Re: [RFC v3 14/22] bpf/cgroup: Make cgroup_bpf_update() return an error code

[Alexei Starovoitov]

On Wed, Sep 14, 2016 at 09:24:07AM +0200, Mickaël Salaün wrote:
> This will be useful to support Landlock for the next commits.
> 
> Signed-off-by: Mickaël Salaün <mic@xxxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: Daniel Mack <daniel@xxxxxxxxxx>
> Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>

I think this is good to do regardless. Sooner or later cgroup_bpf_update
will start rejecting the prog attach. Like we discussed to have a flag
that would dissallow processeses lower in the cgroup hierarchy to install
their own bpf programs.
It will be minimal change to bpf_prog_attach() error handling,
but will greatly help Mickael to build stuff on top.
DanielM can you refactor your patch to do that from the start ?

Thanks!

> ---
>  include/linux/bpf-cgroup.h |  4 ++--
>  kernel/bpf/cgroup.c        |  3 ++-
>  kernel/bpf/syscall.c       | 10 ++++++----
>  kernel/cgroup.c            |  6 ++++--
>  4 files changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/bpf-cgroup.h b/include/linux/bpf-cgroup.h
> index 2234042d7f61..6cca7924ee17 100644
> --- a/include/linux/bpf-cgroup.h
> +++ b/include/linux/bpf-cgroup.h
> @@ -31,13 +31,13 @@ struct cgroup_bpf {
>  void cgroup_bpf_put(struct cgroup *cgrp);
>  void cgroup_bpf_inherit(struct cgroup *cgrp, struct cgroup *parent);
>  
> -void __cgroup_bpf_update(struct cgroup *cgrp,
> +int __cgroup_bpf_update(struct cgroup *cgrp,
>  			 struct cgroup *parent,
>  			 struct bpf_prog *prog,
>  			 enum bpf_attach_type type);
>  
>  /* Wrapper for __cgroup_bpf_update() protected by cgroup_mutex */
> -void cgroup_bpf_update(struct cgroup *cgrp,
> +int cgroup_bpf_update(struct cgroup *cgrp,
>  		       struct bpf_prog *prog,
>  		       enum bpf_attach_type type);
>  
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 782878ec4f2d..7b75fa692617 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -83,7 +83,7 @@ void cgroup_bpf_inherit(struct cgroup *cgrp, struct cgroup *parent)
>   *
>   * Must be called with cgroup_mutex held.
>   */
> -void __cgroup_bpf_update(struct cgroup *cgrp,
> +int __cgroup_bpf_update(struct cgroup *cgrp,
>  			 struct cgroup *parent,
>  			 struct bpf_prog *prog,
>  			 enum bpf_attach_type type)
> @@ -117,6 +117,7 @@ void __cgroup_bpf_update(struct cgroup *cgrp,
>  		bpf_prog_put(old_pinned.prog);
>  		static_branch_dec(&cgroup_bpf_enabled_key);
>  	}
> +	return 0;
>  }
>  
>  /**
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 45a91d511119..c978f2d9a1b3 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -831,6 +831,7 @@ static int bpf_prog_attach(const union bpf_attr *attr)
>  {
>  	struct bpf_prog *prog;
>  	struct cgroup *cgrp;
> +	int result;
>  
>  	if (!capable(CAP_NET_ADMIN))
>  		return -EPERM;
> @@ -858,10 +859,10 @@ static int bpf_prog_attach(const union bpf_attr *attr)
>  		return PTR_ERR(cgrp);
>  	}
>  
> -	cgroup_bpf_update(cgrp, prog, attr->attach_type);
> +	result = cgroup_bpf_update(cgrp, prog, attr->attach_type);
>  	cgroup_put(cgrp);
>  
> -	return 0;
> +	return result;
>  }
>  
>  #define BPF_PROG_DETACH_LAST_FIELD attach_type
> @@ -869,6 +870,7 @@ static int bpf_prog_attach(const union bpf_attr *attr)
>  static int bpf_prog_detach(const union bpf_attr *attr)
>  {
>  	struct cgroup *cgrp;
> +	int result = 0;
>  
>  	if (!capable(CAP_NET_ADMIN))
>  		return -EPERM;
> @@ -883,7 +885,7 @@ static int bpf_prog_detach(const union bpf_attr *attr)
>  		if (IS_ERR(cgrp))
>  			return PTR_ERR(cgrp);
>  
> -		cgroup_bpf_update(cgrp, NULL, attr->attach_type);
> +		result = cgroup_bpf_update(cgrp, NULL, attr->attach_type);
>  		cgroup_put(cgrp);
>  		break;
>  
> @@ -891,7 +893,7 @@ static int bpf_prog_detach(const union bpf_attr *attr)
>  		return -EINVAL;
>  	}
>  
> -	return 0;
> +	return result;
>  }
>  #endif /* CONFIG_CGROUP_BPF */
>  
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 87324ce481b1..48b650a640a9 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -6450,15 +6450,17 @@ static __init int cgroup_namespaces_init(void)
>  subsys_initcall(cgroup_namespaces_init);
>  
>  #ifdef CONFIG_CGROUP_BPF
> -void cgroup_bpf_update(struct cgroup *cgrp,
> +int cgroup_bpf_update(struct cgroup *cgrp,
>  		       struct bpf_prog *prog,
>  		       enum bpf_attach_type type)
>  {
>  	struct cgroup *parent = cgroup_parent(cgrp);
> +	int result;
>  
>  	mutex_lock(&cgroup_mutex);
> -	__cgroup_bpf_update(cgrp, parent, prog, type);
> +	result = __cgroup_bpf_update(cgrp, parent, prog, type);
>  	mutex_unlock(&cgroup_mutex);
> +	return result;
>  }
>  #endif /* CONFIG_CGROUP_BPF */
>  
> -- 
> 2.9.3
>