• J. Bruce Fields's avatar
    vfs: stop d_splice_alias creating directory aliases · 7732a557
    J. Bruce Fields authored
    A directory should never have more than one dentry pointing to it.
    
    But d_splice_alias() will add one if it finds a directory with an
    already-existing non-DISCONNECTED dentry.
    
    I can't find an obvious reproducer, but I also can't see what prevents
    d_splice_alias() from encountering such a case.
    
    It therefore seems safest to allow d_splice_alias to use any dentry it
    finds.
    
    (Prior to the removal of dentry_unhash() from vfs_rmdir(), around v3.0,
    this could cause an nfsd deadlock like this:
    
    	- Somebody attempts to remove a non-empty directory.
    	- The dentry_unhash() in vfs_rmdir() unhashes the dentry
    	  pointing to the non-empty directory.
    	- ->rmdir() then fails with -ENOTEMPTY
    	- Before the vfs_rmdir() caller reaches dput(), an nfsd process
    	  in rename looks up the directory by filehandle; at the end of
    	  that lookup, this dentry is found by d_alloc_anon(), and a
    	  reference is taken on it, preventing dput() from removing it.
    	- A regular lookup of the directory calls d_splice_alias(),
    	  finds only an unhashed (not a DISCONNECTED) dentry, and
    	  insteads adds a new one, so the directory now has two
    	  dentries.
    	- The nfsd process in rename, which was previously looking up
    	  the source directory of the rename, now looks up the target
    	  directory (which is the same), and gets the dentry newly
    	  created by the previous lookup.
    	- The rename, seeing two different dentries, assumes this is a
    	  cross-directory rename and attempts to take the i_mutex on the
    	  directory twice.
    
    That reproducer no longer exists, but I don't think there was anything
    fundamentally incorrect about the vfs_rmdir() behavior there, so I think
    the real fault was here in d_splice_alias().)
    Signed-off-by: default avatarJ. Bruce Fields <bfields@redhat.com>
    Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
    7732a557
dcache.c 80.2 KB