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

[Dan Carpenter <dan.carpenter@linaro.org>]

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>
> > ---
> >  mm/damon/sysfs-schemes.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/damon/sysfs-schemes.c b/mm/damon/sysfs-schemes.c
> > index 23b562df0839..79220aba436f 100644
> > --- 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.

regards,
dan carpenter