[PATCH v6 bpf-next 02/13] bpf: add BPF token delegation mount options to BPF FS

[Hou Tao]

On 9/28/2023 6:57 AM, Andrii Nakryiko wrote:
> Add few new mount options to BPF FS that allow to specify that a given
> BPF FS instance allows creation of BPF token (added in the next patch),
> and what sort of operations are allowed under BPF token. As such, we get
> 4 new mount options, each is a bit mask
>   - `delegate_cmds` allow to specify which bpf() syscall commands are
>     allowed with BPF token derived from this BPF FS instance;
>   - if BPF_MAP_CREATE command is allowed, `delegate_maps` specifies
>     a set of allowable BPF map types that could be created with BPF token;
>   - if BPF_PROG_LOAD command is allowed, `delegate_progs` specifies
>     a set of allowable BPF program types that could be loaded with BPF token;
>   - if BPF_PROG_LOAD command is allowed, `delegate_attachs` specifies
>     a set of allowable BPF program attach types that could be loaded with
>     BPF token; delegate_progs and delegate_attachs are meant to be used
>     together, as full BPF program type is, in general, determined
>     through both program type and program attach type.
>
> Currently, these mount options accept the following forms of values:
>   - a special value "any", that enables all possible values of a given
>   bit set;
>   - numeric value (decimal or hexadecimal, determined by kernel
>   automatically) that specifies a bit mask value directly;
>   - all the values for a given mount option are combined, if specified
>   multiple times. E.g., `mount -t bpf nodev /path/to/mount -o
>   delegate_maps=0x1 -o delegate_maps=0x2` will result in a combined 0x3
>   mask.
>
SNIP
>  	return 0;
> @@ -740,10 +786,14 @@ static int populate_bpffs(struct dentry *parent)
>  static int bpf_fill_super(struct super_block *sb, struct fs_context *fc)
>  {
>  	static const struct tree_descr bpf_rfiles[] = { { "" } };
> -	struct bpf_mount_opts *opts = fc->fs_private;
> +	struct bpf_mount_opts *opts = sb->s_fs_info;
>  	struct inode *inode;
>  	int ret;
>  
> +	/* Mounting an instance of BPF FS requires privileges */
> +	if (fc->user_ns != &init_user_ns && !capable(CAP_SYS_ADMIN))
> +		return -EPERM;
> +
>  	ret = simple_fill_super(sb, BPF_FS_MAGIC, bpf_rfiles);
>  	if (ret)
>  		return ret;
> @@ -765,7 +815,10 @@ static int bpf_get_tree(struct fs_context *fc)
>  
>  static void bpf_free_fc(struct fs_context *fc)
>  {
> -	kfree(fc->fs_private);
> +	struct bpf_mount_opts *opts = fc->s_fs_info;
> +
> +	if (opts)
> +		kfree(opts);
>  }
>  

The NULL check is not needed here, use kfree(fc->s_fs_info) will be enough.
>  static const struct fs_context_operations bpf_context_ops = {
> @@ -787,17 +840,32 @@ static int bpf_init_fs_context(struct fs_context *fc)
>  
>  	opts->mode = S_IRWXUGO;
>  
> -	fc->fs_private = opts;
> +	/* start out with no BPF token delegation enabled */
> +	opts->delegate_cmds = 0;
> +	opts->delegate_maps = 0;
> +	opts->delegate_progs = 0;
> +	opts->delegate_attachs = 0;
> +
> +	fc->s_fs_info = opts;
>  	fc->ops = &bpf_context_ops;
>  	return 0;
>  }
>  
> +static void bpf_kill_super(struct super_block *sb)
> +{
> +	struct bpf_mount_opts *opts = sb->s_fs_info;
> +
> +	kill_litter_super(sb);
> +	kfree(opts);
> +}
> +
>  static struct file_system_type bpf_fs_type = {
>  	.owner		= THIS_MODULE,
>  	.name		= "bpf",
>  	.init_fs_context = bpf_init_fs_context,
>  	.parameters	= bpf_fs_parameters,
> -	.kill_sb	= kill_litter_super,
> +	.kill_sb	= bpf_kill_super,
> +	.fs_flags	= FS_USERNS_MOUNT,
>  };
>  
>  static int __init bpf_init(void)