linux-ext4 - Re: [PATCH] ext4: fix undefined bit shift result in ext4_fill_flex

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Date:	Wed, 3 Oct 2012 11:18:54 -0300
From:	Carlos Maiolino <cmaiolino@...hat.com>
To:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: fix undefined bit shift result in
 ext4_fill_flex_info

On Wed, Oct 03, 2012 at 09:11:47AM +0200, Lukas Czerner wrote:
> The result of the bit shift expression in
> '1 << sbi->s_log_groups_per_flex' can be undefined in the case that
> s_log_groups_per_flex is 31 because the result of the shift is bigger
> than INT_MAX. In reality this probably should not cause much problems
> since we'll end up with INT_MIN which will then be converted into
> 'unsigned int' type, but nevertheless according to the ISO C99 the
> result is actually undefined.
> 
> Fix this by changing the left operand to 'unsigned int' type.
> 
> Note that the commit d50f2ab6f050311dbf7b8f5501b25f0bf64a439b already
> tried to fix the undefined behaviour, but this was missed.
> 
> Thanks to Laszlo Ersek for pointing this out and suggesting the fix.
> 
> Signed-off-by: Lukas Czerner <lczerner@...hat.com>
> Reported-by: Laszlo Ersek <lersek@...hat.com>
> ---
>  fs/ext4/super.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 69c55d4..95b9c8e 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1929,7 +1929,7 @@ static int ext4_fill_flex_info(struct super_block *sb)
>  		sbi->s_log_groups_per_flex = 0;
>  		return 1;
>  	}
> -	groups_per_flex = 1 << sbi->s_log_groups_per_flex;
> +	groups_per_flex = 1U << sbi->s_log_groups_per_flex;
>  
>  	/* We allocate both existing and potentially added groups */
>  	flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) +

Looks good,

Reviewed-by: Carlos Maiolino <cmaiolino@...hat.com>
-- 
--Carlos
--
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