linux-kernel - Re: [patch 3/5] fuse: fix race in llseek

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Message-Id: <E1JqV2D-0004vS-Ny@pomaz-ex.szeredi.hu>
Date:	Mon, 28 Apr 2008 17:12:53 +0200
From:	Miklos Szeredi <miklos@...redi.hu>
To:	jblunck@...e.de
CC:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 3/5] fuse: fix race in llseek

> > From: Miklos Szeredi <mszeredi@...e.cz>
> > 
> > Fuse doesn't use i_mutex to protect setting i_size, and so
> > generic_file_llseek() can be racy: it doesn't use i_size_read().
> > 
> > So do a fuse specific llseek method, which does use i_size_read().
> > 
> 
> Is there any specific reason why we don't use i_size_read() in 
> generic_file_llseek()?

Just to save some cycles on 32bit archs.  Considering that
lseek(SEEK_END) isn't a very performance sensitive operation...

> If no, why don't you fix it instead?

...we could do that.  Does anybody object?

Miklos
----


Some filesystems (e.g. fuse) don't use i_mutex to protect setting
i_size, and so generic_file_llseek() can be racy.

Since lseek(SEEK_END) isn't a very performance sensitive operation, do
this in generic_file_llseek(), instead of requiring such filesystems
to duplicate this function.

Signed-off-by: Miklos Szeredi <mszeredi@...e.cz>
---
 fs/read_write.c |    6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

Index: linux/fs/read_write.c
===================================================================
--- linux.orig/fs/read_write.c	2008-04-17 04:49:44.000000000 +0200
+++ linux/fs/read_write.c	2008-04-28 17:08:25.000000000 +0200
@@ -39,7 +39,11 @@ loff_t generic_file_llseek(struct file *
 	mutex_lock(&inode->i_mutex);
 	switch (origin) {
 		case SEEK_END:
-			offset += inode->i_size;
+			/*
+			 * Some filesystems use a different lock as i_mutex
+			 * to protect writing i_size
+			 */
+			offset += i_size_read(inode);
 			break;
 		case SEEK_CUR:
 			offset += file->f_pos;
--
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