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Subject: Re: [PATCH 3/8] vhost: vringh: use krealloc_array()
From:   Joe Perches <joe@perches.com>
To:     Bartosz Golaszewski <bgolaszewski@baylibre.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Bartosz Golaszewski <brgl@bgdev.pl>,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
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Date:   Tue, 27 Oct 2020 10:08:02 -0700
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On Tue, 2020-10-27 at 17:58 +0100, Bartosz Golaszewski wrote:
> On Tue, Oct 27, 2020 at 5:50 PM Joe Perches <joe@perches.com> wrote:
> > 
> > On Tue, 2020-10-27 at 11:28 -0400, Michael S. Tsirkin wrote:
> > > On Tue, Oct 27, 2020 at 01:17:20PM +0100, Bartosz Golaszewski wrote:
> > > > From: Bartosz Golaszewski <bgolaszewski@baylibre.com>
> > > > 
> > > > Use the helper that checks for overflows internally instead of manually
> > > > calculating the size of the new array.
> > > > 
> > > > Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
> > > 
> > > No problem with the patch, it does introduce some symmetry in the code.
> > 
> > Perhaps more symmetry by using kmemdup
> > ---
> > ?drivers/vhost/vringh.c | 23 ++++++++++-------------
> > ?1 file changed, 10 insertions(+), 13 deletions(-)
> > 
> > diff --git a/drivers/vhost/vringh.c b/drivers/vhost/vringh.c
> > index 8bd8b403f087..99222a3651cd 100644
> > --- a/drivers/vhost/vringh.c
> > +++ b/drivers/vhost/vringh.c
> > @@ -191,26 +191,23 @@ static int move_to_indirect(const struct vringh *vrh,
> > ?static int resize_iovec(struct vringh_kiov *iov, gfp_t gfp)
> > ?{
> > ????????struct kvec *new;
> > -       unsigned int flag, new_num = (iov->max_num & ~VRINGH_IOV_ALLOCATED) * 2;
> > +       size_t new_num = (iov->max_num & ~VRINGH_IOV_ALLOCATED) * 2;
> > +       size_t size;
> > 
> > ????????if (new_num < 8)
> > ????????????????new_num = 8;
> > 
> > -       flag = (iov->max_num & VRINGH_IOV_ALLOCATED);
> > -       if (flag)
> > -               new = krealloc(iov->iov, new_num * sizeof(struct iovec), gfp);
> > -       else {
> > -               new = kmalloc_array(new_num, sizeof(struct iovec), gfp);
> > -               if (new) {
> > -                       memcpy(new, iov->iov,
> > -                              iov->max_num * sizeof(struct iovec));
> > -                       flag = VRINGH_IOV_ALLOCATED;
> > -               }
> > -       }
> > +       if (unlikely(check_mul_overflow(new_num, sizeof(struct iovec), &size)))
> > +               return -ENOMEM;
> > +
> 
> The whole point of using helpers such as kmalloc_array() is not doing
> these checks manually.

Tradeoffs for in readability for overflow and not mistyping or doing
the multiplication of iov->max_num * sizeof(struct iovec) twice.

Just fyi:

the realloc doesn't do a multiplication overflow test as written so the
suggestion is slightly more resistant to defect.