linux-kernel - [PATCH 1/2] vfs: make sure struct filename->iname is word-aligned

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Message-Id: <1455662966-7977-1-git-send-email-linux@rasmusvillemoes.dk>
Date:	Tue, 16 Feb 2016 23:49:24 +0100
From:	Rasmus Villemoes <linux@...musvillemoes.dk>
To:	Alexander Viro <viro@...iv.linux.org.uk>,
	Jeff Layton <jlayton@...chiereds.net>,
	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	Rasmus Villemoes <linux@...musvillemoes.dk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH 1/2] vfs: make sure struct filename->iname is word-aligned

I noticed that offsetof(struct filename, iname) is actually 28 on 64
bit platforms, so we always pass an unaligned pointer to
strncpy_from_user. This is mostly a problem for those 64 bit platforms
without HAVE_EFFICIENT_UNALIGNED_ACCESS, but even on x86_64, unaligned
accesses carry a penalty, especially when done in a loop.

Let's try to ensure we always pass an aligned destination pointer to
strncpy_from_user. I considered making refcnt a long instead of doing
the union thing, and mostly ended up tossing a coin.

Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
---
Cc'ing Linus, not because it's urgent in any way, but because he's
usually interested in strncpy_from_user and he can probably tell me
why this is completely immaterial.

 fs/namei.c         | 2 ++
 include/linux/fs.h | 5 ++++-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index f624d132e01e..bd150fa799a2 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -35,6 +35,7 @@
 #include <linux/fs_struct.h>
 #include <linux/posix_acl.h>
 #include <linux/hash.h>
+#include <linux/bug.h>
 #include <asm/uaccess.h>
 
 #include "internal.h"
@@ -127,6 +128,7 @@ getname_flags(const char __user *filename, int flags, int *empty)
 	struct filename *result;
 	char *kname;
 	int len;
+	BUILD_BUG_ON(offsetof(struct filename, iname) % sizeof(long) != 0);
 
 	result = audit_reusename(filename);
 	if (result)
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ae681002100a..d522e6391855 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2245,7 +2245,10 @@ struct filename {
 	const char		*name;	/* pointer to actual string */
 	const __user char	*uptr;	/* original userland pointer */
 	struct audit_names	*aname;
-	int			refcnt;
+	union {
+		int		refcnt;
+		long		__padding;
+	};
 	const char		iname[];
 };
 
-- 
2.1.4