linux-kernel - [PATCH 09/25] make access() use mnt check

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Message-Id: <20070920195300.328EF882@kernel>
Date:	Thu, 20 Sep 2007 12:53:00 -0700
From:	Dave Hansen <haveblue@...ibm.com>
To:	akpm@...l.org
Cc:	linux-kernel@...r.kernel.org, hch@...radead.org,
	Dave Hansen <haveblue@...ibm.com>
Subject: [PATCH 09/25] make access() use mnt check


It is OK to let access() go without using a mnt_want/drop_write()
pair because it doesn't actually do writes to the filesystem,
and it is inherently racy anyway.  This is a rare case when it is
OK to use __mnt_is_readonly() directly.

Acked-by: Christoph Hellwig <hch@...radead.org>
Signed-off-by: Dave Hansen <haveblue@...ibm.com>
---

 lxc-dave/fs/open.c |   13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff -puN fs/open.c~make-access-use-helper fs/open.c
--- lxc/fs/open.c~make-access-use-helper	2007-09-20 12:16:13.000000000 -0700
+++ lxc-dave/fs/open.c	2007-09-20 12:16:13.000000000 -0700
@@ -457,8 +457,17 @@ asmlinkage long sys_faccessat(int dfd, c
 	if(res || !(mode & S_IWOTH) ||
 	   special_file(nd.dentry->d_inode->i_mode))
 		goto out_path_release;
-
-	if(IS_RDONLY(nd.dentry->d_inode))
+	/*
+	 * This is a rare case where using __mnt_is_readonly()
+	 * is OK without a mnt_want/drop_write() pair.  Since
+	 * no actual write to the fs is performed here, we do
+	 * not need to telegraph to that to anyone.
+	 *
+	 * By doing this, we accept that this access is
+	 * inherently racy and know that the fs may change
+	 * state before we even see this result.
+	 */
+	if (__mnt_is_readonly(nd.mnt))
 		res = -EROFS;
 
 out_path_release:
_
-
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