libfs: Return ENOSPC when the directory offset range is exhausted

BugLink: https://bugs.launchpad.net/bugs/2109640

commit 903dc9c43a155e0893280c7472d4a9a3a83d75a6 upstream.

Testing shows that the EBUSY error return from mtree_alloc_cyclic()
leaks into user space. The ERRORS section of "man creat(2)" says:

>	EBUSY	O_EXCL was specified in flags and pathname refers
>		to a block device that is in use by the system
>		(e.g., it is mounted).

ENOSPC is closer to what applications expect in this situation.

Note that the normal range of simple directory offset values is
2..2^63, so hitting this error is going to be rare to impossible.

Fixes: 6faddda69f ("libfs: Add directory operations for stable offsets")
Cc: stable@vger.kernel.org # v6.9+
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Yang Erkun <yangerkun@huawei.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Link: https://lore.kernel.org/r/20241228175522.1854234-2-cel@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Manuel Diewald <manuel.diewald@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

fs/libfs.c

@@ -294,8 +294,8 @@ int simple_offset_add(struct offset_ctx *octx, struct dentry *dentry)
 	ret = mtree_alloc_cyclic(&octx->mt, &offset, dentry, DIR_OFFSET_MIN,
 				 DIR_OFFSET_MAX, &octx->next_offset,
 				 GFP_KERNEL);
-	if (ret < 0)
-		return ret;
+	if (unlikely(ret < 0))
+		return ret == -EBUSY ? -ENOSPC : ret;
 
 	offset_set(dentry, offset);
 	return 0;