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Subject: Re: [PATCH v2] ovl: use a dedicated semaphore for dir upperfile caching
To:     Miklos Szeredi <miklos@szeredi.hu>,
        Amir Goldstein <amir73il@gmail.com>,
        Xiao Yang <yangx.jy@cn.fujitsu.com>
CC:     linux-unionfs@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org
From:   Icenowy Zheng <icenowy@aosc.io>
Message-ID: <41A054BA-B955-4A17-BC19-31F8A0B2F1B3@aosc.io>
Precedence: bulk


=E4=BA=8E 2021=E5=B9=B41=E6=9C=884=E6=97=A5 GMT+08:00 =E4=B8=8B=E5=8D=883:=
28:35, Icenowy Zheng <icenowy@aosc=2Eio> =E5=86=99=E5=88=B0:
>The function ovl_dir_real_file() currently uses the semaphore of the
>inode to synchronize write to the upperfile cache field=2E
>
>However, this function will get called by ovl_ioctl_set_flags(), which
>utilizes the inode semaphore too=2E 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=2E
>
>Define a dedicated semaphore for the upperfile cache, so that the
>deadlock won't happen=2E
>
>Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR
>ioctls for directories")
>Cc: stable@vger=2Ekernel=2Eorg # v5=2E10
>Signed-off-by: Icenowy Zheng <icenowy@aosc=2Eio>
>---

Sorry for lack of changelog=2E

A missing replacement of inode_lock() is added in v2=2E

> fs/overlayfs/readdir=2Ec | 10 +++++-----
> 1 file changed, 5 insertions(+), 5 deletions(-)
>
>diff --git a/fs/overlayfs/readdir=2Ec b/fs/overlayfs/readdir=2Ec
>index 01620ebae1bd=2E=2Efa1844ff8db4 100644
>--- a/fs/overlayfs/readdir=2Ec
>+++ b/fs/overlayfs/readdir=2Ec
>@@ -56,6 +56,7 @@ struct ovl_dir_file {
> 	struct list_head *cursor;
> 	struct file *realfile;
> 	struct file *upperfile;
>+	struct semaphore upperfile_sem;
> };
>=20
>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 =3D file_inode(file);
>-
> 		realfile =3D 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 =3D ovl_dir_open_realfile(file, &upperpath);
>=20
>-			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 =3D od->upperfile;
> 			}
>-			inode_unlock(inode);
>+			up(&od->upperfile_sem);
> 		}
> 	}
>=20
>@@ -959,6 +958,7 @@ static int ovl_dir_open(struct inode *inode, struct
>file *file)
> 	od->realfile =3D realfile;
> 	od->is_real =3D ovl_dir_is_real(file->f_path=2Edentry);
> 	od->is_upper =3D OVL_TYPE_UPPER(type);
>+	sema_init(&od->upperfile_sem, 1);
> 	file->private_data =3D od;
>=20
> 	return 0;