linux-ext4 - Re: [PATCH] ext4: avoid oversized shift in ext4_fill_flex

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Message-Id: <C0DF217A-64C8-49E3-B979-652DB2C362C3@gmail.com>
Date:	Mon, 26 Dec 2011 00:20:59 -0700
From:	Andreas Dilger <aedilger@...il.com>
To:	Xi Wang <xi.wang@...il.com>
Cc:	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Xi Wang <xi.wang@...il.com>
Subject: Re: [PATCH] ext4: avoid oversized shift in ext4_fill_flex_info()

On 2011-12-25, at 23:36, Xi Wang <xi.wang@...il.com> wrote:

> Commit 503358ae fixed a division by zero, but groups_per_flex still
> overflows due to an oversized shift, given a large s_log_groups_per_flex
> like 36.  (1 << 36) is undefined in C; the result may vary depending
> on the architecture, e.g., 16 on x86, thus bypassing the sanity check
> (groups_per_flex < 2).

While this is true in theory, it is not possible to have 2^32 groups per flex group. This would mean 2^32 block bitmaps and inode bitmaps in a single group, which is impossible. 

> Signed-off-by: Xi Wang <xi.wang@...il.com>
> ---
> fs/ext4/super.c |    9 +++++----
> 1 files changed, 5 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 3e1329e..6deaf41 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -2010,14 +2010,15 @@ static int ext4_fill_flex_info(struct super_block *sb)
>    size_t size;
>    int i;
> 
> -    sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
> -    groups_per_flex = 1 << sbi->s_log_groups_per_flex;
> -
> -    if (groups_per_flex < 2) {
> +    if (sbi->s_es->s_log_groups_per_flex == 0 ||
> +        sbi->s_es->s_log_groups_per_flex >= 32) {
>        sbi->s_log_groups_per_flex = 0;
>        return 1;
>    }
> 
> +    sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
> +    groups_per_flex = 1 << 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) +
>            ((le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) + 1) <<
> -- 
> 1.7.5.4
> 
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