linux-kernel - Re: [PATCH] vfs: partially sanitize i

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Date: Tue, 11 Jun 2024 20:56:40 +1000
From: Dave Chinner <david@...morbit.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: Jan Kara <jack@...e.cz>, brauner@...nel.org, viro@...iv.linux.org.uk,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] vfs: partially sanitize i_state zeroing on inode creation

On Tue, Jun 11, 2024 at 12:23:59PM +0200, Mateusz Guzik wrote:
> On Tue, Jun 11, 2024 at 12:02:22PM +0200, Jan Kara wrote:
> > On Tue 11-06-24 06:15:40, Mateusz Guzik wrote:
> > > new_inode used to have the following:
> > > 	spin_lock(&inode_lock);
> > > 	inodes_stat.nr_inodes++;
> > > 	list_add(&inode->i_list, &inode_in_use);
> > > 	list_add(&inode->i_sb_list, &sb->s_inodes);
> > > 	inode->i_ino = ++last_ino;
> > > 	inode->i_state = 0;
> > > 	spin_unlock(&inode_lock);
> > > 
> > > over time things disappeared, got moved around or got replaced (global
> > > inode lock with a per-inode lock), eventually this got reduced to:
> > > 	spin_lock(&inode->i_lock);
> > > 	inode->i_state = 0;
> > > 	spin_unlock(&inode->i_lock);
> > > 
> > > But the lock acquire here does not synchronize against anyone.
> > > 
> > > Additionally iget5_locked performs i_state = 0 assignment without any
> > > locks to begin with and the two combined look confusing at best.
> > > 
> > > It looks like the current state is a leftover which was not cleaned up.
> > > 
> > > Ideally it would be an invariant that i_state == 0 to begin with, but
> > > achieving that would require dealing with all filesystem alloc handlers
> > > one by one.
> > > 
> > > In the meantime drop the misleading locking and move i_state zeroing to
> > > alloc_inode so that others don't need to deal with it by hand.
> > > 
> > > Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> > 
> > Good point. But the initialization would seem more natural in
> > inode_init_always(), wouldn't it? And that will also address your "FIXME"
> > comment.
> > 
> 
> My point is that by the time the inode is destroyed some of the fields
> like i_state should be set to a well-known value, this one preferably
> plain 0.
>
> I did not patch inode_init_always because it is exported and xfs uses it
> in 2 spots, only one of which zeroing the thing immediately after.
> Another one is a little more involved, it probably would not be a
> problem as the value is set altered later anyway, but I don't want to
> mess with semantics of the func if it can be easily avoided.

Better to move the zeroing to inode_init_always(), do the basic
save/restore mod to xfs_reinit_inode(), and let us XFS people worry
about whether inode_init_always() is the right thing to be calling
in their code...

All you'd need to do in xfs_reinit_inode() is this

+	unsigned long	state = inode->i_state;

	.....
	error = inode_init_always(mp->m_super, inode);
	.....
+	inode->i_state = state;
	.....

And it should behave as expected.

-Dave.
-- 
Dave Chinner
david@...morbit.com