[PATCH v7] erofs: add 'fsoffset' mount option to specify filesystem offset

[Sheng Yong]

On 5/19/25 11:54, Gao Xiang wrote:
> Hi Yong,
> 
> On Sat, May 17, 2025 at 05:05:43PM +0800, Sheng Yong wrote:
>> From: Sheng Yong <shengyong1 at xiaomi.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
>> or a dm 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.
>>
>> To address this issue, this patch adds a new mount option `fsoffset=x'
>> to accept a start offset for the primary device. The offset should be
>> aligned to the block size. EROFS will add this offset before performing
>> read requests.
>>
>> Signed-off-by: Sheng Yong <shengyong1 at xiaomi.com>
>> Signed-off-by: Wang Shuai <wangshuai12 at xiaomi.com>
> 
> I applied the following diff to fulfill the Hongbo's previous suggestion
> and refine an error message:

Hi, Xiang,

I'm fine with the update, thank you for improving the change :-)

thanks,
shengyong

> 
> diff --git a/Documentation/filesystems/erofs.rst b/Documentation/filesystems/erofs.rst
> index 11b0f8635f04..d93b30287110 100644
> --- a/Documentation/filesystems/erofs.rst
> +++ b/Documentation/filesystems/erofs.rst
> @@ -128,7 +128,7 @@ device=%s              Specify a path to an extra device to be used together.
>   fsid=%s                Specify a filesystem image ID for Fscache back-end.
>   domain_id=%s           Specify a domain ID in fscache mode so that different images
>                          with the same blobs under a given domain ID can share storage.
> -fsoffset=%lu           Specify image offset for the primary device.
> +fsoffset=%lu           Specify block-aligned filesystem offset for the primary device.
>   ===================    =========================================================
>   
>   Sysfs Entries
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index 3185bb90f549..e1e9f06e8342 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -654,9 +654,9 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
>   	}
>   
>   	if (sbi->dif0.fsoff) {
> -		if (sbi->dif0.fsoff & ((1 << sbi->blkszbits) - 1))
> -			return invalfc(fc, "fsoffset %llu not aligned to block size",
> -				       sbi->dif0.fsoff);
> +		if (sbi->dif0.fsoff & (sb->s_blocksize - 1))
> +			return invalfc(fc, "fsoffset %llu is not aligned to block size %lu",
> +				       sbi->dif0.fsoff, sb->s_blocksize);
>   		if (erofs_is_fscache_mode(sb))
>   			return invalfc(fc, "cannot use fsoffset in fscache mode");
>   	}