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

[Alexander Larsson <alexl@redhat.com>]

On Thu, 2026-01-01 at 04:42 +0800, Gao Xiang wrote:
> 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>
> ---

Acked-by: Alexander Larsson <alexl@redhat.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;
>  			}
>  		}

-- 
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 Alexander Larsson                                            Red Hat,
Inc 
       alexl@redhat.com            alexander.larsson@gmail.com 
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