[PATCH v2] Add BPF_SYNCHRONIZE_MAPS bpf(2) command

[Daniel Colascione <dancol@google.com>]

BPF_SYNCHRONIZE_MAPS waits for the release of any references to a BPF
map made by a BPF program that is running at the time the
BPF_SYNCHRONIZE_MAPS command is issued. The purpose of this command is
to provide a means for userspace to replace a BPF map with another,
newer version, then ensure that no component is still using the "old"
map before manipulating the "old" map in some way.

Signed-off-by: Daniel Colascione <dancol@google.com>
---
 include/uapi/linux/bpf.h |  9 +++++++++
 kernel/bpf/syscall.c     | 13 +++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index b7db3261c62d..5b27e9117d3e 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -75,6 +75,14 @@ struct bpf_lpm_trie_key {
 	__u8	data[0];	/* Arbitrary size */
 };
 
+/* BPF_SYNCHRONIZE_MAPS waits for the release of any references to a
+ * BPF map made by a BPF program that is running at the time the
+ * BPF_SYNCHRONIZE_MAPS command is issued. The purpose of this command
+ * is to provide a means for userspace to replace a BPF map with
+ * another, newer version, then ensure that no component is still
+ * using the "old" map before manipulating the "old" map in some way.
+ */
+
 /* BPF syscall commands, see bpf(2) man-page for details. */
 enum bpf_cmd {
 	BPF_MAP_CREATE,
@@ -98,6 +106,7 @@ enum bpf_cmd {
 	BPF_BTF_LOAD,
 	BPF_BTF_GET_FD_BY_ID,
 	BPF_TASK_FD_QUERY,
+	BPF_SYNCHRONIZE_MAPS,
 };
 
 enum bpf_map_type {
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index a31a1ba0f8ea..8bbd1a5d01d1 100644
--- a/kernel/bpf/syscall.c
+++ b/kernel/bpf/syscall.c
@@ -2274,6 +2274,19 @@ 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_MAPS) {
+		if (uattr != NULL || size != 0)
+			return -EINVAL;
+		err = security_bpf(cmd, NULL, 0);
+		if (err < 0)
+			return err;
+		/* BPF programs always enter a critical section while
+		 * they have a map reference outstanding.
+		 */
+		synchronize_rcu();
+		return 0;
+	}
+
 	err = bpf_check_uarg_tail_zero(uattr, sizeof(attr), size);
 	if (err)
 		return err;
-- 
2.18.0.233.g985f88cf7e-goog