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Date:   Tue, 21 Jul 2020 11:58:26 +0200
From:   David Sterba <dsterba@suse.cz>
To:     Qu Wenruo <quwenruo.btrfs@gmx.com>
Cc:     dsterba@suse.cz, Qu Wenruo <wqu@suse.com>,
        linux-btrfs@vger.kernel.org, Christian Zangl <coralllama@gmail.com>
Subject: Re: [PATCH 1/2] btrfs-progs: convert: Prevent bit overflow for
 cctx->total_bytes
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On Tue, Jul 21, 2020 at 07:51:00AM +0800, Qu Wenruo wrote:
> 
> 
> On 2020/7/21 上午12:09, David Sterba wrote:
> > On Mon, Jul 20, 2020 at 08:51:08PM +0800, Qu Wenruo wrote:
> >> --- a/convert/source-ext2.c
> >> +++ b/convert/source-ext2.c
> >> @@ -87,7 +87,8 @@ static int ext2_open_fs(struct btrfs_convert_context *cctx, const char *name)
> >>  	cctx->fs_data = ext2_fs;
> >>  	cctx->blocksize = ext2_fs->blocksize;
> >>  	cctx->block_count = ext2_fs->super->s_blocks_count;
> >> -	cctx->total_bytes = ext2_fs->blocksize * ext2_fs->super->s_blocks_count;
> >> +	cctx->total_bytes = (u64)ext2_fs->blocksize *
> >> +			    (u64)ext2_fs->super->s_blocks_count;
> > 
> > Do you need to cast both? Once one of the types is wide enough for the
> > result, there should be no loss.
> > 
> I just want to be extra safe.

Typecasts in code raise questions why are they needed, 'to be extra'
safe is not a good reason. One typecast in multiplication/shifts is a
common pattern to widen the result but two look more like lack of
understanding of the integer promotion rules.