nbd: avoid race with udev during setup #1741
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It's just a file descriptor to keep the device in use. There is nothing loopback specific. It will be reused in the nbd case so rename it to something more generic. Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
With the NBD_CFLAG_DISCONNECT_ON_CLOSE flag set, the nbd device will be automatically removed when there are no more users after it was was opened the first time. The idea here is, that the nbd device is created, and then immediately mounted (possibly with dm-verity or dm-crypt in between). Wenn the bundle ist later unmounted then the nbd device is automatically removed. Unfortunately this is racy: udev will see the new device and open it immediately. If it closes the device before it is mounted (or dm-verity or dm-crypt is configured) then the nbd device is removed immediately and mounting will fail. So instead create the nbd device without NBD_CFLAG_DISCONNECT_ON_CLOSE, open it to keep it busy and then reconfigure the device to set NBD_CFLAG_DISCONNECT_ON_CLOSE. The file descriptor is kept until the bundle is mounted. Note that with this change, the nbd device will not be fully removed if rauc dies between creating the nbd device and reconfiguring it, or if reconfiguring fails. Signed-off-by: Michael Olbrich <m.olbrich@pengutronix.de>
Codecov ReportAttention: Patch coverage is
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## master #1741 +/- ##
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- Coverage 84.52% 84.50% -0.02%
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Files 76 76
Lines 22350 22362 +12
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+ Hits 18891 18898 +7
- Misses 3459 3464 +5
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Oh, an interesting finding! 👍 Haven't seen it myself yet in a system, but the problem description makes sense to me. Maybe we should also add a small comment in the code on why the open and nbd reconfiguration is required? |
jluebbe
reviewed
Jun 23, 2025
| @@ -203,6 +200,27 @@ gboolean r_nbd_setup_device(RaucNBDDevice *nbd_dev, GError **error) | |||
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| nbd_dev->dev = g_strdup_printf("/dev/nbd%"G_GUINT32_FORMAT, nbd_dev->index); | |||
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| *devicefd = open(nbd_dev->dev, O_RDWR|O_CLOEXEC); | |||
| res = FALSE; | ||
| g_set_error(error, G_IO_ERROR, G_IO_ERROR_FAILED, "netlink send_sync failed"); | ||
| goto out; | ||
| } |
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Don't we need to wait/process the response here?
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With the
NBD_CFLAG_DISCONNECT_ON_CLOSEflag set, the nbd device will beautomatically removed when there are no more users after it was was
opened the first time.
The idea here is, that the nbd device is created, and then immediately
mounted (possibly with dm-verity or dm-crypt in between). Wenn the
bundle ist later unmounted then the nbd device is automatically removed.
Unfortunately this is racy: udev will see the new device and open it
immediately. If it closes the device before it is mounted (or dm-verity
or dm-crypt is configured) then the nbd device is removed immediately
and mounting will fail.
So instead create the nbd device without
NBD_CFLAG_DISCONNECT_ON_CLOSE,open it to keep it busy and then reconfigure the device to set
NBD_CFLAG_DISCONNECT_ON_CLOSE.The file descriptor is kept until the bundle is mounted.
Note that with this change, the nbd device will not be fully removed if
rauc dies between creating the nbd device and reconfiguring it, or if
reconfiguring fails.