[RFC] Add BPF_SYNCHRONIZE bpf(2) command

[Daniel Colascione <dancol@google.com>]

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@google.com>
---
 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;
+	}
+
 	err = bpf_check_uarg_tail_zero(uattr, sizeof(attr), size);
 	if (err)
 		return err;
-- 
2.18.0.203.gfac676dfb9-goog