[PATCH] Re: Race in fs/proc/generic.c:make_inode_number()

From: Tom Leete <tleete@mountain.net>
To: Linus Torvalds <torvalds@transmeta.com>,
	Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: linux-kernel@vger.kernel.org
Subject: [PATCH] Re: Race in fs/proc/generic.c:make_inode_number()
Date: Thu, 05 Apr 2001 10:36:10 -0400	[thread overview]
Message-ID: <3ACC82DA.11D76D45@mountain.net> (raw)
In-Reply-To: <3ACBFF4C.97AA345F@mountain.net>

I wrote:
> 
> The proc_alloc_map bitfield is unprotected by any lock, and
> find_first_zero_bit() is not atomic. Concurrent module loading can race
> here.

Hello,

Here is a patch for this. It looks like callers are always in user context
(kmalloc flag GFP_KERNEL), so I used a light spinlock.

Cheers,
Tom
-- 
The Daemons lurk and are dumb. -- Emerson

--- linux-2.4.3/fs/proc/generic.c.orig	Thu Apr  5 10:03:02 2001
+++ linux-2.4.3/fs/proc/generic.c	Thu Apr  5 10:22:48 2001
@@ -192,13 +192,22 @@
 
 static unsigned char proc_alloc_map[PROC_NDYNAMIC / 8];
 
+spinlock_t proc_alloc_map_lock = RW_LOCK_UNLOCKED;
+
 static int make_inode_number(void)
 {
-	int i = find_first_zero_bit((void *) proc_alloc_map, PROC_NDYNAMIC);
-	if (i<0 || i>=PROC_NDYNAMIC) 
-		return -1;
+	int i;
+	spin_lock(&proc_alloc_map_lock);
+	i = find_first_zero_bit((void *) proc_alloc_map, PROC_NDYNAMIC);
+	if (i<0 || i>=PROC_NDYNAMIC) {
+		i = -1;
+		goto out;
+	}
 	set_bit(i, (void *) proc_alloc_map);
-	return PROC_DYNAMIC_FIRST + i;
+	i += PROC_DYNAMIC_FIRST;
+out:
+	spin_unlock(&proc_alloc_map_lock);
+	return i;
 }
 
 static int proc_readlink(struct dentry *dentry, char *buffer, int buflen)
