Kernel-4.14: With ubuntu-18.04 building rootfs images and booting gives SQUASHFS error: xz decompression failed, data probably corrupt

[Phillip Lougher]

On 14/11/2021 07:06, Pintu Agarwal wrote:
> + Adding squashfs-devel to get opinion from squashfs side.
> 
> On Fri, 12 Nov 2021 at 12:48, Pintu Agarwal <pintu.ping at gmail.com> wrote:
>>
>> Hi,
>>
>> On Tue, 9 Nov 2021 at 21:04, Pintu Agarwal <pintu.ping at gmail.com> wrote:
>>
>>>>> We only get these squashfs errors flooded in the boot logs:
>>>>> {{{
>>>>> ....
>>>>> [    5.153479] device-mapper: init: dm-0 is ready
>>>>> [    5.334282] VFS: Mounted root (squashfs filesystem) readonly on device 253:0.
>>>>> ....
>>>>> [    8.954120] SQUASHFS error: xz decompression failed, data probably corrupt
>>>>> [    8.954153] SQUASHFS error: squashfs_read_data failed to read block 0x1106
>>>>> [    8.970316] SQUASHFS error: Unable to read data cache entry [1106]
>>>>> [    8.970349] SQUASHFS error: Unable to read page, block 1106, size 776c
>>>>> [    8.980298] SQUASHFS error: Unable to read data cache entry [1106]
>>>>> [    8.981911] SQUASHFS error: Unable to read page, block 1106, size 776c
>>>>> [    8.988280] SQUASHFS error: Unable to read data cache entry [1106]
>>>>> ....
>>>>> }}}
>>>>>
>>
>> One more observation:
>> When I disable FEC flag in bootloader, I see the below error:
>> [    8.360791] device-mapper: verity: 253:0: data block 2 is corrupted
>> [    8.361134] device-mapper: verity: 253:0: data block 3 is corrupted
>> [    8.366016] SQUASHFS error: squashfs_read_data failed to read block 0x1106
>> [    8.379652] SQUASHFS error: Unable to read data cache entry [1106]
>> [    8.379680] SQUASHFS error: Unable to read page, block 1106, size 7770
>>
>> Also, now I see that the decompress error is gone, but the read error
>> is still there.
>>
>> This seems to me that dm-verity detects some corrupted blocks but with
>> FEC it auto corrects itself, how when dm-verity auto corrects itself,
>> the squashfs decompression algorithm somehow could not understand it.
>>
>> So, it seems like there is some mis-match between the way FEC
>> correction and the squashfs decompression happens ?
>>
>> Is this issue seen by anybody else here ?
>>
> 
> The squashfs version used by Kernel:
> [    0.355958] squashfs: version 4.0 (2009/01/31) Phillip Lougher
> 
> The squashfs version available on Ubuntu:
> mksquashfs version 4.3-git (2014/06/09)
> 
> The squashfs version used by Yocto 2.6:
> squashfs-tools/0001-squashfs-tools-Allow-setting-selinux-xattrs-through-.patch:61:
>     printf("mksquashfs version 4.3-git (2014/09/12)\n");
> 
> We create dm-verity squashfs image using version 4.3 whereas, the
> kernel uses 4.0 version to decompress it.
> Is there something missing here?
> 
> When FEC (Forward Error Correction) comes into picture, then squashfs
> decompress fails.
> When we remove FEC flag from dm-verity then decompress works but read
> error still occurs.
> This seems as if something is missing either in FEC handling or either
> in squashfs decompress logic.
> 
> Just wanted to know if there are any fixes already available in the
> mainline for this ?
> 
> 

As Squashfs maintainer I want you to stop randomly blaming anything and 
everything here.  You won't fix anything doing that.

In a previous email you stated


> 
> One quick observation:
> This issue is seen only when we enable dm-verity in our bootloader and
> cross-building the bootloader/kernel (with Yocto 2.6 toolchain
> arm-oe-linux-gnueabi-) on Ubuntu 18.04.
> The issue is *NOT* seen (on the same device) when building the
> dm-verity enabled kernel on Ubuntu 16.04.
>
> Is it something to do with the cross-toolchain difference between
> Ubuntu 16 and 18 ?
> 

If that is the case, then it is not an issue with Squashfs or any
kernel code, it is a build time issue and *that* is where you should
be concentrating your efforts.  Find out what differences are there.

You don't seem to understand that a Squashfs filesystem generated
by any Mksquashfs 4.X is mountable *without* errors on any kernel
since 2.6.29 (January 2009).  Looking for mismatches between
Mksquashfs and/or kernel version and blaming that for the above 
different behaviour is a complete waste of time.

Phillip