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@google.com>
---
 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