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

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Date:   Mon, 04 Jan 2021 16:14:12 +0800
From:   Icenowy Zheng <icenowy@...c.io>
To:     Miklos Szeredi <miklos@...redi.hu>,
        Amir Goldstein <amir73il@...il.com>,
        Xiao Yang <yangx.jy@...fujitsu.com>
CC:     linux-unionfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        stable@...r.kernel.org
Subject: 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@...c.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@...r.kernel.org # v5.10
>Signed-off-by: Icenowy Zheng <icenowy@...c.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;