Re: [PATCH v2] selftests: Open /dev/udmabuf O_RDONLY

[T.J. Mercier]

On Thu, Jul 2, 2026 at 1:53 PM Bobby Eshleman <bobbyeshleman@xxxxxxxxx> wrote:
>
> On Wed, Jul 01, 2026 at 12:22:08PM -0700, T.J. Mercier wrote:
> > Write permissions on the /dev/udmabuf device file are not required to
> > issue ioctls and allocate udmabufs. Applications should be opening this
> > file as O_RDONLY. The BPF dmabuf_iter selftest already does this. [1]
> >
> > Users are pointing to these selftests as examples of how use udmabuf,
> > and encountering permission errors on systems where write permissions
> > are not available on /dev/udmabuf. Apply the principle of least
> > privilege to selftests which use udmabuf by removing the write access
> > mode from drivers/dma-buf/udmabuf.c and drivers/net/hw/ncdevmem.c.
> >
> > [1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/testing/selftests/bpf/prog_tests/dmabuf_iter.c?h=v7.1#n49
> >
> > Signed-off-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
> > ---
> >  tools/testing/selftests/drivers/dma-buf/udmabuf.c | 2 +-
> >  tools/testing/selftests/drivers/net/hw/ncdevmem.c | 2 +-
> >  2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/tools/testing/selftests/drivers/dma-buf/udmabuf.c b/tools/testing/selftests/drivers/dma-buf/udmabuf.c
> > index d78aec662586..ced0b95c876c 100644
> > --- a/tools/testing/selftests/drivers/dma-buf/udmabuf.c
> > +++ b/tools/testing/selftests/drivers/dma-buf/udmabuf.c
> > @@ -140,7 +140,7 @@ int main(int argc, char *argv[])
> >       ksft_print_header();
> >       ksft_set_plan(7);
> >
> > -     devfd = open("/dev/udmabuf", O_RDWR);
> > +     devfd = open("/dev/udmabuf", O_RDONLY);
> >       if (devfd < 0) {
> >               ksft_print_msg(
> >                       "%s: [skip,no-udmabuf: Unable to access DMA buffer device file]\n",
> > diff --git a/tools/testing/selftests/drivers/net/hw/ncdevmem.c b/tools/testing/selftests/drivers/net/hw/ncdevmem.c
> > index e098d6534c3c..8114a29692fd 100644
> > --- a/tools/testing/selftests/drivers/net/hw/ncdevmem.c
> > +++ b/tools/testing/selftests/drivers/net/hw/ncdevmem.c
> > @@ -149,7 +149,7 @@ static struct memory_buffer *udmabuf_alloc(size_t size)
> >
> >       ctx->size = size;
> >
> > -     ctx->devfd = open("/dev/udmabuf", O_RDWR);
> > +     ctx->devfd = open("/dev/udmabuf", O_RDONLY);
> >       if (ctx->devfd < 0) {
> >               pr_err("[skip,no-udmabuf: Unable to access DMA buffer device file]");
> >               goto err_free_ctx;
> >
> > base-commit: fbb7ad31ab376c5101b2ac7205fad0344fd2de60
> > --
> > 2.55.0.rc0.799.gd6f94ed593-goog
> >
>
> Reviewed-by: Bobby Eshleman <bobbyeshleman@xxxxxxxx>

Thanks Bobby.

Shuah, would you mind picking this up?