Re: [PATCH 1/1] ext4: fix potential negative array index in do_split()

[Lukas Czerner <lczerner@redhat.com>]

On Fri, Jun 19, 2020 at 08:41:22AM +0200, Lukas Czerner wrote:
> On Wed, Jun 17, 2020 at 02:19:04PM -0500, Eric Sandeen wrote:
> > If for any reason a directory passed to do_split() does not have enough
> > active entries to exceed half the size of the block, we can end up
> > iterating over all "count" entries without finding a split point.
> > 
> > In this case, count == move, and split will be zero, and we will
> > attempt a negative index into map[].
> > 
> > Guard against this by detecting this case, and falling back to
> > split-to-half-of-count instead; in this case we will still have
> > plenty of space (> half blocksize) in each split block.
> > 
> > Fixes: ef2b02d3e617 ("ext34: ensure do_split leaves enough free space in both blocks")
> > Signed-off-by: Eric Sandeen <sandeen@redhat.com>
> > ---
> > 
> > diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> > index a8aca4772aaa..8b60881f07ee 100644
> > --- a/fs/ext4/namei.c
> > +++ b/fs/ext4/namei.c
> > @@ -1858,7 +1858,7 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
> >  			     blocksize, hinfo, map);
> >  	map -= count;
> >  	dx_sort_map(map, count);
> > -	/* Split the existing block in the middle, size-wise */
> > +	/* Ensure that neither split block is over half full */
> >  	size = 0;
> >  	move = 0;
> >  	for (i = count-1; i >= 0; i--) {
> > @@ -1868,8 +1868,18 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
> >  		size += map[i].size;
> >  		move++;
> >  	}
> > -	/* map index at which we will split */
> > -	split = count - move;
> > +	/*
> > +	 * map index at which we will split
> > +	 *
> > +	 * If the sum of active entries didn't exceed half the block size, just
> > +	 * split it in half by count; each resulting block will have at least
> > +	 * half the space free.
> > +	 */
> > +	if (i > 0)
> > +		split = count - move;
> > +	else
> > +		split = count/2;
> 
> Won't we have exactly the same problem as we did before your commit
> ef2b02d3e617cb0400eedf2668f86215e1b0e6af ? Since we do not know how much
> space we actually moved we might have not made enough space for the new
> entry ?
> 
> Also since we have the move == count when the problem appears then it's
> clear that we never hit the condition
> 
> 1865 →       →       /* is more than half of this entry in 2nd half of the block? */
> 1866 →       →       if (size + map[i].size/2 > blocksize/2)
> 1867 →       →       →       break;
> 
> in the loop. This is surprising but it means the the entries must have
> gaps between them that are small enough that we can't fit the entry
> right in ? Should not we try to compact it before splitting, or is it
> the case that this should have been done somewhere else ?

The other possibility is that map[i].size is not right and indeed there
seems to be a bug in dx_make_map()

map_tail->size = le16_to_cpu(de->rec_len);

should be

map_tail->size = ext4_rec_len_from_disk(de->rec_len, blocksize));

right ? Otherwise with large enough records the size will be smaller
than it really is.

A quick look at fs/ext4/namei.c reveals couple of places there rec_len
is used without the conversion and we should check whether it needs
fixing.

-Lukas



> 
> If we really want ot be fair and we want to split it right in the middle
> of the entries size-wise then we need to keep track of of sum of the
> entries and decide based on that, not blocksize/2. But maybe the problem
> could be solved by compacting the entries together because the condition
> seems to rely on that.
> 
> -Lukas
> 
> > +
> >  	hash2 = map[split].hash;
> >  	continued = hash2 == map[split - 1].hash;
> >  	dxtrace(printk(KERN_INFO "Split block %lu at %x, %i/%i\n",
> > 
> > 
>