erofs: add 'fsoffset' mount option to specify filesystem offset

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@xiaomi.com>
Signed-off-by: Wang Shuai <wangshuai12@xiaomi.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250517090544.2687651-1-shengyong1@xiaomi.com
[ Gao Xiang: minor update on documentation and the error message. ]
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/erofs/internal.h b/fs/erofs/internal.h
index 4ac188d5d894..a32c03a80c70 100644
--- a/fs/erofs/internal.h
+++ b/fs/erofs/internal.h
@@ -44,7 +44,7 @@ struct erofs_device_info {
 	struct erofs_fscache *fscache;
 	struct file *file;
 	struct dax_device *dax_dev;
-	u64 dax_part_off;
+	u64 fsoff, dax_part_off;
 
 	erofs_blk_t blocks;
 	erofs_blk_t uniaddr;
@@ -199,6 +199,7 @@ enum {
 struct erofs_buf {
 	struct address_space *mapping;
 	struct file *file;
+	u64 off;
 	struct page *page;
 	void *base;
 };