[PATCH v2] ovl: use a dedicated semaphore for dir upperfile caching

[Icenowy Zheng]

The function ovl_dir_real_file() currently uses the semaphore of the
inode to synchronize write to the upperfile cache field.

However, this function will get called by ovl_ioctl_set_flags(), which
utilizes the inode semaphore too. In this case ovl_dir_real_file() will
try to claim a lock that is owned by a function in its call stack, which
won't get released before ovl_dir_real_file() returns.

Define a dedicated semaphore for the upperfile cache, so that the
deadlock won't happen.

Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for 
directories")
Cc: sta...@vger.kernel.org # v5.10
Signed-off-by: Icenowy Zheng <icen...@aosc.io>
---
 fs/overlayfs/readdir.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 01620ebae1bd..fa1844ff8db4 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -56,6 +56,7 @@ struct ovl_dir_file {
        struct list_head *cursor;
        struct file *realfile;
        struct file *upperfile;
+       struct semaphore upperfile_sem;
 };
 
 static struct ovl_cache_entry *ovl_cache_entry_from_node(struct rb_node *n)
@@ -874,8 +875,6 @@ struct file *ovl_dir_real_file(const struct file *file, 
bool want_upper)
         * Need to check if we started out being a lower dir, but got copied up
         */
        if (!od->is_upper) {
-               struct inode *inode = file_inode(file);
-
                realfile = READ_ONCE(od->upperfile);
                if (!realfile) {
                        struct path upperpath;
@@ -883,10 +882,10 @@ struct file *ovl_dir_real_file(const struct file *file, 
bool want_upper)
                        ovl_path_upper(dentry, &upperpath);
                        realfile = ovl_dir_open_realfile(file, &upperpath);
 
-                       inode_lock(inode);
+                       down(&od->upperfile_sem);
                        if (!od->upperfile) {
                                if (IS_ERR(realfile)) {
-                                       inode_unlock(inode);
+                                       up(&od->upperfile_sem);
                                        return realfile;
                                }
                                smp_store_release(&od->upperfile, realfile);
@@ -896,7 +895,7 @@ struct file *ovl_dir_real_file(const struct file *file, 
bool want_upper)
                                        fput(realfile);
                                realfile = od->upperfile;
                        }
-                       inode_unlock(inode);
+                       up(&od->upperfile_sem);
                }
        }
 
@@ -959,6 +958,7 @@ static int ovl_dir_open(struct inode *inode, struct file 
*file)
        od->realfile = realfile;
        od->is_real = ovl_dir_is_real(file->f_path.dentry);
        od->is_upper = OVL_TYPE_UPPER(type);
+       sema_init(&od->upperfile_sem, 1);
        file->private_data = od;
 
        return 0;
-- 
2.28.0