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

[SeongJae Park <sj@kernel.org>]

On Tue, 22 Apr 2025 13:44:39 +0300 Dan Carpenter <dan.carpenter@linaro.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@nfschina.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.


Thanks,
SJ

> 
> regards,
> dan carpenter