[PATCH] erofs: don't bother with s_stack_depth increasing for now

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

Previously, commit d53cd891f0e4 ("erofs: limit the level of fs stacking
for file-backed mounts") bumped `s_stack_depth` by one to avoid kernel
stack overflow, but it breaks composefs mounts, which need erofs+ovl^2
sometimes (and such setups are already used in production for quite long
time) since `s_stack_depth` can be 3 (i.e., FILESYSTEM_MAX_STACK_DEPTH
needs to change from 2 to 3).

After a long discussion on GitHub issues [1] about possible solutions,
it seems there is no need to support nesting file-backed mounts as one
conclusion (especially when increasing FILESYSTEM_MAX_STACK_DEPTH to 3).
So let's disallow this right now, since there is always a way to use
loopback devices as a fallback.

Then, I started to wonder about an alternative EROFS quick fix to
address the composefs mounts directly for this cycle: since EROFS is the
only fs to support file-backed mounts and other stacked fses will just
bump up `FILESYSTEM_MAX_STACK_DEPTH`, just check that `s_stack_depth`
!= 0 and the backing inode is not from EROFS instead.

At least it works for all known file-backed mount use cases (composefs,
containerd, and Android APEX for some Android vendors), and the fix is
self-contained.

Let's defer increasing FILESYSTEM_MAX_STACK_DEPTH for now.

Fixes: d53cd891f0e4 ("erofs: limit the level of fs stacking for file-backed mounts")
Closes: https://github.com/coreos/fedora-coreos-tracker/issues/2087 [1]
Closes: https://lore.kernel.org/r/CAFHtUiYv4+=+JP_-JjARWjo6OwcvBj1wtYN=z0QXwCpec9sXtg@mail.gmail.com
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Alexander Larsson <alexl@redhat.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
---
 fs/erofs/super.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index 937a215f626c..0cf41ed7ced8 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -644,14 +644,20 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
 		 * fs contexts (including its own) due to self-controlled RO
 		 * accesses/contexts and no side-effect changes that need to
 		 * context save & restore so it can reuse the current thread
-		 * context.  However, it still needs to bump `s_stack_depth` to
-		 * avoid kernel stack overflow from nested filesystems.
+		 * context.
+		 * However, we still need to prevent kernel stack overflow due
+		 * to filesystem nesting: just ensure that s_stack_depth is 0
+		 * to disallow mounting EROFS on stacked filesystems.
+		 * Note: s_stack_depth is not incremented here for now, since
+		 * EROFS is the only fs supporting file-backed mounts for now.
+		 * It MUST change if another fs plans to support them, which
+		 * may also require adjusting FILESYSTEM_MAX_STACK_DEPTH.
 		 */
 		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 ||
+			    inode->i_sb->s_stack_depth) {
+				erofs_err(sb, "file-backed mounts cannot be applied to stacked fses");
 				return -ENOTBLK;
 			}
 		}
-- 
2.43.5