Date:	Fri, 24 Feb 2012 20:37:03 +0100 (CET)
From:	Lukas Czerner <lczerner@...hat.com>
To:	Eric Sandeen <sandeen@...hat.com>
cc:	Lukas Czerner <lczerner@...hat.com>, linux-ext4@...r.kernel.org,
	tytso@....edu
Subject: Re: [PATCH] ext3: fix start and len arguments handling in
 ext3_trim_fs()

On Fri, 24 Feb 2012, Eric Sandeen wrote:

> On 2/24/12 10:33 AM, Lukas Czerner wrote:
> > The overflow can happen when we are calling get_group_no_and_offset()
> > which stores the result of do_div() in 32 bit long type. However the
> > result might be bigger than that if big blocknr is passed in. This will
> > most likely happen when calling FITRIM with the default argument len =
> > ULLONG_MAX.
> 
> I think that it's not the do_div() that is the problem; it's:
> 
> static void ext3_get_group_no_and_offset(struct super_block *sb,
>         ext3_fsblk_t blocknr, unsigned long *blockgrpp, ext3_grpblk_t *offsetp)
> 
> and
> 
> typedef unsigned long ext3_fsblk_t;
> 
> so if we do:
> 
>         ext3_get_group_no_and_offset(sb, (ext3_fsblk_t) (start + len),
>                                      &last_group, &last_block);
> 
> with:
> 
>         uint64_t start, len, minlen, trimmed;
> 
> we may overflow the "blocknr" arg well before we get to the do_div, for a large start or len.
> 
> so in the end this makes sense:
> 
> > +	if (unlikely(end >= max_blks))
> > +		end = max_blks - 1;
> 
> but I think the commit message needs fixing :)
> 
> -Eric

You're right I screwed the description. Will resend next week.

Thanks for review Eric.
-Lukas

> 
> > Fix this by using "end" variable instead of "start+len" as it is easier
> > to get right and specifically check that the end is not beyond the end
> > of the file system, so we are sure that the result of
> > get_group_no_and_offset() will not overflow. Otherwise truncate it to
> > the size of the file system.
> > 
> > Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> > ---
> >  fs/ext3/balloc.c |   59 +++++++++++++++++++++++++++--------------------------
> >  1 files changed, 30 insertions(+), 29 deletions(-)
> > 
> > diff --git a/fs/ext3/balloc.c b/fs/ext3/balloc.c
> > index a203892..2ad59c6 100644
> > --- a/fs/ext3/balloc.c
> > +++ b/fs/ext3/balloc.c
> > @@ -2091,71 +2091,72 @@ err_out:
> >   */
> >  int ext3_trim_fs(struct super_block *sb, struct fstrim_range *range)
> >  {
> > -	ext3_grpblk_t last_block, first_block, free_blocks;
> > -	unsigned long first_group, last_group;
> > -	unsigned long group, ngroups;
> > +	ext3_grpblk_t last_block, first_block;
> > +	unsigned long group, first_group, last_group;
> >  	struct ext3_group_desc *gdp;
> >  	struct ext3_super_block *es = EXT3_SB(sb)->s_es;
> > -	uint64_t start, len, minlen, trimmed;
> > +	uint64_t start, minlen, end, trimmed = 0;
> > +	ext3_fsblk_t first_data_blk =
> > +			le32_to_cpu(EXT3_SB(sb)->s_es->s_first_data_block);
> >  	ext3_fsblk_t max_blks = le32_to_cpu(es->s_blocks_count);
> >  	int ret = 0;
> >  
> > -	start = (range->start >> sb->s_blocksize_bits) +
> > -		le32_to_cpu(es->s_first_data_block);
> > -	len = range->len >> sb->s_blocksize_bits;
> > +	start = range->start >> sb->s_blocksize_bits;
> > +	end = start + (range->len >> sb->s_blocksize_bits) - 1;
> >  	minlen = range->minlen >> sb->s_blocksize_bits;
> > -	trimmed = 0;
> >  
> > -	if (unlikely(minlen > EXT3_BLOCKS_PER_GROUP(sb)))
> > +	if (unlikely(minlen > EXT3_BLOCKS_PER_GROUP(sb)) ||
> > +	    unlikely(start >= max_blks))
> >  		return -EINVAL;
> > -	if (start >= max_blks)
> > -		return -EINVAL;
> > -	if (start + len > max_blks)
> > -		len = max_blks - start;
> > +	if (unlikely(end >= max_blks))
> > +		end = max_blks - 1;
> > +	if (end <= first_data_blk)
> > +		return 0;
> > +	if (start < first_data_blk)
> > +		start = first_data_blk;
> >  
> > -	ngroups = EXT3_SB(sb)->s_groups_count;
> >  	smp_rmb();
> >  
> >  	/* Determine first and last group to examine based on start and len */
> >  	ext3_get_group_no_and_offset(sb, (ext3_fsblk_t) start,
> >  				     &first_group, &first_block);
> > -	ext3_get_group_no_and_offset(sb, (ext3_fsblk_t) (start + len),
> > +	ext3_get_group_no_and_offset(sb, (ext3_fsblk_t) end,
> >  				     &last_group, &last_block);
> > -	last_group = (last_group > ngroups - 1) ? ngroups - 1 : last_group;
> > -	last_block = EXT3_BLOCKS_PER_GROUP(sb);
> >  
> > -	if (first_group > last_group)
> > -		return -EINVAL;
> > +	/* The last block to discard in the group */
> > +	end = EXT3_BLOCKS_PER_GROUP(sb);
> >  
> >  	for (group = first_group; group <= last_group; group++) {
> >  		gdp = ext3_get_group_desc(sb, group, NULL);
> >  		if (!gdp)
> >  			break;
> >  
> > -		free_blocks = le16_to_cpu(gdp->bg_free_blocks_count);
> > -		if (free_blocks < minlen)
> > +		if (le16_to_cpu(gdp->bg_free_blocks_count) < minlen)
> >  			continue;
> >  
> >  		/*
> >  		 * For all the groups except the last one, last block will
> >  		 * always be EXT3_BLOCKS_PER_GROUP(sb), so we only need to
> > -		 * change it for the last group in which case first_block +
> > -		 * len < EXT3_BLOCKS_PER_GROUP(sb).
> > +		 * change it for the last group, note that last_block is
> > +		 * already computed earlier by ext3_get_group_no_and_offset()
> >  		 */
> > -		if (first_block + len < EXT3_BLOCKS_PER_GROUP(sb))
> > -			last_block = first_block + len;
> > -		len -= last_block - first_block;
> > +		if (group == last_group)
> > +			end = last_block;
> >  
> >  		ret = ext3_trim_all_free(sb, group, first_block,
> > -					last_block, minlen);
> > +					 end, minlen);
> >  		if (ret < 0)
> >  			break;
> > -
> >  		trimmed += ret;
> > +
> > +		/*
> > +		 * For every group except the first one, we are sure
> > +		 * that the first block to discard will be block #0.
> > +		 */
> >  		first_block = 0;
> >  	}
> >  
> > -	if (ret >= 0)
> > +	if (ret > 0)
> >  		ret = 0;
> >  	range->len = trimmed * sb->s_blocksize;
> >  
> 
> 

-- 
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Powered by blists - more mailing lists