ACK: [SRU][R][PATCH 0/1] dma-buf filesystem flags fix

[Yufeng Gao]

On 8/4/26 10:32, AceLan Kao wrote:
> From: "Chia-Lin Kao (AceLan)" <acelan.kao at canonical.com>
>
> BugLink: https://bugs.launchpad.net/bugs/2139656
>
> [Impact]
> The VFS layer now enforces that inodes flagged with S_ANON_INODE must be located on
> filesystems with the SB_I_NOEXEC flag set. The dmabuf (Direct Memory Access Buffer)
> filesystem creates inodes using alloc_anon_inode(), which sets the S_ANON_INODE flag
> but does not set SB_I_NOEXEC and SB_I_NODEV on the filesystem.
>
> This causes a kernel warning when a dmabuf is memory-mapped, which can occur in
> multimedia applications such as GStreamer's v4l2src (Video4Linux source) element.
> The warning does not cause a crash but indicates a VFS policy violation that will
> eventually become a fatal error in future kernel versions.
>
> Error log:
> ```
> [ 60.061328] WARNING: CPU: 2 PID: 2803 at fs/exec.c:125 path_noexec+0xa0/0xd0
> ...
> [ 60.061637] do_mmap+0x2b5/0x680
> ```
>
> Affected scenarios: Any system running multimedia applications that use video capture
> or memory-mapped dmabuf operations, particularly with Video4Linux source elements or
> similar hardware-accelerated media processing.
>
> [Fix]
> Set the SB_I_NOEXEC and SB_I_NODEV flags on the dmabuf filesystem context. This follows
> the same pattern previously applied to other anonymous inode filesystems such as
> secretmem (commit 98f99394a104c) and the anon_inode filesystem (commit ce7419b6cf23d).
>
> The fix is minimal - adding two flag assignments in drivers/dma-buf/dma-buf.c:
> - fc->s_iflags |= SB_I_NOEXEC;
> - fc->s_iflags |= SB_I_NODEV;
>
> These changes align the dmabuf filesystem with the VFS enforcement introduced in
> commit 1e7ab6f67824 ("anon_inode: rework assertions").
>
> Upstream patch: https://lore.kernel.org/lkml/20260114011917.241196-1-acelan.kao@canonical.com/
>
> [Test Plan]
> 1. Install a system with a video capture device (USB webcam, integrated camera, or
>     Video4Linux-compatible device)
>
> 2. Install GStreamer and the v4l2src plugin:
>     $ sudo apt-get install gstreamer1.0-plugins-good gstreamer1.0-plugins-base
>
> 3. Run a GStreamer pipeline that uses v4l2src to capture video:
>     $ gst-launch-1.0 v4l2src ! video/x-raw, width=640, height=480, framerate=30/1 ! videoconvert ! autovideosink
>
> 4. Check the kernel log for warnings:
>     $ dmesg | grep -i "path_noexec\|SB_I_NOEXEC"
>
> Expected result without fix:
> - WARNING message appears in dmesg at fs/exec.c:125 (path_noexec+0xa0/0xd0)
>    indicating anonymous inode on non-compliant filesystem
>
> Expected result with fix:
> - No warning appears; video capture works correctly without VFS policy violations
>
> [Where problems could occur]
> The dmabuf filesystem itself could be affected if the SB_I_NOEXEC or SB_I_NODEV flags
> cause unexpected behavior with the filesystem's inode operations.
>
> If there are edge cases where the dmabuf filesystem needs to allow execution or device
> access for specific operations, this fix could prevent those operations from working.
> Symptoms would include dmabuf allocations failing, memory mapping failing, or hardware
> acceleration features becoming unavailable.
>
> The SB_I_NODEV flag prevents device node creation on the filesystem (which should never
> occur on a pseudo-filesystem), and SB_I_NOEXEC prevents execution (which is the correct
> behavior for a memory buffer filesystem). These are defensive flags used by all
> similar anonymous inode filesystems, so the regression risk is minimal, but if there
> are hidden dependencies on the absence of these flags, dmabuf clients could fail.
>
> Monitor for:
> - Memory mapping failures in applications using dmabuf
> - Failures to create dmabuf file descriptors
> - DMA operations failing to map memory regions
> - Video capture or media acceleration features becoming unavailable
>
> [Other Info]
> This is a backport of a fix for the upstream kernel. The patch follows the exact
> approach used in previous commits to other anonymous inode filesystems:
> - commit 98f99394a104c ("secretmem: use SB_I_NOEXEC")
> - commit ce7419b6cf23d ("anon_inode: raise SB_I_NODEV and SB_I_NOEXEC")
>
> The warning was introduced by commit 1e7ab6f67824 ("anon_inode: rework assertions")
> which added enforcement in the VFS layer that anonymous inodes must have these flags
> set on their host filesystem.
>
> Chia-Lin Kao (AceLan) (1):
>    UBUNTU: SAUCE: dma-buf: set SB_I_NOEXEC and SB_I_NODEV on dmabuf
>      filesystem
>
>   drivers/dma-buf/dma-buf.c | 2 ++
>   1 file changed, 2 insertions(+)
>
Acked-by: Yufeng Gao <yufeng.gao at canonical.com>