Date:	Fri, 9 Nov 2012 15:03:47 +0100 (CET)
From:	Lukáš Czerner <lczerner@...hat.com>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	tytso@....edu, zab@...hat.com
Subject: Re: [PATCH v2] ext4: Prevent race while waling extent tree

On Fri, 9 Nov 2012, Dmitry Monakhov wrote:

> Date: Fri, 09 Nov 2012 16:27:20 +0400
> From: Dmitry Monakhov <dmonakhov@...nvz.org>
> To: Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org
> Cc: tytso@....edu, zab@...hat.com, Lukas Czerner <lczerner@...hat.com>
> Subject: Re: [PATCH v2] ext4: Prevent race while waling extent tree
> 
> On Fri,  9 Nov 2012 11:38:53 +0100, Lukas Czerner <lczerner@...hat.com> wrote:
> > Currently ext4_ext_walk_space() only takes i_data_sem for read when
> > searching for the extent at given block with ext4_ext_find_extent().
> > Then it drops the lock and the extent tree can be changed at will.
> > However later on we're searching for the 'next' extent, but the extent
> > tree might already have changed, so the information might not be
> > accurate.
> > 
> > In fact we can hit BUG_ON(end <= start) if the extent got inserted into
> > the tree after the one we found and before the block we were searching
> > for. This has been reproduced by running xfstests 225 in loop on s390x
> > architecture, but theoretically we could hit this on any other
> > architecture as well, but probably not as often.
> > 
> > ext4_ext_walk_space() is currently only used from ext4_fiemap().
> > 
> > Fix this by extending the critical section to include
> > ext4_ext_next_allocated_block() as well. It means that if there are any
> > operation going on on the particular inode, the fiemap will return
> > inaccurate data. However this will also fix the concerns about starving
> > writers to the extent tree, because we will put and reacquire the
> > semaphore with every iteration. This will not be particularly fast, but
> > fiemap is not critical operation.
> See comments below
> > 
> > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> > ---
> > v2: Extend the critical section rather than put the whole function under
> >     the lock.
> > 
> >  fs/ext4/extents.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index 7011ac9..d444281 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -1978,7 +1978,6 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
> >  		/* find extent for this block */
> >  		down_read(&EXT4_I(inode)->i_data_sem);
> >  		path = ext4_ext_find_extent(inode, block, path);
> > -		up_read(&EXT4_I(inode)->i_data_sem);
> >  		if (IS_ERR(path)) {
> >  			err = PTR_ERR(path);
> >  			path = NULL;
> First of all: you should drop i_data_sem here, and in all other error
> handlers

Right, that what I get for rushing things up, sorry.

> > @@ -1993,6 +1992,7 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
> >  		}
> >  		ex = path[depth].p_ext;
> >  		next = ext4_ext_next_allocated_block(path);
> > +		up_read(&EXT4_I(inode)->i_data_sem);
> >  
> >  		exists = 0;
> >  		if (!ex) {
> > -- 
> > 1.7.7.6
> Also i believe that BUG_ON is still possible because after you drop
> i_data_sem, path[depth].p_ext may contains semi-random data
> (for example after i_depth change) so your previous fix was more
> intrusive, but 100% safe. IMHO it is safe to drop sem a bit later
> right after you have finished with 'path' on current iteration
> for example like this(caution i'm not test this patch):

This still isn't good enough because we're passing the reference to
the extent down to the callback function. And since it could move
since we acquired it, there might be garbage, or some other data
present leading to exposing it to the user space.

What we should do is to copy the extent to the local variable and
pass the pointed to it down to the callback function. This way it
should be safe.

-Lukas

> diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> index 7011ac9..2d2d2af 100644
> --- a/fs/ext4/extents.c
> +++ b/fs/ext4/extents.c
> @@ -1978,10 +1978,10 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
>  		/* find extent for this block */
>  		down_read(&EXT4_I(inode)->i_data_sem);
>  		path = ext4_ext_find_extent(inode, block, path);
> -		up_read(&EXT4_I(inode)->i_data_sem);
>  		if (IS_ERR(path)) {
>  			err = PTR_ERR(path);
>  			path = NULL;
> +			up_read(&EXT4_I(inode)->i_data_sem);
>  			break;
>  		}
>  
> @@ -1989,6 +1989,7 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
>  		if (unlikely(path[depth].p_hdr == NULL)) {
>  			EXT4_ERROR_INODE(inode, "path[%d].p_hdr == NULL", depth);
>  			err = -EIO;
> +			up_read(&EXT4_I(inode)->i_data_sem);
>  			break;
>  		}
>  		ex = path[depth].p_ext;
> @@ -2028,6 +2029,8 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
>  			BUG();
>  		}
>  		BUG_ON(end <= start);
> +		up_read(&EXT4_I(inode)->i_data_sem);
> +		BUG_ON(end <= start);
>  
>  		if (!exists) {
>  			cbex.ec_block = start;
> @@ -2045,7 +2048,6 @@ static int ext4_ext_walk_space(struct inode *inode, ext4_lblk_t block,
>  			break;
>  		}
>  		err = func(inode, next, &cbex, ex, cbdata);
> -		ext4_ext_drop_refs(path);
>  
>  		if (err < 0)
>  			break;
> 
> > 
> > --
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