[PATCH v2] erofs: don't bother with s_stack_depth increasing for now

[Sheng Yong]

On 1/7/26 01:05, 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 when stacking an unlimited number of EROFS on top of
> each other.
> 
> This fix breaks composefs mounts, which need EROFS+ovl^2 sometimes
> (and such setups are already used in production for quite a long time).
> 
> One way to fix this regression is to bump FILESYSTEM_MAX_STACK_DEPTH
> from 2 to 3, but proving that this is safe in general is a high bar.
> 
> After a long discussion on GitHub issues [1] about possible solutions,
> one conclusion is that there is no need to support nesting file-backed
> EROFS mounts on stacked filesystems, because there is always the option
> to use loopback devices as a fallback.
> 
> As a quick fix for the composefs regression for this cycle, instead of
> bumping `s_stack_depth` for file backed EROFS mounts, we disallow
> nesting file-backed EROFS over EROFS and over filesystems with
> `s_stack_depth` > 0.
> 
> This works for all known file-backed mount use cases (composefs,
> containerd, and Android APEX for some Android vendors), and the fix is
> self-contained.
> 
> Essentially, we are allowing one extra unaccounted fs stacking level of
> EROFS below stacking filesystems, but EROFS can only be used in the read
> path (i.e. overlayfs lower layers), which typically has much lower stack
> usage than the write path.
> 
> We can consider increasing FILESYSTEM_MAX_STACK_DEPTH later, after more
> stack usage analysis or using alternative approaches, such as splitting
> the `s_stack_depth` limitation according to different combinations of
> stacking.
> 
> Fixes: d53cd891f0e4 ("erofs: limit the level of fs stacking for file-backed mounts")
> Reported-by: Dusty Mabe <dusty at dustymabe.com>
> Reported-by: Timothée Ravier <tim at siosm.fr>
> Closes: https://github.com/coreos/fedora-coreos-tracker/issues/2087 [1]
> Reported-by: "Alekséi Naidénov" <an at digitaltide.io>
> Closes: https://lore.kernel.org/r/CAFHtUiYv4+=+JP_-JjARWjo6OwcvBj1wtYN=z0QXwCpec9sXtg@mail.gmail.com
> Acked-by: Amir Goldstein <amir73il at gmail.com>
> Cc: Alexander Larsson <alexl at redhat.com>
> Cc: Christian Brauner <brauner at kernel.org>
> Cc: Miklos Szeredi <mszeredi at redhat.com>
> Cc: Sheng Yong <shengyong1 at xiaomi.com>
> Cc: Zhiguo Niu <niuzhiguo84 at gmail.com>
> Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
> ---
> v2:
>   - Update commit message (suggested by Amir in 1-on-1 talk);
>   - Add proper `Reported-by:`.
> 
>   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 ||

Hi, Xiang

In Android APEX scenario, apex images formatted as EROFS are packed in
system.img which is also EROFS format. As a result, it will always fail
to do APEX-file-backed mount since `inode->i_sb->s_op == &erofs_sops'
is true.
Any thoughts to handle such scenario?

thanks,
shengyong

> +			    inode->i_sb->s_stack_depth) {
> +				erofs_err(sb, "file-backed mounts cannot be applied to stacked fses");
>   				return -ENOTBLK;
>   			}
>   		}