Subject: Re: [PATCH v3 3/6] bpf: add BPF_PROG_ATTACH and BPF_PROG_DETACH commands

From: Daniel Borkmann <[email-masked]>
Date: Tue, 30 Aug 2016 01:00:07 +0200

On 08/26/2016 09:58 PM, Daniel Mack wrote:
> Extend the bpf(2) syscall by two new commands, BPF_PROG_ATTACH and
> BPF_PROG_DETACH which allow attaching and detaching eBPF programs
> to a target.
>
> On the API level, the target could be anything that has an fd in
> userspace, hence the name of the field in union bpf_attr is called
> 'target_fd'.
>
> When called with BPF_ATTACH_TYPE_CGROUP_INET_{E,IN}GRESS, the target is
> expected to be a valid file descriptor of a cgroup v2 directory which
> has the bpf controller enabled. These are the only use-cases
> implemented by this patch at this point, but more can be added.
>
> If a program of the given type already exists in the given cgroup,
> the program is swapped automically, so userspace does not have to drop
> an existing program first before installing a new one, which would
> otherwise leave a gap in which no program is attached.
>
> For more information on the propagation logic to subcgroups, please
> refer to the bpf cgroup controller implementation.
>
> The API is guarded by CAP_NET_ADMIN.
>
> Signed-off-by: Daniel Mack <[email-masked]>
> syscall

^^^ slipped in?

> ---
>   include/uapi/linux/bpf.h |  9 ++++++
>   kernel/bpf/syscall.c     | 83 ++++++++++++++++++++++++++++++++++++++++++++++++
>   2 files changed, 92 insertions(+)
>
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 1d5db42..4cc2dcf 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -73,6 +73,8 @@ enum bpf_cmd {
>   	BPF_PROG_LOAD,
>   	BPF_OBJ_PIN,
>   	BPF_OBJ_GET,
> +	BPF_PROG_ATTACH,
> +	BPF_PROG_DETACH,
>   };
>
>   enum bpf_map_type {
> @@ -147,6 +149,13 @@ union bpf_attr {
>   		__aligned_u64	pathname;
>   		__u32		bpf_fd;
>   	};
> +
> +	struct { /* anonymous struct used by BPF_PROG_ATTACH/DETACH commands */
> +		__u32		target_fd;	/* container object to attach to */
> +		__u32		attach_bpf_fd;	/* eBPF program to attach */
> +		__u32		attach_type;	/* BPF_ATTACH_TYPE_* */
> +		__u64		attach_flags;

Could we just do ...

__u32 dst_fd;
__u32 src_fd;
__u32 attach_type;

... and leave flags out, since unused anyway? Also see below.

> +	};
>   } __attribute__((aligned(8)));
>
>   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index 228f962..cc4d603 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -822,6 +822,79 @@ static int bpf_obj_get(const union bpf_attr *attr)
>   	return bpf_obj_get_user(u64_to_ptr(attr->pathname));
>   }
>
> +#ifdef CONFIG_CGROUP_BPF

#define BPF_PROG_ATTACH_LAST_FIELD attach_type
#define BPF_PROG_DETACH_LAST_FIELD BPF_PROG_ATTACH_LAST_FIELD

> +static int bpf_prog_attach(const union bpf_attr *attr)
> +{
> +	struct bpf_prog *prog;
> +
> +	if (!capable(CAP_NET_ADMIN))
> +		return -EPERM;
> +
> +	/* Flags are unused for now */
> +	if (attr->attach_flags != 0)
> +		return -EINVAL;

Correct way would be to:

	if (CHECK_ATTR(BPF_PROG_ATTACH))
		return -EINVAL;

> +
> +	switch (attr->attach_type) {
> +	case BPF_ATTACH_TYPE_CGROUP_INET_INGRESS:
> +	case BPF_ATTACH_TYPE_CGROUP_INET_EGRESS: {
> +		struct cgroup *cgrp;
> +
> +		prog = bpf_prog_get_type(attr->attach_bpf_fd,
> +					 BPF_PROG_TYPE_CGROUP_SOCKET_FILTER);
> +		if (IS_ERR(prog))
> +			return PTR_ERR(prog);
> +
> +		cgrp = cgroup_get_from_fd(attr->target_fd);
> +		if (IS_ERR(cgrp)) {
> +			bpf_prog_put(prog);
> +			return PTR_ERR(cgrp);
> +		}
> +
> +		cgroup_bpf_update(cgrp, prog, attr->attach_type);
> +		cgroup_put(cgrp);
> +
> +		break;
> +	}
> +
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +
> +static int bpf_prog_detach(const union bpf_attr *attr)
> +{
> +	if (!capable(CAP_NET_ADMIN))
> +		return -EPERM;
> +
> +	/* Flags are unused for now */
> +	if (attr->attach_flags != 0)
> +		return -EINVAL;

	if (CHECK_ATTR(BPF_PROG_DETACH))
		return -EINVAL;

> +	switch (attr->attach_type) {
> +	case BPF_ATTACH_TYPE_CGROUP_INET_INGRESS:
> +	case BPF_ATTACH_TYPE_CGROUP_INET_EGRESS: {
> +		struct cgroup *cgrp;
> +
> +		cgrp = cgroup_get_from_fd(attr->target_fd);
> +		if (IS_ERR(cgrp))
> +			return PTR_ERR(cgrp);
> +
> +		cgroup_bpf_update(cgrp, NULL, attr->attach_type);
> +		cgroup_put(cgrp);
> +
> +		break;
> +	}
> +
> +	default:
> +		return -EINVAL;
> +	}
> +
> +	return 0;
> +}
> +#endif /* CONFIG_CGROUP_BPF */
> +
>   SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, size)
>   {
>   	union bpf_attr attr = {};
> @@ -888,6 +961,16 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
>   	case BPF_OBJ_GET:
>   		err = bpf_obj_get(&attr);
>   		break;
> +
> +#ifdef CONFIG_CGROUP_BPF
> +	case BPF_PROG_ATTACH:
> +		err = bpf_prog_attach(&attr);
> +		break;
> +	case BPF_PROG_DETACH:
> +		err = bpf_prog_detach(&attr);
> +		break;
> +#endif
> +
>   	default:
>   		err = -EINVAL;
>   		break;
>