Re: [RFC] Add BPF_SYNCHRONIZE bpf(2) command

[Alexei Starovoitov]

On Fri, Jul 06, 2018 at 06:56:16PM -0700, Daniel Colascione wrote:
> BPF_SYNCHRONIZE waits for any BPF programs active at the time of
> BPF_SYNCHRONIZE to complete, allowing userspace to ensure atomicity of
> RCU data structure operations with respect to active programs. For
> example, userspace can update a map->map entry to point to a new map,
> use BPF_SYNCHRONIZE to wait for any BPF programs using the old map to
> complete, and then drain the old map without fear that BPF programs
> may still be updating it.
> 
> Signed-off-by: Daniel Colascione <dancol@xxxxxxxxxx>
> ---
>  include/uapi/linux/bpf.h |  1 +
>  kernel/bpf/syscall.c     | 14 ++++++++++++++
>  2 files changed, 15 insertions(+)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index b7db3261c62d..4365c50e8055 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -98,6 +98,7 @@ enum bpf_cmd {
>  	BPF_BTF_LOAD,
>  	BPF_BTF_GET_FD_BY_ID,
>  	BPF_TASK_FD_QUERY,
> +	BPF_SYNCHRONIZE,
>  };
>  
>  enum bpf_map_type {
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index d10ecd78105f..60ec7811846e 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -2272,6 +2272,20 @@ SYSCALL_DEFINE3(bpf, int, cmd, union bpf_attr __user *, uattr, unsigned int, siz
>  	if (sysctl_unprivileged_bpf_disabled && !capable(CAP_SYS_ADMIN))
>  		return -EPERM;
>  
> +	if (cmd == BPF_SYNCHRONIZE) {
> +		if (uattr != NULL || size != 0)
> +			return -EINVAL;
> +		err = security_bpf(cmd, NULL, 0);
> +		if (err < 0)
> +			return err;
> +		/* BPF programs are run with preempt disabled, so
> +		 * synchronize_sched is sufficient even with
> +		 * RCU_PREEMPT.
> +		 */
> +		synchronize_sched();
> +		return 0;

I don't think it's necessary. sys_membarrier() can do this already
and some folks use it exactly for this use case.