[PATCH] 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>
---
 fs/erofs/super.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 16fa3b853f1b6..bc9ab5a6234a0 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -825,6 +825,13 @@ static int erofs_sb_share_test_super(struct super_block *sb,
 		return 0;
 	if (!sbi->dif0.file || !new_sbi->dif0.file)
 		return 0;
+	/*
+	 * extra_devices is not part of sbi->opt, so the memcmp below cannot
+	 * see device= options.  @sb may have extra_devices set from the
+	 * on-disk device table, so only the incoming context is tested.
+	 */
+	if (new_sbi->devs->extra_devices)
+		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));

base-commit: 1137aa577a35e2684e8046c3e53f920e79c7cc60
-- 
2.43.0