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

于 2021年1月4日 GMT+08:00 下午3:28:35, Icenowy Zheng icenowy@aosc.io 写到:

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: stable@vger.kernel.org # v5.10 Signed-off-by: Icenowy Zheng icenowy@aosc.io

---

Sorry for lack of changelog.

A missing replacement of inode_lock() is added in v2.

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;