On Fri, Apr 18, 2025 at 10:15:01AM +0800, Xilin Wu wrote:
> On 2025/4/7 17:54:15, Christian Brauner wrote:
> > This allows the VFS to not trip over anonymous inodes and we can add
> > asserts based on the mode into the vfs. When we report it to userspace
> > we can simply hide the mode to avoid regressions. I've audited all
> > direct callers of alloc_anon_inode() and only secretmen overrides i_mode
> > and i_op inode operations but it already uses a regular file.
> >
> > Fixes: af153bb63a336 ("vfs: catch invalid modes in may_open()")
> > Cc: <stable@xxxxxxxxxxxxxxx> # all LTS kernels
> > Reported-by: syzbot+5d8e79d323a13aa0b248@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://lore.kernel.org/all/67ed3fb3.050a0220.14623d.0009.GAE@xxxxxxxxxx
> > Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
>
> Hi Christian and FSdevel list,
>
> I'm reporting a regression introduced in the linux-next tree, which I've
> tracked down using `git bisect` to this commit.
>
> Starting with `linux-next` tag `next-20250408` (up to the latest
> `next-20250417` I tested), attempting to start an SDDM Wayland session
> (using KDE Plasma's KWin) results in a black screen. The `kwin_wayland` and
> `sddm-helper-start-wayland` processes both enter a state consuming 100% CPU
> on a single core indefinitely, preventing the login screen from appearing.
> The last known good kernel is `next-20250407`.
>
> Using `strace` on the `kwin_wayland` process revealed it's stuck in a tight
> loop involving `ppoll` and `ioctl`:
>
> ```
> ioctl(29, FIONREAD, [0]) = 0
> ppoll([{fd=10, events=POLLIN}, {fd=37, events=POLLIN}, {fd=9,
> events=POLLIN}, {fd=5, events=POLLIN}, {fd=29, events=POLLIN}, {fd=17,
> events=POLLIN}, {fd=3, events=POLLIN}], 7, NULL, NULL, 8) = 1 ([{fd=29,
> revents=POLLIN}])
> ioctl(29, FIONREAD, [0]) = 0
> ppoll([{fd=10, events=POLLIN}, {fd=37, events=POLLIN}, {fd=9,
> events=POLLIN}, {fd=5, events=POLLIN}, {fd=29, events=POLLIN}, {fd=17,
> events=POLLIN}, {fd=3, events=POLLIN}], 7, NULL, NULL, 8) = 1 ([{fd=29,
> revents=POLLIN}])
> # ... this repeats indefinitely at high frequency
> ```
>
> Initially, I suspected a DRM issue, but checking the file descriptor
> confirmed it's an `inotify` instance:
>
> ```
> $ sudo ls -l /proc/<kwin_pid>/fd/29
> lr-x------ 1 sddm sddm 64 Apr 17 14:03 /proc/<kwin_pid>/fd/29 ->
> anon_inode:inotify
> ```
>
> Reverting this commit resolves the issue, allowing SDDM and KWin Wayland to
> start normally.
>
> Could you please take a look at this? Let me know if you need any further
> information or testing from my side.
I'll take a look latest Tuesday.
