linux-ext4 - Re: [PATCH 5/5] libext2fs: avoid 32-bit overflow in ext2fs

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Date:	Tue, 15 Jan 2013 23:33:31 +0800
From:	Zheng Liu <gnehzuil.liu@...il.com>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 5/5] libext2fs: avoid 32-bit overflow in
 ext2fs_initialize with a 512M cluster size

On Mon, Jan 14, 2013 at 07:37:12PM -0500, Theodore Ts'o wrote:
> If the user attemps to create a 512MB cluster, we need to adjust the
> defaults to avoid a 32-bit overflow of s_blocks_per_group.  Also check
> to make sure that the caller of ext2fs_initialize() has not given a
> value of s_clusters_per_group that would result in an overflow of
> s_blocks_per_group.
> 
> Signed-off-by: "Theodore Ts'o" <tytso@....edu>

The patch itself looks good to me.  Feel free to add:
Reviewed-by: Zheng Liu <wenqing.lz@...bao.com>

FWIW, I wonder why we need to add such complex logical to handle a
corner case.  I guess no one wants to use a 512MB cluster.  So changing
max cluster size from 512MB to 256MB is very simple and straightfoward.

Regards,
                                                - Zheng

> ---
>  lib/ext2fs/initialize.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/lib/ext2fs/initialize.c b/lib/ext2fs/initialize.c
> index b0c15d2..5afdc27 100644
> --- a/lib/ext2fs/initialize.c
> +++ b/lib/ext2fs/initialize.c
> @@ -207,6 +207,8 @@ errcode_t ext2fs_initialize(const char *name, int flags,
>  		super->s_log_block_size;
>  
>  	if (bigalloc_flag) {
> +		unsigned long long bpg;
> +
>  		if (param->s_blocks_per_group &&
>  		    param->s_clusters_per_group &&
>  		    ((param->s_clusters_per_group * EXT2FS_CLUSTER_RATIO(fs)) !=
> @@ -220,12 +222,19 @@ errcode_t ext2fs_initialize(const char *name, int flags,
>  			super->s_clusters_per_group = 
>  				param->s_blocks_per_group /
>  				EXT2FS_CLUSTER_RATIO(fs);
> -		else
> +		else if (super->s_log_cluster_size + 15 < 32)
>  			super->s_clusters_per_group = fs->blocksize * 8;
> +		else
> +			super->s_clusters_per_group = (fs->blocksize - 1) * 8;
>  		if (super->s_clusters_per_group > EXT2_MAX_CLUSTERS_PER_GROUP(super))
>  			super->s_clusters_per_group = EXT2_MAX_CLUSTERS_PER_GROUP(super);
> -		super->s_blocks_per_group = EXT2FS_C2B(fs,
> -				       super->s_clusters_per_group);
> +		bpg = EXT2FS_C2B(fs,
> +			(unsigned long long) super->s_clusters_per_group);
> +		if (bpg >= (((unsigned long long) 1) << 32)) {
> +			retval = EXT2_ET_INVALID_ARGUMENT;
> +			goto cleanup;
> +		}
> +		super->s_blocks_per_group = bpg;
>  	} else {
>  		set_field(s_blocks_per_group, fs->blocksize * 8);
>  		if (super->s_blocks_per_group > EXT2_MAX_BLOCKS_PER_GROUP(super))
> -- 
> 1.7.12.rc0.22.gcdd159b
> 
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