linux-kernel - [PATCH] Fix i_mutex handling in nfsd readdir.

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Message-Id: <1240013753.21423.86.camel@macbook.infradead.org>
Date:	Sat, 18 Apr 2009 01:15:53 +0100
From:	David Woodhouse <dwmw2@...radead.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	bfields@...i.umich.edu, hooanon05@...oo.co.jp, hch@...radead.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: [PATCH] Fix i_mutex handling in nfsd readdir.

Commit 14f7dd63 ("Copy XFS readdir hack into nfsd code") introduced a
bug to generic code which had been extant for a long time in the XFS
version -- it started to call through into lookup_one_len() and hence
into the file systems' ->lookup() methods without i_mutex held on the
directory.

This patch fixes it by locking the directory's i_mutex again before
calling the filldir functions. The original deadlocks which commit
14f7dd63 was designed to avoid are still avoided, because they were due
to fs-internal locking, not i_mutex.

While we're at it, fix the return type of nfsd_buffered_readdir() which
should be a __be32 not an int -- it's an NFS errno, not a Linux errno.
And return nfserrno(-ENOMEM) when allocation fails, not just -ENOMEM.
Sparse would have caught both of those if it wasn't so busy bitching
about __cold__.

Reported-by: J. R. Okajima <hooanon05@...oo.co.jp>
Signed-off-by: David Woodhouse <David.Woodhouse@...el.com>
---

Commit 05f4f678b introduced a similar problem for NFSv4 which I'm not
going to attempt to deal with before getting some sleep...

diff --git a/fs/namei.c b/fs/namei.c
index b8433eb..78f253c 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -1248,6 +1248,8 @@ struct dentry *lookup_one_len(const char *name, struct dentry *base, int len)
 	int err;
 	struct qstr this;
 
+	WARN_ON_ONCE(!mutex_is_locked(&base->d_inode->i_mutex));
+
 	err = __lookup_one_len(name, &this, base, len);
 	if (err)
 		return ERR_PTR(err);
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index ab93fcf..44a68e1 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1885,8 +1885,8 @@ static int nfsd_buffered_filldir(void *__buf, const char *name, int namlen,
 	return 0;
 }
 
-static int nfsd_buffered_readdir(struct file *file, filldir_t func,
-				 struct readdir_cd *cdp, loff_t *offsetp)
+static __be32 nfsd_buffered_readdir(struct file *file, filldir_t func,
+				    struct readdir_cd *cdp, loff_t *offsetp)
 {
 	struct readdir_data buf;
 	struct buffered_dirent *de;
@@ -1896,11 +1896,12 @@ static int nfsd_buffered_readdir(struct file *file, filldir_t func,
 
 	buf.dirent = (void *)__get_free_page(GFP_KERNEL);
 	if (!buf.dirent)
-		return -ENOMEM;
+		return nfserrno(-ENOMEM);
 
 	offset = *offsetp;
 
 	while (1) {
+		struct inode *dir_inode = file->f_path.dentry->d_inode;
 		unsigned int reclen;
 
 		cdp->err = nfserr_eof; /* will be cleared on successful read */
@@ -1919,22 +1920,35 @@ static int nfsd_buffered_readdir(struct file *file, filldir_t func,
 		if (!size)
 			break;
 
+		/*
+		 * Various filldir functions may end up calling back into
+		 * lookup_one_len() and the file system's ->lookup() method.
+		 * These expect i_mutex to be held, as it would within readdir.
+		 */
+		host_err = mutex_lock_killable(&dir_inode->i_mutex);
+		if (host_err)
+			break;
+
 		de = (struct buffered_dirent *)buf.dirent;
 		while (size > 0) {
 			offset = de->offset;
 
 			if (func(cdp, de->name, de->namlen, de->offset,
 				 de->ino, de->d_type))
-				goto done;
+				break;
 
 			if (cdp->err != nfs_ok)
-				goto done;
+				break;
 
 			reclen = ALIGN(sizeof(*de) + de->namlen,
 				       sizeof(u64));
 			size -= reclen;
 			de = (struct buffered_dirent *)((char *)de + reclen);
 		}
+		mutex_unlock(&dir_inode->i_mutex);
+		if (size > 0) /* We bailed out early */
+			break;
+
 		offset = vfs_llseek(file, 0, SEEK_CUR);
 	}
 


-- 
David Woodhouse                            Open Source Technology Centre
David.Woodhouse@...el.com                              Intel Corporation

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