[PATCH v2] erofs: limit the level of fs stacking for file-backed mounts

[Hongbo Li]

Hi Xiang,

On 2025/11/22 14:23, Gao Xiang wrote:
> Otherwise, it could cause potential kernel stack overflow (e.g., EROFS
> mounting itself).
> 
> Reviewed-by: Sheng Yong <shengyong1 at xiaomi.com>
> Fixes: fb176750266a ("erofs: add file-backed mount support")
> Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
> ---
> Change since v1:
>   - Return -ENOTBLK instead of -EINVAL since userspace tools like
>     util-linux will fall back to using loop to mount again.
> 
>     Don't use -ELOOP compared to other stacked fses, since -ENOTBLK is
>     more suitable: it means the kernel can't handle it anymore.
> 
>   fs/erofs/super.c | 16 ++++++++++++++++
>   1 file changed, 16 insertions(+)
> 
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index f3f8d8c066e4..2db534f76464 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -639,6 +639,22 @@ static int erofs_fc_fill_super(struct super_block *sb, struct fs_context *fc)
>   
>   	sbi->blkszbits = PAGE_SHIFT;
>   	if (!sb->s_bdev) {
> +		/*
> +		 * (File-backed mounts) EROFS claims it's safe to nest other
> +		 * 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.
> +		 */
> +		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");

Since it will success once the max stack depth is exceeded, a warning 
would be better? Otherwise it looks good me.

Reviewed-by: Hongbo Li <lihongbo22 at huawei.com>

Thanks,
Hongbo

> +				return -ENOTBLK;
> +			}
> +		}
>   		sb->s_blocksize = PAGE_SIZE;
>   		sb->s_blocksize_bits = PAGE_SHIFT;
>