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This is detected by the commit in v5.3-rc1,
	d6fc9fcbaa65 2019-07-08 kbuild: compile-test exported headers to
		ensure they are self-contained
But the issue is not for v5.3-rc1 only I am afraid.  So this commit is
brought to aufs4.14 which is my current development base.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
(cherry picked from commit d38615f7fef1c65e30e3cc418daa9e19c93ed98f)

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
(cherry picked from commit 4596850541ed7144f7fea951d02f0f6251c4a997)

The scenario is very similar to previous commit
"aufs: bugfix, ignore the being freed dynop object".
One exception is that this commit is for sbinfo object which is managed
by kobject (instead of kref).

In order to enter the plink-maintenance mode, users write an ID to
"/proc/fs/aufs/plink_maint" (this path is defined as macros in
include/uapi/linux/aufs_type.h).  If someone else is unmounting the
aufs mount corresponding that ID, then the searcher task may find a
being freed sbinfo object.
The problem and the fix is very similar to previous commit
"aufs: bugfix, ignore the being freed dynop object".
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
(cherry picked from commit 949b498ae30797b19b9e7ac9b230815f31ffe378)

Aufs DYNOP (Dynamically customizable FS operations) object is managed by
kref, and when its counter reaches zero, the callback function removes
the object from the internal list which is protected by a spinlock and
then frees the object.
Here there is a small time window between
A: the counter reaches zero, and
B: require the lock to remove the object from the list.
If someone else acquires the lock and searches the list, it may find the
counter-zero'ed object which means the object is being freed.
This commit ignores the object whose counter is already zero.
Reported-and-tested-by: Kirill Kolyshkin <kolyshkin@gmail.com>
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
(cherry picked from commit b633d7b2635b9615fe294b85257d05008e3747a3)

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
(cherry picked from commit 9dbd45984201453ecebcc03a91699878aa38239d)

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
(cherry picked from commit 94c692425480664a02aaac3d6f7d496ea1d05c1f)

At the mount-time, XINO files are created and removed very soon.  When
multiple mounts with being given the same XINO file path executed in
parallel, some of them may fail in creating XINO due to EEXIST.
Introducing a local mutex, make it serialized.
By default, XINO is created at the top dir of the first writable
branch.  In this case, the new mutex won't be used.

Obviously this is an unnecessary overhead when the XINO file path is not
same, and such lock should be done by inode_lock() for the parent dir.
Actually au_xino_create2() behaves in this manner.  Then why didn't I
apply the same way to this au_xino_create()?  It is just because of my
laziness.  Calling VFS filp_open() here is easy for me.
Reported-by: Kirill Kolyshkin <kolyshkin@gmail.com>
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
(cherry picked from commit 30d9273f2a1ce331b1a79b770bdb4c493919a673)

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Although I am not sure module_exit is necessary for the statically
linked module, here is it.  I hope it does no harm.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

They are historical options and a branch attribute.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Fuse doesn't want the callers to access the inode attributes without
issuing stat, and it is not assured that they are valid after lookup or
iget().
The inode attribute is critical for aufs, and aufs decided to call stat
every time for fuse.
Of course, it makes aufs slow. But when the branch fs is not fuse, stat
is not called.

Currently, only FUSE implements ->poll(), and aufs supports it.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Basically ramfs is limited for its size, and is not suitable for aufs RW
branch. But people sometimes use it as RW branch without knowing it or
with knowing it.
This configuration is for those who knows what he is doing.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Special support for filesystems which acquires an inode mutex at final
closing a file, eg, hfsplus.
This trick is very simple and stupid, just to open the file before really
necessary open to tell hfsplus that this is not the final closing.
The caller should call au_h_open_pre() after acquiring the inode mutex,
and au_h_open_post() after releasing it.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Still I am not sure how this feature breaches the security. Some people
say it doesn't matter. But I don't know.
Anyway upon the request from the users, aufs implements it as a module
option.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

It is ok that the branch is loopback-mounted.
But it had a problem if the backend fs-image is placed on another
branch, and aufs had prohibited such nested branch for a long time due
to the recursive lookup by 'loopN' daemon.
I don't remember when it was, but the daemon stopped such recursive
behaviour, but aufs is still prohibitting the nested branch via
loopback-mount.
Upon the request from users, aufs will allow the loopback-mounted branch
on another branch by another patch (aufs4-loopback.patch in
aufs4-standalone.git).
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

This feature automatically handles MVDOWN in other commits.
In user-space, a daemon monitors the free space of the branch and issues
MVDOWN ioctl automatically when necessary. The main role is in
user-space and several options are implemented.
For a branch to join the FHSM circle, a new attribute 'fhsm' should be
specified.

See also the document in this commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

When these attributes are specified and aufs tries opening a file on that
branch, aufs copies/moves it up.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Support for the options of MVDOWN feature, which allows to overwrite the
existing entry, and writing to the branch even if its permission is RO.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Another ioctl feature, move-down.
The behaviour is, as you can guess, the opposite of copy-up.
The feature called FHSM (file-based hierarchical storage management, in
later commit) uses this ioctl aggressively.

See also the document in this commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Because of some inode is in use, the deletion of a branch can fail.
For those who wants to test the inode is busy or not, aufs provides an
ioctl, and a utility 'aubusy' in aufs-util.git.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

For a directory which has millions of files, aufs VDIR consumes
much memory. In this case, RDU (readdir(3) in user-space) is definitely
better.
If you enable CONFIG_AUFS_RDU at compiling aufs, install libau.so from
aufs-util.git, and set some environment variables, then you can use this
feature. When readdir(3) in libau.so receives an aufs dir, it issues
ioctl(2) instead of regular readdir(3).
All merging and whiteout handling are done in userspace.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Provide info about the branches, which will be used from user-space.
This is essentially equivalent to the entries under sysfs
(/sys/fs/aufs/si_*/).
But the ioctl behaviour is atomic and never confuse the matching of the
branch id.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Provide a file descriptor corresponding the specified writable branch.
The file descriptor will be used from user-space such as FHSM and
libau.so. For details, see aufs-util.git.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Print the current data status for debugging.
The trigger key is a module parameter and you can freely change. The
default is 'a' of course.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Simple checks when the module is loaded.
This feature is compiled when CONFIG_AUFS_DEBUG is enabled.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Aufs provides some info via debugfs such as
- the branch path
- the current number of pseudo-links
- the size and the number of consumed blocks by XINO, XIB and XIGEN.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

These are very old options.
Since Unionfs created 'diropq' unconditionally in mkdir(2), old users
may expect the same behaviour. But there are cases where 'diropq' is
unnecessary. The aufs default behaviour is to create 'diropq' only when
it is necessary.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Generally aufs hides the name of whiteouts. But in some cases, to show
them is very useful for users. For instance, creating a new middle layer
(branch) by merging existing layers.

See also the document in this commit.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

While most people (especially who use tmpfs as top writable branch)
doesn't care, I care and think it can be a security problem.
For example, when the lower readonly branch may contain
/etc/{passwd,shadow} and the permission bits of the upper empty
branch is world-writable, then a malicious user can make these files
manually with by-passing aufs.
Aufs can do nothing but produce a warning.

For details, see aufs manual in aufs-util.git.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

Introduce the new mount options, dirren and nodirren, which activates
and deactivates DIRREN feature.
In remount and unmount, the inum-list per branch should be flushed to
the file.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

When aufs meets a new dir inode on a branch in lookup, it tests whether
the inode is in the list which the branch has. If the inode is found, it
means the dir has ever been logically renamed and there is some info
about the name under that dir. Then aufs tries loading the info, and
continues looking up using the before-renamed name on the lower
branches.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>

When DIRREN is enabled and activated, the error case where
aufs rename(2) used to return EXDEV will be gone.
Aufs rename(2) registers the renaming dir inum to the list in the
branch, creates the detailed info file, and returns a success.

If udba=notify option is specified with dirren, the internal detection
may not work correctly since aufs may not be able to find the target
name.
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>