On Tue, 2020-10-27 at 17:58 +0100, Bartosz Golaszewski wrote:

On Tue, Oct 27, 2020 at 5:50 PM Joe Perches [off-list ref] wrote:

quoted

On Tue, 2020-10-27 at 11:28 -0400, Michael S. Tsirkin wrote:

quoted

On Tue, Oct 27, 2020 at 01:17:20PM +0100, Bartosz Golaszewski wrote:

quoted

From: Bartosz Golaszewski <redacted>

Use the helper that checks for overflows internally instead of manually
calculating the size of the new array.

Signed-off-by: Bartosz Golaszewski <redacted>

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.