Re: [f2fs-dev] [PATCH] f2fs: fix multiple f2fs_add_link() calls having same name

[Jaegeuk Kim <jaegeuk@kernel.org>]

On 02/14, Jaegeuk Kim wrote:
> On 02/15, Chao Yu wrote:
> > On 2017/2/15 2:03, Jaegeuk Kim wrote:
> > > VFS uses f2fs_lookup() to decide f2fs_add_link() call during file creation.
> > > But, if there is a race condition, f2fs_add_link() can be called multiple
> > > times, resulting in multiple dentries with a same name. This patches fixes
> > > it by adding __f2fs_find_entry() in f2fs_add_link() path.
> > 
> > As during ->lookup, i_mutex will be held all the time, so there is no race
> > condition in between different file creators?
> 
> Hehe, yup.
> I dropped this patch.

It turns out sdcardfs has a race condition between lookup and create, and it
calls vfs_create() twice. Workaround by f2fs would be needed for whole AOSP as
well. And I added to check current to avoid performance regression.

Thanks,

>From 2f433dbfa491550e666a3d08f5a59ac5bed42b0f Mon Sep 17 00:00:00 2001
From: Jaegeuk Kim <jaegeuk@kernel.org>
Date: Tue, 14 Feb 2017 09:54:37 -0800
Subject: [PATCH] f2fs: fix multiple f2fs_add_link() calls having same name

It turns out a stakable filesystem like sdcardfs in AOSP can trigger multiple
vfs_create() to lower filesystem. In that case, f2fs will add multiple dentries
having same name which breaks filesystem consistency.

Until upper layer fixes, let's work around by f2fs, which shows actually not
much performance regression.

Cc: <stable@vger.kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
 fs/f2fs/dir.c  | 26 ++++++++++++++++++++++++--
 fs/f2fs/f2fs.h |  1 +
 2 files changed, 25 insertions(+), 2 deletions(-)

diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index 827c5daef4fc..4f5b3bb514c6 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -207,9 +207,11 @@ static struct f2fs_dir_entry *find_in_level(struct inode *dir,
 		f2fs_put_page(dentry_page, 0);
 	}
 
+	/* This is to increase the speed of f2fs_create */
 	if (!de && room && F2FS_I(dir)->chash != namehash) {
 		F2FS_I(dir)->chash = namehash;
 		F2FS_I(dir)->clevel = level;
+		F2FS_I(dir)->task = current;
 	}
 
 	return de;
@@ -643,14 +645,34 @@ int __f2fs_add_link(struct inode *dir, const struct qstr *name,
 				struct inode *inode, nid_t ino, umode_t mode)
 {
 	struct fscrypt_name fname;
+	struct page *page = NULL;
+	struct f2fs_dir_entry *de = NULL;
 	int err;
 
 	err = fscrypt_setup_filename(dir, name, 0, &fname);
 	if (err)
 		return err;
 
-	err = __f2fs_do_add_link(dir, &fname, inode, ino, mode);
-
+	/*
+	 * An immature stakable filesystem shows a race condition between lookup
+	 * and create. If we have same task when doing lookup and create, it's
+	 * definitely fine as expected by VFS normally. Otherwise, let's just
+	 * verify on-disk dentry one more time, which guarantees filesystem
+	 * consistency more.
+	 */
+	if (current != F2FS_I(dir)->task) {
+		de = __f2fs_find_entry(dir, &fname, &page);
+		F2FS_I(dir)->task = NULL;
+	}
+	if (de) {
+		f2fs_dentry_kunmap(dir, page);
+		f2fs_put_page(page, 0);
+		err = -EEXIST;
+	} else if (!IS_ERR(page)) {
+		err = __f2fs_do_add_link(dir, &fname, inode, ino, mode);
+	} else {
+		err = PTR_ERR(page);
+	}
 	fscrypt_free_filename(&fname);
 	return err;
 }
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index d263dade5e4c..cc22dc458896 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -454,6 +454,7 @@ struct f2fs_inode_info {
 	atomic_t dirty_pages;		/* # of dirty pages */
 	f2fs_hash_t chash;		/* hash value of given file name */
 	unsigned int clevel;		/* maximum level of given file name */
+	struct task_struct *task;	/* lookup and create consistency */
 	nid_t i_xattr_nid;		/* node id that contains xattrs */
 	loff_t	last_disk_size;		/* lastly written file size */
 
-- 
2.11.0