[PATCH v3] Add BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES bpf(2) command

[Daniel Colascione <dancol@google.com>]

BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES waits for the release of all
references to maps active at the instant the
BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES is
issued. BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES waits only for the
expiration of map references obtained by BPF programs from other maps.

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 | 14 ++++++++++++++
 kernel/bpf/syscall.c     | 13 +++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index b7db3261c62d..ca3cfca76edc 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -75,6 +75,19 @@ struct bpf_lpm_trie_key {
 	__u8	data[0];	/* Arbitrary size */
 };
 
+/* BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES waits for the release of all
+ * references to maps active at the instant the
+ * BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES is
+ * issued. BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES waits only for the
+ * expiration of map references obtained by BPF programs from
+ * other maps.
+ *
+ * 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 +111,7 @@ enum bpf_cmd {
 	BPF_BTF_LOAD,
 	BPF_BTF_GET_FD_BY_ID,
 	BPF_TASK_FD_QUERY,
+	BPF_SYNCHRONIZE_MAP_TO_MAP_REFERENCES,
 };
 
 enum bpf_map_type {
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index a31a1ba0f8ea..bc9a0713f47d 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_MAP_TO_MAP_REFERENCES) {
+		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;
-- 
2.18.0.345.g5c9ce644c3-goog