linux-kernel - Re: [PATCH v3 2/2] erofs: add 'offset' mount option for file-backed & bdev-based mounts

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Message-ID: <d4eae031-8fbb-45e2-bdf4-f3a8a034b8ad@linux.alibaba.com>
Date: Mon, 7 Apr 2025 23:49:31 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: Karel Zak <kzak@...hat.com>, Sheng Yong <shengyong2021@...il.com>
Cc: xiang@...nel.org, chao@...nel.org, zbestahu@...il.com,
 jefflexu@...ux.alibaba.com, dhavale@...gle.com,
 linux-erofs@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
 Sheng Yong <shengyong1@...omi.com>, Wang Shuai <wangshuai12@...omi.com>
Subject: Re: [PATCH v3 2/2] erofs: add 'offset' mount option for file-backed &
 bdev-based mounts

Hi Karel,

On 2025/4/7 19:40, Karel Zak wrote:
> On Mon, Apr 07, 2025 at 07:05:51PM +0800, Sheng Yong wrote:
>> From: Sheng Yong <shengyong1@...omi.com>
>>
>> When attempting to use an archive file, such as APEX on android,
>> as a file-backed mount source, it fails because EROFS image within
>> the archive file does not start at offset 0. As a result, a loop
>> device is still needed to attach the image file at an appropriate
>> offset first. Similarly, if an EROFS image within a block device
>> does not start at offset 0, it cannot be mounted directly either.
> 
> Does it work with mount(8)? The mount option offset= has been defined
> for decades as userspace-only and is used for loop devices. If I
> remember correctly, libmount does not send the option to the kernel at
> all. The option also triggers loop device usage by mount(8).
> 
> In recent years, we use the "X-" prefix for userspace options.
> Unfortunately, loop=, offset=, and sizelimit= are older than any
> currently used convention (I see the option in mount code from year
> 1998).
> 
> We can improve it in libmount and add any if-erofs hack there, but my
> suggestion is to select a better name for the mount option. For
> example, erofsoff=, erostart=, fsoffset=, start=, or similar.

Thanks for your suggestion!

it's somewhat weird to use erofsprefix here, I think fsoffset
may be fine.

Thanks,
Gao Xiang

> 
>      Karel
> 
>