linux-kernel - Re: [PATCH] mm/damon/sysfs-schemes: using kmalloc_array() and size

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Message-ID: <501ea9b1-017b-4517-8de4-7056803e7127@wanadoo.fr>
Date: Tue, 22 Apr 2025 20:50:01 +0200
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: SeongJae Park <sj@...nel.org>, Dan Carpenter <dan.carpenter@...aro.org>
Cc: Su Hui <suhui@...china.com>, akpm@...ux-foundation.org,
 damon@...ts.linux.dev, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 kernel-janitors@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] mm/damon/sysfs-schemes: using kmalloc_array() and
 size_add()

Le 22/04/2025 à 20:23, SeongJae Park a écrit :
> On Tue, 22 Apr 2025 13:44:39 +0300 Dan Carpenter <dan.carpenter@...aro.org> wrote:
> 
>> On Tue, Apr 22, 2025 at 01:38:05PM +0300, Dan Carpenter wrote:
>>> On Mon, Apr 21, 2025 at 02:24:24PM +0800, Su Hui wrote:
>>>> It's safer to using kmalloc_array() and size_add() because it can
>>>> prevent possible overflow problem.
>>>>
>>>> Signed-off-by: Su Hui <suhui@...china.com>
> [...]
>>>> --- a/mm/damon/sysfs-schemes.c
>>>> +++ b/mm/damon/sysfs-schemes.c
>>>> @@ -465,7 +465,8 @@ static ssize_t memcg_path_store(struct kobject *kobj,
>>>>   {
>>>>   	struct damon_sysfs_scheme_filter *filter = container_of(kobj,
>>>>   			struct damon_sysfs_scheme_filter, kobj);
>>>> -	char *path = kmalloc(sizeof(*path) * (count + 1), GFP_KERNEL);
>>>> +	char *path = kmalloc_array(size_add(count, 1), sizeof(*path),
>>>> +				   GFP_KERNEL);
>>>
>>> Count is clamped in rw_verify_area().
>>>
>>> Smatch does a kind of ugly hack to handle rw_verify_area() which is that
>>> it says neither the count nor the pos can be more than 1G.  And obviously
>>> files which are larger than 2GB exist but pretending they don't silences
>>> all these integer overflow warnings.
>>>
>>
>> Actually rw_verify_area() ensures that "pos + count" can't overflow.  But
>> here we are multiplying.  Fortunately, we are multiplying by 1 so that's
>> safe and also count can't be larger than PAGE_SIZE here which is safe as
>> well.
> 
> Thank you for adding these details, Dan.  I understand the size_add() change
> can make warnings slience, though it is not really fixing a real bug.  So I
> believe there is no action item to make a change to this patch.  Maybe making
> the commit message more clarified can be helpful, though?
> 
> Please let me know if I'm misunderstanding your point and/or you want some
> changes.

As sizeof(*path) = 1, maybe, just change it to:
	char *path = kmalloc(count + 1, GFP_KERNEL);

CJ
> 
> 
> Thanks,
> SJ
> 
>>
>> regards,
>> dan carpenter
> 
>