linux-ext4 - Re: [PATCH 7/7] libext2fs: enforce the block group descriptor size in ext2fs

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Message-Id: <CD3F0352-A252-4427-AE7B-F71160BF77BC@whamcloud.com>
Date:	Wed, 1 Aug 2012 13:45:32 -0700
From:	Andreas Dilger <adilger@...mcloud.com>
To:	Theodore Ts'o <tytso@....edu>
Cc:	Ext4 Developers List <linux-ext4@...r.kernel.org>,
	"tony@...eyournoodle.com" <tony@...eyournoodle.com>
Subject: Re: [PATCH 7/7] libext2fs: enforce the block group descriptor size in ext2fs_open()

On 2012-07-31, at 13:09, Theodore Ts'o <tytso@....edu> wrote:
> On Tue, Jul 31, 2012 at 11:38:47AM -0700, Andreas Dilger wrote:
>>> +    /* Enforce the block group descriptor size */
>>> +    if (fs->super->s_feature_incompat & EXT4_FEATURE_INCOMPAT_64BIT) {
>>> +        if (fs->super->s_desc_size != EXT2_MIN_DESC_SIZE_64BIT) {
>> 
>> It doesn't necessarily make sense to limit this to the minimum size, only that it is at least the minimum size. 
> 
> I'm not at all convinced that the ext2fs library will do the right
> thing if the block group size is larger than what we expect.  At the
> very least we need to make sure it is a power of two, but even so I'd
> want to audit the code and do some experiments before I would hang my
> hat on this actually working correctly...

I'm pretty sure that we've always checked that it is a power-of-two value. The problem with this change is that it makes it much harder to increase the size again in the future .

It would definitely need another feature flag, but it isn't clear without more investigation (that i can't do on my phone) if a COMPAT flag would be enough (which we would likely have anyway if we needed a larger descriptor) or if we need a more restrictive feature flag. 

Cheers, Andreas--
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