Re: [PATCH] virtio_blk: fix snprintf truncation compiler warning

[Stefan Hajnoczi]

On Tue, 5 Dec 2023 at 08:54, David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Stefan Hajnoczi
> > Sent: 04 December 2023 14:08
> >
> > Commit 4e0400525691 ("virtio-blk: support polling I/O") triggers the
> > following gcc 13 W=1 warnings:
> >
> > drivers/block/virtio_blk.c: In function ‘init_vq’:
> > drivers/block/virtio_blk.c:1077:68: warning: ‘%d’ directive output may be truncated writing between 1
> > and 11 bytes into a region of size 7 [-Wformat-truncation=]
> >  1077 |                 snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> >       |                                                                    ^~
> > drivers/block/virtio_blk.c:1077:58: note: directive argument in the range [-2147483648, 65534]
> >  1077 |                 snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> >       |                                                          ^~~~~~~~~~~~~
> > drivers/block/virtio_blk.c:1077:17: note: ‘snprintf’ output between 11 and 21 bytes into a destination
> > of size 16
> >  1077 |                 snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> >       |                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > This is a false positive because the lower bound -2147483648 is
> > incorrect. The true range of i is [0, num_vqs - 1] where 0 < num_vqs <
> > 65536.
> >
> > The code mixes int, unsigned short, and unsigned int types in addition
> > to using "%d" for an unsigned value. Use unsigned short and "%u"
> > consistently to solve the compiler warning.
> >
> > Cc: Suwan Kim <suwan.kim027@xxxxxxxxx>
> > Reported-by: kernel test robot <lkp@xxxxxxxxx>
> > Closes: https://lore.kernel.org/oe-kbuild-all/202312041509.DIyvEt9h-lkp@xxxxxxxxx/
> > Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> > ---
> >  drivers/block/virtio_blk.c | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> > index d53d6aa8ee69..47556d8ccc32 100644
> > --- a/drivers/block/virtio_blk.c
> > +++ b/drivers/block/virtio_blk.c
> > @@ -1019,12 +1019,12 @@ static void virtblk_config_changed(struct virtio_device *vdev)
> >  static int init_vq(struct virtio_blk *vblk)
> >  {
> >       int err;
> > -     int i;
> > +     unsigned short i;
> >       vq_callback_t **callbacks;
> >       const char **names;
> >       struct virtqueue **vqs;
> >       unsigned short num_vqs;
> > -     unsigned int num_poll_vqs;
> > +     unsigned short num_poll_vqs;
> >       struct virtio_device *vdev = vblk->vdev;
> >       struct irq_affinity desc = { 0, };
> >
> > @@ -1068,13 +1068,13 @@ static int init_vq(struct virtio_blk *vblk)
> >
> >       for (i = 0; i < num_vqs - num_poll_vqs; i++) {
>
> Ugg doing arithmetic on char/short is likely to generate horrid
> code (especially on non-x86).
> Hint, there will be explicit masking and/or sign/zero extension.
>
> Even the array index might add extra code (although there'll be
> an explicit sign extend to 64bit with the current code).
>
> There really ought to be a better way to make gcc STFU.
>
> In this case 'unsigned int i' might be enough since gcc seems
> to have a small enough upper bound.

Sounds good, I'll send a v2 that uses unsigned int. The core virtio
code uses unsigned int for virtqueue indices too.

Stefan

>
>         David
>
>
> >               callbacks[i] = virtblk_done;
> > -             snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%d", i);
> > +             snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req.%u", i);
> >               names[i] = vblk->vqs[i].name;
> >       }
> >
> >       for (; i < num_vqs; i++) {
> >               callbacks[i] = NULL;
> > -             snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%d", i);
> > +             snprintf(vblk->vqs[i].name, VQ_NAME_LEN, "req_poll.%u", i);
> >               names[i] = vblk->vqs[i].name;
> >       }
> >
> > --
> > 2.43.0
>
> -
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