linux-kernel - Re: [patch] ext4: underflow in alignment check

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Message-ID: <20160620160204.GG6882@quack2.suse.cz>
Date:	Mon, 20 Jun 2016 18:02:04 +0200
From:	Jan Kara <jack@...e.cz>
To:	Dan Carpenter <dan.carpenter@...cle.com>
Cc:	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	linux-ext4@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-janitors@...r.kernel.org
Subject: Re: [patch] ext4: underflow in alignment check

On Thu 16-06-16 10:07:09, Dan Carpenter wrote:
> My static checker complains that this can underflow if arg is negative
> which is true.
> 
> Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>

How come? (1 << 30) fits even into 32-bit signed type. So where's the
problem?

								Honza

> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index b1a3471..a2a17e9 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -1576,7 +1576,7 @@ static int handle_mount_opt(struct super_block *sb, char *opt, int token,
>  	} else if (token == Opt_min_batch_time) {
>  		sbi->s_min_batch_time = arg;
>  	} else if (token == Opt_inode_readahead_blks) {
> -		if (arg && (arg > (1 << 30) || !is_power_of_2(arg))) {
> +		if (arg && (arg > (1U << 30) || !is_power_of_2(arg))) {
>  			ext4_msg(sb, KERN_ERR,
>  				 "EXT4-fs: inode_readahead_blks must be "
>  				 "0 or a power of 2 smaller than 2^31");
> --
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-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR