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VFS/nfsd/cachefiles/ovl: add start_creating() and end_creating()

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start_creating() is similar to simple_start_creating() but is not so
simple.
It takes a qstr for the name, includes permission checking, and does NOT
report an error if the name already exists, returning a positive dentry
instead.

This is currently used by nfsd, cachefiles, and overlayfs.

end_creating() is called after the dentry has been used.
end_creating() drops the reference to the dentry as it is generally no
longer needed.  This is exactly the first section of end_creating_path()
so that function is changed to call the new end_creating()

These calls help encapsulate locking rules so that directory locking can
be changed.

Occasionally this change means that the parent lock is held for a
shorter period of time, for example in cachefiles_commit_tmpfile().
As this function now unlocks after an unlink and before the following
lookup, it is possible that the lookup could again find a positive
dentry, so a while loop is introduced there.

In overlayfs the ovl_lookup_temp() function has ovl_tempname()
split out to be used in ovl_start_creating_temp().  The other use
of ovl_lookup_temp() is preparing for a rename.  When rename handling
is updated, ovl_lookup_temp() will be removed.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: NeilBrown <neil@brown.name>
Link: https://patch.msgid.link/20251113002050.676694-5-neilb@ownmail.net
Tested-by: syzbot@syzkaller.appspotmail.com
Signed-off-by: Christian Brauner <brauner@kernel.org>
This commit is contained in:
NeilBrown 2025-11-13 11:18:27 +11:00 • committed by Christian Brauner
commit 7ab96df840
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12 changed files with 215 additions and 160 deletions
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