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cb383f0668filesystem may be modified in the context of fanotify permission events
(e.g. by HSM service), so assert that sb freeze protection is not held.
If the assertion fails, then the following deadlock would be possible:
CPU0 CPU1 CPU2
-------------------------------------------------------------------------
file_start_write()#0
...
fsnotify_perm()
fanotify_get_response() => (read event and fill file)
...
... freeze_super()
... sb_wait_write()
...
vfs_write()
file_start_write()#1
This example demonstrates a use case of an hierarchical storage management
(HSM) service that uses fanotify permission events to fill the content of
a file before access, while a 3rd process starts fsfreeze.
This creates a circular dependeny:
file_start_write()#0 => fanotify_get_response =>
file_start_write()#1 =>
sb_wait_write() =>
file_end_write()#0
Where file_end_write()#0 can never be called and none of the threads can
make progress.
The assertion is checked for both MAY_READ and MAY_WRITE permission
hooks in preparation for a pre-modify permission event.
The assertion is not checked for an open permission event, because
do_open() takes mnt_want_write() in O_TRUNC case, meaning that it is not
safe to write to filesystem in the content of an open permission event.
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Link: https://lore.kernel.org/r/20231212094440.250945-5-amir73il@gmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.