[elbe-devel] [PATCH] elbepack: imgutils: protect against race between losetup() and udev

[John Ogness]

On 2025-02-19, Thomas Weißschuh <thomas.weissschuh at linutronix.de> wrote:
> Udev or on of its plugins reconfigures new loop devices.
> Partitions are rescanned, leading to the removal and readding of
> their dev nodes.
> Depending on the timing, ELBE tries to access the partion devnode
> while it has been removed but not yet recreated.
>
> Use a file lock to synchronize the accesses between udev and ELBE.
> It's not necessary to wait for the udev updates as devtmpfs will create
> the device nodes on its own.

You do not need udev for the device nodes to be created, but you want to
synchronize with udev? I do not understand the reasoning here. Either
the device nodes exist or they do not. If udev is not involved in their
creation, then why even care about udev?

> Reported-by: John Ogness <jogness at linutronix.de>
> Link: https://systemd.io/BLOCK_DEVICE_LOCKING
> Signed-off-by: Thomas Weißschuh <thomas.weissschuh at linutronix.de>
> ---
>  elbepack/imgutils.py | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/elbepack/imgutils.py b/elbepack/imgutils.py
> index 635b3e68e7a906e1acc2b5e4f145f7aadcc9dcff..b060121c4590d4235a0fbc65c97c0b0c82cd95ce 100644
> --- a/elbepack/imgutils.py
> +++ b/elbepack/imgutils.py
> @@ -3,6 +3,7 @@
>  # SPDX-FileCopyrightText: 2024 Linutronix GmbH
>  
>  import contextlib
> +import fcntl
>  import subprocess
>  
>  from elbepack.shellhelper import ELBE_LOGGING, do, run
> @@ -16,7 +17,10 @@ def losetup(dev, extra_args=[]):
>      ).stdout.decode('ascii').rstrip('\n')
>  
>      try:
> -        yield loopdev
> +        with open(loopdev) as f:
> +            # protect against races with udev
> +            fcntl.flock(f, fcntl.LOCK_EX)
> +            yield loopdev

And where is the unlock?

John Ogness