linux-ext4 - Re: [PATCH] ext4: fix error when itable blocks is greater than s_itb_per

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Date:   Sat, 6 Aug 2022 12:02:22 +0800
From:   Michael Wu <michael@...winnertech.com>
To:     Theodore Ts'o <tytso@....edu>, Lukas Czerner <lczerner@...hat.com>
Cc:     adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        allwinner-opensource-support@...winnertech.com
Subject: Re: [PATCH] ext4: fix error when itable blocks is greater than
 s_itb_per_group

On 8/4/2022 10:19 AM, Theodore Ts'o wrote:
> On Wed, Aug 03, 2022 at 09:18:59AM +0200, Lukas Czerner wrote:
>>
>> Hi Michael,
>>
>> mke2fs is making sure that we completely fill the inote table blocks.
>> This is a corrupted image and so AFAICT ext4 is doing the right thing
>> here. There does not seem to be a problem to fix, unless you can somehow
>> trick mke2fs to make a file system like this.
> 
> Several years ago, android was shipping a bogus/busted
> reimeplementation of mke2fs, reportedly because a certain founder of
> Android (cough, Andy Rubin, cough) was alergic to the GPL.  ("The
> problem with GPL in embedded systems [such as smartphones and tablets]
> is that it's viral...")  This bogus reimplementation would create file
> systems where the number of inodes per block group was a multiple of 4
> instead of 8.  But, it was under the BSD license, so it was all good!   :-/
> 
> This bogus reimplementation of mkfs would, 50% of the time, create
> busted file systems which couldn't be fixed, if they got corrupted, by
> e2fsck.  This is because e2fsprogs' allocation bitmap code assumes
> that you can back the bitarray into a single contiguous memory block
> --- and this doesn't work if the number of inodes per block group is
> not a multiple of 8.  If the file system got corrupted, the only
> recourse was to wipe the user partition and the user would lose any
> data that wasn't backed up to the cloud.
> 
> This has since been fixed for quite some time, but if there is some
> low-end Android manufacturer is using an ancient version of AOSP, this
> could be happening even in 2022 --- but that doesn't mean we need to
> support such broken file systems.  As far as I'm concerned the only
> way to make valid Android ext4 system images is the combination of
> mke2fs and e2fsdroid, which is what modern versions of AOSP do.
> 
>      	       	     	     	    	     	- Ted

Dear Ted & Lukas,
Thanks for your clarification. I did several tests, turned outs Ted was 
right. I'm clear now.

-- 
Regards,
Michael Wu