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Message-ID: <1b0ce59a-30a7-46e3-9c63-72c3f8d44b77@linux.alibaba.com>
Date: Tue, 25 Mar 2025 09:37:39 +0800
From: Gao Xiang <hsiangkao@...ux.alibaba.com>
To: Sheng Yong <shengyong2021@...il.com>
Cc: linux-kernel@...r.kernel.org, Sheng Yong <shengyong1@...omi.com>,
Wang Shuai <wangshuai12@...omi.com>, Gao Xiang <xiang@...nel.org>,
Chao Yu <chao@...nel.org>, Yue Hu <zbestahu@...il.com>,
Jeffle Xu <jefflexu@...ux.alibaba.com>, Sandeep Dhavale
<dhavale@...gle.com>, linux-erofs mailing list <linux-erofs@...ts.ozlabs.org>
Subject: Re: [RFC PATCH] erofs: add start offset for file-backed mount
Hi Yong!
On 2025/3/24 10:28, 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.
>
> To address this issue, this patch parses the `source' parameter in
> EROFS to accept a start offset for the file-backed mount. The format
> is `/path/to/archive_file:offs', where `offs' represents the start
> offset. EROFS will add this offset before performing read requests.
>
> Signed-off-by: Sheng Yong <shengyong1@...omi.com>
> Signed-off-by: Wang Shuai <wangshuai12@...omi.com>
Sorry for late reply. Yeah, that is a great feature.
> ---
...
> @@ -411,6 +412,31 @@ static bool erofs_fc_set_dax_mode(struct fs_context *fc, unsigned int mode)
> #endif
> }
>
> +static loff_t erofs_fc_parse_source(struct fs_context *fc,
> + struct fs_parameter *param)
> +{
> + const char *devname = param->string;
> + const char *fofs_start __maybe_unused;
> + loff_t fofs = 0;
> +
> + if (!devname || !*devname)
> + return invalfc(fc, "Empty source");
> +
> +#ifdef CONFIG_EROFS_FS_BACKED_BY_FILE
> + fofs_start = strchr(devname, ':');
> + if (!fofs_start)
> + goto out;
> + if (kstrtoll(fofs_start + 1, 0, &fofs) < 0)
> + return invalfc(fc, "Invalid filebacked offset %s", fofs_start);
> + fc->source = kstrndup(devname, fofs_start - devname, GFP_KERNEL);
> + return fofs;
> +out:
> +#endif
> + fc->source = devname;
> + param->string = NULL;
> + return fofs;
> +}
Could we just add a new mount option for this, and apply this feature
for both file-backed mounts and bdev-based mounts?
Twist source option is not quite clean on my side.
> +
> static int erofs_fc_parse_param(struct fs_context *fc,
> struct fs_parameter *param)
> {
> @@ -507,6 +533,11 @@ static int erofs_fc_parse_param(struct fs_context *fc,
> errorfc(fc, "%s option not supported", erofs_fs_parameters[opt].name);
> #endif
> break;
> + case Opt_source:
> + sbi->dif0.fofs = erofs_fc_parse_source(fc, param);
> + if (sbi->dif0.fofs < 0)
> + return -EINVAL;
> + break;
> }
> return 0;
> }
> @@ -697,6 +728,10 @@ static int erofs_fc_get_tree(struct fs_context *fc)
> file = filp_open(fc->source, O_RDONLY | O_LARGEFILE, 0);
> if (IS_ERR(file))
> return PTR_ERR(file);
> + if (sbi->dif0.fofs + PAGE_SIZE >= i_size_read(file_inode(file))) {
> + fput(file);
> + return invalf(fc, "Start offset too large");
> + }
I guess we need to verify the offset is block-aligned too.
Thanks,
Gao Xiang
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