[PATCH] erofs: fix file-backed mounts no longer working on EROFS partitions

[Gao Xiang <hsiangkao@linux.alibaba.com>]

Sheng Yong reported [1] that Android APEX images didn't work with
[PATCH v2] of upstream commit 072a7c7cdbea ("erofs: don't bother
with s_stack_depth increasing for now") because "EROFS-formatted APEX
file images can be stored within an EROFS-formatted Android system
partition."

In response, I sent a quick fat-fingered [PATCH v3] to address the
report.  Unfortunately, the updated condition was incorrect:

         if (erofs_is_fileio_mode(sbi)) {
-            sb->s_stack_depth =
-                file_inode(sbi->dif0.file)->i_sb->s_stack_depth + 1;
-            if (sb->s_stack_depth > FILESYSTEM_MAX_STACK_DEPTH) {
-                erofs_err(sb, "maximum fs stacking depth exceeded");
+            inode = file_inode(sbi->dif0.file);
+            if ((inode->i_sb->s_op == &erofs_sops && !sb->s_bdev) || 
+                inode->i_sb->s_stack_depth) {

The condition `!sb->s_bdev` is always true for all file-backed EROFS
mounts, making the check effectively a no-op.

The real fix tested and confirmed by Sheng Yong [2] at that time was
[PATCH v3 RESEND], which correctly ensures the following EROFS^2 setup
works:
    EROFS (on a block device) + EROFS (file-backed mount)

But sadly I screwed it up again by upstreaming the outdated [PATCH v3]
and I should be blamed.

This patch applies the same logic as the delta between the upstream
[PATCH v3] and the real fix [PATCH v3 RESEND].

Reported-by: Sheng Yong <shengyong1@xiaomi.com>
Closes: https://lore.kernel.org/r/3acec686-4020-4609-aee4-5dae7b9b0093@gmail.com [1]
Fixes: 072a7c7cdbea ("erofs: don't bother with s_stack_depth increasing for now")
Link: https://lore.kernel.org/r/243f57b8-246f-47e7-9fb1-27a771e8e9e8@gmail.com [2]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
---
Hi Linus,

As suggested by Amir, I send out the patch to fix the broken fix.

If possible, could you help apply this patch directly?

If you perfer another pull request I will do later too after a sleep,
but I guess I will just repeat my stupid mistake again in the pull
request and the tag message.

Thanks,
Gao Xiang

 fs/erofs/super.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index e93264034b5d..5136cda5972a 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -655,7 +655,8 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 		 */
 		if (erofs_is_fileio_mode(sbi)) {
 			inode = file_inode(sbi->dif0.file);
-			if ((inode->i_sb->s_op == &erofs_sops && !sb->s_bdev) ||
+			if ((inode->i_sb->s_op == &erofs_sops &&
+			     !inode->i_sb->s_bdev) ||
 			    inode->i_sb->s_stack_depth) {
 				erofs_err(sb, "file-backed mounts cannot be applied to stacked fses");
 				return -ENOTBLK;
-- 
2.43.5