linux-kernel - Re: [PATCH 1/4] fs: befs: Remove redundant validation from befs_find_brun

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Date:	Sat, 2 Jul 2016 14:34:43 +0100
From:	Salah Triki <salah.triki@....org>
To:	Luis de Bethencourt <luisbg@....samsung.com>
Cc:	Salah Triki <salah.triki@...il.com>, akpm@...ux-foundation.org,
	viro@...iv.linux.org.uk, mhocko@...e.com, vdavydov@...tuozzo.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/4] fs: befs: Remove redundant validation from
 befs_find_brun_direct

On Sat, Jul 02, 2016 at 12:38:18PM +0100, Luis de Bethencourt wrote:
> On 02/07/16 09:05, Salah Triki wrote:
> > The only caller of befs_find_brun_direct is befs_fblock2brun, which
> > already validates that the block is within the range of direct blocks.
> > So remove the duplicate validation.
> > 
> > Signed-off-by: Salah Triki <salah.triki@...il.com>
> > ---
> >  fs/befs/datastream.c | 8 --------
> >  1 file changed, 8 deletions(-)
> > 
> > diff --git a/fs/befs/datastream.c b/fs/befs/datastream.c
> > index 26cc417..e224b9a 100644
> > --- a/fs/befs/datastream.c
> > +++ b/fs/befs/datastream.c
> > @@ -249,17 +249,9 @@ befs_find_brun_direct(struct super_block *sb, const befs_data_stream *data,
> >  	int i;
> >  	const befs_block_run *array = data->direct;
> >  	befs_blocknr_t sum;
> > -	befs_blocknr_t max_block =
> > -	    data->max_direct_range >> BEFS_SB(sb)->block_shift;
> >  
> >  	befs_debug(sb, "---> %s, find %lu", __func__, (unsigned long)blockno);
> >  
> > -	if (blockno > max_block) {
> > -		befs_error(sb, "%s passed block outside of direct region",
> > -			   __func__);
> > -		return BEFS_ERR;
> > -	}
> > -
> >  	for (i = 0, sum = 0; i < BEFS_NUM_DIRECT_BLOCKS;
> >  	     sum += array[i].len, i++) {
> >  		if (blockno >= sum && blockno < sum + (array[i].len)) {
> > 
> 
> Hi Salah,
> 
> These aren't the same check though. If we ignore the BEFS_SB(sb)->block_shift just to
> comparing them, we can consider the checks to be the following.
> 
> In befs_fblock2brun():
> if (fblock < data->max_direct_range)
> 
> In befs_find_brun_direct():
> if (fblock > data->max_direct_range)
> 
> Notice how one checks if the block is past the range, and the other checks if it isn't
> before it.
> 
> They also looked similar to me the first time I saw them and I had to double-check :)
> 
> Thanks,
> Luis
> 
> 

the two checks could not be evaluated to true at the same time, and since befs_find_brun_direct 
is called only when the first check is passed, the second check will be always evaluated to false.
So I think the second check is useless, am I right ? 

Salah