b/sound/usb/line6/midi.c
>>>> index 0838632c788e..abcf58f46673 100644
>>>> --- a/sound/usb/line6/midi.c
>>>> +++ b/sound/usb/line6/midi.c
>>>> @@ -283,13 +283,18 @@ int line6_init_midi(struct usb_line6 *line6)
>>>>  
>>>>  	err = line6_midibuf_init(&line6midi->midibuf_in, MIDI_BUFFER_SIZE, 0);
>>>>  	if (err < 0)
>>>> -		return err;
>>>> +		goto error;
>>>>  
>>>>  	err = line6_midibuf_init(&line6midi->midibuf_out, MIDI_BUFFER_SIZE, 1);
>>>>  	if (err < 0)
>>>> -		return err;
>>>> +		goto error;
>>>>  
>>>>  	line6->line6midi = line6midi;
>>>>  	return 0;
>>>> +
>>>> +error:
>>>> +	kfree(line6midi);
>>>> +	return err;
>>>> +
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(line6_init_midi);
>>>> -- 
>>>> 2.34.1
>>>>