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We got a report that adding a fanotify filsystem watch prevents tail -f
from receiving events.
Reproducer:
1. Create 3 windows / login sessions. Become root in each session.
2. Choose a mounted filesystem that is pretty quiet; I picked /boot.
3. In the first window, run: fsnotifywait -S -m /boot
4. In the second window, run: echo data >> /boot/foo
5. In the third window, run: tail -f /boot/foo
6. Go back to the second window and run: echo more data >> /boot/foo
7. Observe that the tail command doesn't show the new data.
8. In the first window, hit control-C to interrupt fsnotifywait.
9. In the second window, run: echo still more data >> /boot/foo
10. Observe that the tail command in the third window has now printed
the missing data.
When stracing tail, we observed that when fanotify filesystem mark is
set, tail does get the inotify event, but the event is receieved with
the filename:
read(4, "\1\0\0\0\2\0\0\0\0\0\0\0\20\0\0\0foo\0\0\0\0\0\0\0\0\0\0\0\0\0",
50) = 32
This is unexpected, because tail is watching the file itself and not its
parent and is inconsistent with the inotify event received by tail when
fanotify filesystem mark is not set:
read(4, "\1\0\0\0\2\0\0\0\0\0\0\0\0\0\0\0", 50) = 16
The inteference between different fsnotify groups was caused by the fact
that the mark on the sb requires the filename, so the filename is passed
to fsnotify(). Later on, fsnotify_handle_event() tries to take care of
not passing the filename to groups (such as inotify) that are interested
in the filename only when the parent is watching.
But the logic was incorrect for the case that no group is watching the
parent, some groups are watching the sb and some watching the inode.
Reported-by: Miklos Szeredi <miklos@szeredi.hu>
Fixes:
|
||
|---|---|---|
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| Documentation | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| LICENSES | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
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 reStructuredText 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.