linux-kernel - Re: [PATCH] erofs: support idmapped mounts

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Message-ID: <20220517090622.4wrtrjmzknh66bci@wittgenstein>
Date:   Tue, 17 May 2022 11:06:22 +0200
From:   Christian Brauner <brauner@...nel.org>
To:     Chao Yu <chao@...nel.org>
Cc:     xiang@...nel.org, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Chao Yu <chao.yu@...o.com>,
        fsdevel@...r.kernel.org
Subject: Re: [PATCH] erofs: support idmapped mounts

On Tue, May 17, 2022 at 03:32:10PM +0800, Chao Yu wrote:
> This patch enables idmapped mounts for erofs, since all dedicated helpers
> for this functionality existsm, so, in this patch we just pass down the
> user_namespace argument from the VFS methods to the relevant helpers.
> 
> Simple idmap example on erofs image:
> 
> 1. mkdir dir
> 2. touch dir/file
> 3. mkfs.erofs erofs.img dir
> 4. mount -t erofs -o loop erofs.img  /mnt/erofs/
> 
> 5. ls -ln /mnt/erofs/
> total 0
> -rw-rw-r-- 1 1000 1000 0 May 17 15:26 file
> 
> 6. mount-idmapped --map-mount b:0:1001:1 /mnt/erofs/ /mnt/scratch_erofs/
> 
> 7. ls -ln /mnt/scratch_erofs/
> total 0
> -rw-rw-r-- 1 65534 65534 0 May 17 15:26 file

Your current example maps id 0 in the filesystem to id 1001 in the
mount. But since no files with id 0 exist in the filesystem you're
illustrating that unmapped ids are correctly reported as overflow{g,u}id.

I think what you'd rather want to show is something like this:

5. ls -ln /mnt/erofs/
total 0
-rw-rw-r-- 1 1000 1000 0 May 17 15:26 file

6. mount-idmapped --map-mount b:1000:1001:1 /mnt/erofs/ /mnt/scratch_erofs/

7. ls -ln /mnt/scratch_erofs/
total 0
-rw-rw-r-- 1 1001 1001 0 May 17 15:26 file

where id 1000 in the filesystem maps to id 1001 in the mount.

> 
> Signed-off-by: Chao Yu <chao.yu@...o.com>
> ---

Overall this is currently the smallest patch to support idmapped mounts.

Is erofs integrated with xfstests in any way?
For read-only filesystems we probably only need to verify that {g,u}id
are correctly reported. All the writable aspects are irrelevant.

Looks good,
Reviewed-by: Christian Brauner (Microsoft) <brauner@...nel.org>

>  fs/erofs/inode.c | 2 +-
>  fs/erofs/super.c | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/erofs/inode.c b/fs/erofs/inode.c
> index e8b37ba5e9ad..5320bf52c1ce 100644
> --- a/fs/erofs/inode.c
> +++ b/fs/erofs/inode.c
> @@ -370,7 +370,7 @@ int erofs_getattr(struct user_namespace *mnt_userns, const struct path *path,
>  	stat->attributes_mask |= (STATX_ATTR_COMPRESSED |
>  				  STATX_ATTR_IMMUTABLE);
>  
> -	generic_fillattr(&init_user_ns, inode, stat);
> +	generic_fillattr(mnt_userns, inode, stat);
>  	return 0;
>  }
>  
> diff --git a/fs/erofs/super.c b/fs/erofs/super.c
> index 0c4b41130c2f..7dc5f2e8ddee 100644
> --- a/fs/erofs/super.c
> +++ b/fs/erofs/super.c
> @@ -781,7 +781,7 @@ static struct file_system_type erofs_fs_type = {
>  	.name           = "erofs",
>  	.init_fs_context = erofs_init_fs_context,
>  	.kill_sb        = erofs_kill_sb,
> -	.fs_flags       = FS_REQUIRES_DEV,
> +	.fs_flags       = FS_REQUIRES_DEV | FS_ALLOW_IDMAP,
>  };
>  MODULE_ALIAS_FS("erofs");
>  
> -- 
> 2.25.1
>