linux-kernel - Re: [v3][PATCH 5/5] Merge code for single andmultiple-instancemounts

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Message-ID: <20090511221502.GA32530@linux.vnet.ibm.com>
Date:	Mon, 11 May 2009 15:15:02 -0700
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	Marc Dionne <marc.c.dionne@...il.com>
Cc:	Eric Paris <eparis@...hat.com>, Eric Paris <eparis@...isplace.org>,
	Christoph Hellwig <hch@....de>, Andrew Morton <akpm@...l.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, serue@...ibm.com,
	hpa@...or.com, sukadev@...ibm.com, linux-kernel@...r.kernel.org,
	jbacik@...hat.com
Subject: Re: [v3][PATCH 5/5] Merge code for single
	andmultiple-instancemounts

Marc Dionne [marc.c.dionne@...il.com] wrote:
> On 05/07/2009 07:21 PM, Sukadev Bhattiprolu wrote:
>> Er. I meant add following lines after the memset().
>>
>> |         opts->mode    = DEVPTS_DEFAULT_MODE;
>> |         opts->ptmxmode = DEVPTS_DEFAULT_PTMX_MODE;
>
> Adding those two lines (with . instead of ->) does fix the issue for me, 
> thanks.
>
> | > Of course we would still need to understand if/
> | > why this patch changes the settings.
>
> That particular patch changed things because the original memcpy in 
> new_pts_mount() did this:
>
> memcpy(&fsi->mount_opts, opts, sizeof(opts));
>
> where opts was a pointer, not a structure.  So only the first few bytes of 
> the blank opts actually got copied over.  The patch moved this memcpy to 
> devpts_get_sb() and in the process fixed the sizeof error.

Here is a cleaner fix - When user space (/etc/rc.sysinit on RHEL5) mounts
devpts, this problem might be masked since the mount system call could pass
a non-NULL 'data' parmeter to devpts_get_sb().

I tested this patch by directly calling the system call mount() with a NULL
data parameter. If you/Eric/Peter can confirm that this works for you, I will
send this patch to Andrew.

Thanks,
---
>From 2f7746f4df78ff57125c4714f0cd64e739ccf804 Mon Sep 17 00:00:00 2001
From: Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Date: Mon, 11 May 2009 13:11:11 -0700
Subject: [PATCH] devpts: Correctly set default options

devpts_get_sb() calls memset(0) to clear mount options and calls
parse_mount_options() if user specified any mount options. The memset(0) is
bogus since the 'mode' and 'ptmxmode' options are non-zero by default.
parse_mount_options() restores options to default anyway and can properly deal
with NULL mount options.

So in devpts_get_sb() remove memset(0) and call parse_mount_options() even for
NULL mount options.

Bug reported by Eric Paris: http://lkml.org/lkml/2009/5/7/448.

Signed-off-by: Sukadev Bhattiprolu (sukadev@...ibm.com)
---
 fs/devpts/inode.c |   18 ++++++++++++------
 1 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/fs/devpts/inode.c b/fs/devpts/inode.c
index 63a4a59..b7a954e 100644
--- a/fs/devpts/inode.c
+++ b/fs/devpts/inode.c
@@ -90,6 +90,15 @@ static inline struct super_block *pts_sb_from_inode(struct inode *inode)
 #define PARSE_MOUNT	0
 #define PARSE_REMOUNT	1
 
+/*
+ * parse_mount_options():
+ * 	Set @opts to mount options specified in @data. If an option is not
+ * 	specified in @data, set it to its default value. The exception is
+ * 	'newinstance' option which can only be set/cleared on a mount (i.e.
+ * 	cannot be changed during remount).
+ *
+ * Note: @data may be NULL (in which case all options are set to default).
+ */
 static int parse_mount_options(char *data, int op, struct pts_mount_opts *opts)
 {
 	char *p;
@@ -355,12 +364,9 @@ static int devpts_get_sb(struct file_system_type *fs_type,
 	struct pts_mount_opts opts;
 	struct super_block *s;
 
-	memset(&opts, 0, sizeof(opts));
-	if (data) {
-		error = parse_mount_options(data, PARSE_MOUNT, &opts);
-		if (error)
-			return error;
-	}
+	error = parse_mount_options(data, PARSE_MOUNT, &opts);
+	if (error)
+		return error;
 
 	if (opts.newinstance)
 		s = sget(fs_type, NULL, set_anon_super, NULL);
--
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