[libcamera-devel] [PATCH] v4l2: Accept read-only buffers mappings and require MAP_SHARED

[Laurent Pinchart]

V4L2 is happy to map buffers read-only for capture devices (but rejects
write-only mappings). We can support this as the dmabuf mmap()
implementation supports it. This change fixes usage of the V4L2
compatibility layer with OpenCV.

While at it, attempt to validate the other flags. videobuf2 requires
MAP_SHARED and doesn't check other flags, so mimic the same behaviour.
While unlikly, other flags could get rejected by other kernel layers for
V4L2 buffers but not for dmabuf. This can be handled later if the need
arises.

Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
---
 src/v4l2/v4l2_camera_proxy.cpp | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

Nejc, could you please test this patch ? It's slightly different than
the one I've shared on the IRC channel. If you don't mind appearing in
the git development history, you can reply with

Tested-by: Name <email at address>

and I'll add that (as well as a corresponding Reported-by tag) to the
commit before pushing it.

diff --git a/src/v4l2/v4l2_camera_proxy.cpp b/src/v4l2/v4l2_camera_proxy.cpp
index f3470a6d312a..ebe7601a4ba6 100644
--- a/src/v4l2/v4l2_camera_proxy.cpp
+++ b/src/v4l2/v4l2_camera_proxy.cpp
@@ -104,8 +104,17 @@ void *V4L2CameraProxy::mmap(V4L2CameraFile *file, void *addr, size_t length,
 
 	MutexLocker locker(proxyMutex_);
 
-	/* \todo Validate prot and flags properly. */
-	if (prot != (PROT_READ | PROT_WRITE)) {
+	/*
+	 * Mimic the videobuf2 behaviour, which requires PROT_READ. Reject
+	 * PROT_EXEC as it makes no sense.
+	 */
+	if (!(prot & PROT_READ) || prot & PROT_EXEC) {
+		errno = EINVAL;
+		return MAP_FAILED;
+	}
+
+	/* videobuf2 also requires MAP_SHARED. */
+	if (!(flags & MAP_SHARED)) {
 		errno = EINVAL;
 		return MAP_FAILED;
 	}

base-commit: 7ea52d2b586144fdc033a3ffc1c4a4bbb99b5440
-- 
Regards,

Laurent Pinchart