Hi Yong!
On 2025/3/24 10:28, Sheng Yong wrote:
From: Sheng Yong <shengyo...@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
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 <shengyo...@xiaomi.com>
Signed-off-by: Wang Shuai <wangshua...@xiaomi.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
