linux-kernel - Re: [PATCH 01/10] firewire-net: Use kmalloc_array() in fwnet_broadcast

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Date:   Sat, 24 Sep 2016 17:29:00 +0200
From:   SF Markus Elfring <elfring@...rs.sourceforge.net>
To:     Stefan Richter <stefanr@...6.in-berlin.de>
Cc:     linux1394-devel@...ts.sourceforge.net,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org,
        Julia Lawall <julia.lawall@...6.fr>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH 01/10] firewire-net: Use kmalloc_array() in
 fwnet_broadcast_start()

>> @@ -1103,8 +1103,7 @@ static int fwnet_broadcast_start(struct fwnet_device *dev)
>>  
>>  	max_receive = 1U << (dev->card->max_receive + 1);
>>  	num_packets = (FWNET_ISO_PAGE_COUNT * PAGE_SIZE) / max_receive;
>> -
>> -	ptrptr = kmalloc(sizeof(void *) * num_packets, GFP_KERNEL);
>> +	ptrptr = kmalloc_array(num_packets, sizeof(*ptrptr), GFP_KERNEL);
>>  	if (!ptrptr) {
>>  		retval = -ENOMEM;
>>  		goto failed;
> 
> Coccinelle enabled you to determine that kmalloc_array /could/ be used here.

A script for the semantic patch language pointed hundreds of source files out
with such software update opportunities.

> But whether it /should/ be used here is another question, and it is
> not addressed in your changelog.

I can expand the corresponding description when it will be desired.

> (You state that there is an "issue" but do not explain.)

Do you prefer an other wording for such an update candidate?

> kmalloc_array is a kmalloc wrapper which adds an inline check for integer
> overflow.  So, can sizeof(void *) * num_packets ever overflow size_t?
> 
> If yes,

Is there a probability that the calculated number of packets will become
too big for the preferred system limits anyhow?

> 	do we want a runtime check here (which kmalloc_array provides),

Did you notice the information from the commit "mm: faster kmalloc_array(), kcalloc()"
(from 2016-07-26) already?
https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git/commit/?id=91c6a05f72a996bee5133e76374ab3ad7d3b9b72

> 	or do we want a compile-time check?

I guess that some software developers and subsystem maintainers are looking
for a bit more clarification around involved design dependencies.

> If no,
> 	then the remaining benefit of the patch is that it is more obvious
> 	to the reader that dev->broadcast_rcv_buffer_ptrs is an array,

How do you value such a kind of source code annotation?

> 	but possibly at the cost of superfluous code.

How do you think about to care for a bit more consistent use of Linux programming interfaces?

> 	Is gcc's optimizer able to resolve kmalloc_array's check at compile time
> 	as always false, such that the superfluous code is eliminated as dead code?

Which versions of compiler implementations would you like to check further?

> I believe I know answers to this but prefer to hear what you as the patch
> author think about it.

I presented another update suggestion also for this software module as a result
from a general source code search pattern.
The corresponding change acceptance varies and is evolving as usual.

Regards,
Markus