[PATCH v2] erofs: don't reuse a superblock for a mount that passes device=

[Zhan Xusheng <zhanxusheng1024@gmail.com>]

From: Zhan Xusheng <zhanxusheng@xiaomi.com>

superblock_share together with device= is rejected in
erofs_fc_fill_super():

	if (test_opt(&sbi->opt, SUPERBLOCK_SHARE) &&
	    sbi->devs->extra_devices)
		return -EINVAL;

but extra_devices is kept in sbi->devs, not in sbi->opt, and
erofs_sb_share_test_super() compares only the backing inode, the
fsoffset and sbi->opt.  So once the image is mounted with
superblock_share, a second mount with superblock_share,device= matches
that superblock and get_tree_super() reuses it without calling
fill_super() at all.

The same command line therefore either fails with -EINVAL or succeeds
with device= quietly dropped, depending on whether a compatible mount
already exists.

Refuse to match a context that brought device= options, so that it gets
a superblock of its own and fill_super() decides as it does today.

Only the incoming context can be tested: a shared superblock may have
extra_devices set itself, because erofs_scan_devices() fills the device
table from the image when no device= was given.

Fixes: cd4108319bb3 ("erofs: reuse superblock for file-backed mounts")
Signed-off-by: Zhan Xusheng <zhanxusheng@xiaomi.com>
---
v2: fold the check into the return statement and drop the comment
    (Gao Xiang)
v1: https://lore.kernel.org/all/20261021083826.1870937-1-zhanxusheng1024@gmail.com/

 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 16fa3b853f1b..8ca44b1c0bd1 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -827,7 +827,8 @@ static int erofs_sb_share_test_super(struct super_block *sb,
 		return 0;
 	return file_inode(sbi->dif0.file) == file_inode(new_sbi->dif0.file) &&
 	       sbi->dif0.fsoff == new_sbi->dif0.fsoff &&
-	       !memcmp(&sbi->opt, &new_sbi->opt, sizeof(sbi->opt));
+	       !memcmp(&sbi->opt, &new_sbi->opt, sizeof(sbi->opt)) &&
+	       !new_sbi->devs->extra_devices;
 }
 
 static int erofs_file_get_tree(struct fs_context *fc)

base-commit: 52dceb88082b4c7983d1c8c7de1c181f018055ac
-- 
2.43.0