Re: [BUG]Missing i_sb NULL pointer check in destroy_inode()

[Mingming Cao <cmm@us.ibm.com>]

On Tue, 2003-11-25 at 00:36, Christoph Hellwig wrote:
> On Mon, Nov 24, 2003 at 11:00:38AM -0800, Mingming Cao wrote:
> > Hello, Andrew, Marcelo,
> > 
> > destroy_inode() dereferences inode->i_sb without checking if it is NULL.
> > This is inconsistent with its caller: iput() and clear_inode(),  both of
> > which check inode->i_sb before dereferencing it. Since iput() calls
> > destroy_inode() after calling file system's .clear_inode method(via
> > clear_inode()),  some file systems might choose to clear the i_sb in the
> > .clear_inode super block operation. This results in a crash in
> > destroy_inode().
> > 
> > This issue exists in both 2.6, 2.4 and 2.4 kernel.  A simple fix against
> > 2.6.0-test9 is included below. 2.4 based fix should be very similar to
> > this one.  Please take a look and consider include it.  
> 
> inode->i_sb can't be NULL.  We should remove all those checks.
> 
Sorry I can not agree with this. Maybe the inode->i_sb should not be
NULL, but the kernel still allows the file system to do so.  In fact
JFS's diReadSpecial() function clears the inode->i_sb to NULL before
calling iput().  

Acutally iput() in 2.6 is missing the check too.(in 2.4 the check is
there).  Here is the the incremental fix for 2.6 only:

diff -urNp linux-2.6.0-test10/fs/inode.c a/fs/inode.c
--- linux-2.6.0-test10/fs/inode.c	2003-11-23 17:33:24.000000000 -0800
+++ a/fs/inode.c	2003-11-26 13:59:34.000000000 -0800
@@ -1084,13 +1084,13 @@ static inline void iput_final(struct ino
 void iput(struct inode *inode)
 {
 	if (inode) {
-		struct super_operations *op = inode->i_sb->s_op;
-
+		struct super_block *sb = inode->i_sb;
+		
 		if (inode->i_state == I_CLEAR)
 			BUG();
 
-		if (op && op->put_inode)
-			op->put_inode(inode);
+		if (sb && sb->op && sb->op->put_inode)
+			sb->op->put_inode(inode);
 
 		if (atomic_dec_and_lock(&inode->i_count, &inode_lock))
 			iput_final(inode);