linux-kernel - Re: [PATCH] DLM: fix a couple of races

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Message-Id: <1178611258.7476.5.camel@quoit>
Date:	Tue, 08 May 2007 09:00:58 +0100
From:	Steven Whitehouse <steve@...gwyn.com>
To:	Satyam Sharma <ssatyam@....iitk.ac.in>
Cc:	linux-kernel@...r.kernel.org, teigland@...hat.com,
	linux-cluster@...hat.com, joel.becker@...cle.com
Subject: Re: [PATCH] DLM: fix a couple of races

Hi,

Added to the GFS2 -nmw git tree, thanks. Please remember to add a
Signed-off-by line for future patches - I've added it for you this time,

Steve.

On Fri, 2007-05-04 at 21:49 +0530, Satyam Sharma wrote:
> Hi,
> 
> There are the following two trivially-fixed races in fs/dlm/config.c:
> 
> 1. The configfs subsystem semaphore must be held by the caller when 
> calling config_group_find_obj(). It's needed to walk the subsystem 
> hierarchy without racing with a simultaneous mkdir(2) or rmdir(2). I 
> looked around to see if there was some other way we were avoiding this 
> race, but couldn't find any.
> 
> 2. get_comm() does hold the subsystem semaphore but lets go too soon -- 
> before grabbing a reference on the found config_item. A concurrent 
> rmdir(2) could come and release the comm after the up() but before the 
> config_item_get().
> 
> Patch that fixes both these bugs below.
> 
> Cheers,
> S
> 
> PS: For some reason, configfs still uses a struct semaphore (as a binary 
> semaphore) for configfs_subsystem.su_sem. Someone with free time should 
> convert that to a struct mutex, say configfs_subsystem.su_mtx -- which is 
> the preferred way to use (binary) mutexes presently. CC'ing Joel Becker on 
> this.
> 
> ---
> 
> Fix two races in fs/dlm/config.c:
> 
> (1) Grab the configfs subsystem semaphore before calling 
> config_group_find_obj() in get_space(). This solves a potential race 
> between get_space() and concurrent mkdir(2) or rmdir(2).
> 
> (2) Grab a reference on the found config_item _while_ holding the configfs 
> subsystem semaphore in get_comm(), and not after it. This solves a 
> potential race between get_comm() and concurrent rmdir(2).
> 
>   fs/dlm/config.c |   15 +++++++++++----
>   1 file changed, 11 insertions(+), 4 deletions(-)
> 
>  	Signed-off-by: Satyam Sharma <ssatyam@....iitk.ac.in>
> 
> ---
> 
> diff -ruNp linux-2.6.21.1/fs/dlm/config.c linux-2.6.21.1~patch/fs/dlm/config.c
> --- linux-2.6.21.1/fs/dlm/config.c	2007-04-26 08:38:32.000000000 +0530
> +++ linux-2.6.21.1~patch/fs/dlm/config.c	2007-05-04 21:08:54.000000000 +0530
> @@ -744,9 +744,16 @@ static ssize_t node_weight_write(struct
> 
>   static struct space *get_space(char *name)
>   {
> +	struct config_item *i;
> +
>   	if (!space_list)
>   		return NULL;
> -	return to_space(config_group_find_obj(space_list, name));
> +
> +	down(&space_list->cg_subsys->su_sem);
> +	i = config_group_find_obj(space_list, name);
> +	up(&space_list->cg_subsys->su_sem);
> +
> +	return to_space(i);
>   }
> 
>   static void put_space(struct space *sp)
> @@ -772,20 +779,20 @@ static struct comm *get_comm(int nodeid,
>   			if (cm->nodeid != nodeid)
>   				continue;
>   			found = 1;
> +			config_item_get(i);
>   			break;
>   		} else {
>   			if (!cm->addr_count ||
>   			    memcmp(cm->addr[0], addr, sizeof(*addr)))
>   				continue;
>   			found = 1;
> +			config_item_get(i);
>   			break;
>   		}
>   	}
>   	up(&clusters_root.subsys.su_sem);
> 
> -	if (found)
> -		config_item_get(i);
> -	else
> +	if (!found)
>   		cm = NULL;
>   	return cm;
>   }
> 
> -
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