[PATCH] erofs: ensure regular inodes for file-backed mounts

[Gao Xiang]

Only regular inodes are allowed for file-backed mounts, not directories
(as seen in the original syzbot case) or special inodes.

Also ensure that .read_folio() is implemented on the underlying fs.

Fixes: fb176750266a ("erofs: add file-backed mount support")
Reported-by: syzbot+001306cd9c92ce0df23f at syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/00000000000011bdde0622498ee3@google.com
Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
---
 fs/erofs/super.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 666873f745da..7c3467dcc5b4 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -714,7 +714,10 @@ static int erofs_fc_get_tree(struct fs_context *fc)
 		if (IS_ERR(sbi->fdev))
 			return PTR_ERR(sbi->fdev);
 
-		return get_tree_nodev(fc, erofs_fc_fill_super);
+		if (S_ISREG(file_inode(sbi->fdev)->i_mode) &&
+		    sbi->fdev->f_mapping->a_ops->read_folio)
+			return get_tree_nodev(fc, erofs_fc_fill_super);
+		fput(sbi->fdev);
 	}
 #endif
 	return ret;
-- 
2.43.5