[PATCH bpf-next 2/3] bpf: Add KF_DEPRECATED kfunc flag

[David Vernet <void@manifault.com>]

Now that we have our kfunc lifecycle expectations clearly documented,
and that KF_DEPRECATED is documented as an optional method for kfunc
developers and maintainers to provide a deprecation story to BPF users,
we need to actually implement the flag.

This patch adds KF_DEPRECATED, and updates the verifier to issue a
verifier log message if a deprecated kfunc is called. Currently, a BPF
program either has to fail to verify, or be loaded with log level 2 in
order to see the message. We could eventually enhance this to always
be logged regardless of log level or verification status, or we could
instead emit a warning to dmesg. This seems like the least controversial
option for now.

A subsequent patch will add a selftest that verifies this behavior.

Signed-off-by: David Vernet <void@manifault.com>
---
 include/linux/btf.h   | 1 +
 kernel/bpf/verifier.c | 8 ++++++++
 2 files changed, 9 insertions(+)

diff --git a/include/linux/btf.h b/include/linux/btf.h
index 49e0fe6d8274..a0ea788ee9b0 100644
--- a/include/linux/btf.h
+++ b/include/linux/btf.h
@@ -71,6 +71,7 @@
 #define KF_SLEEPABLE    (1 << 5) /* kfunc may sleep */
 #define KF_DESTRUCTIVE  (1 << 6) /* kfunc performs destructive actions */
 #define KF_RCU          (1 << 7) /* kfunc only takes rcu pointer arguments */
+#define KF_DEPRECATED   (1 << 8) /* kfunc is slated to be removed or deprecated */
 
 /*
  * Tag marking a kernel function as a kfunc. This is meant to minimize the
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 4cc0e70ee71e..22adcf24f9e1 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -8511,6 +8511,11 @@ static bool is_kfunc_rcu(struct bpf_kfunc_call_arg_meta *meta)
 	return meta->kfunc_flags & KF_RCU;
 }
 
+static bool is_kfunc_deprecated(const struct bpf_kfunc_call_arg_meta *meta)
+{
+	return meta->kfunc_flags & KF_DEPRECATED;
+}
+
 static bool is_kfunc_arg_kptr_get(struct bpf_kfunc_call_arg_meta *meta, int arg)
 {
 	return arg == 0 && (meta->kfunc_flags & KF_KPTR_GET);
@@ -9646,6 +9651,9 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
 			mark_btf_func_reg_size(env, regno, t->size);
 	}
 
+	if (is_kfunc_deprecated(&meta))
+		verbose(env, "calling deprecated kfunc %s\n", func_name);
+
 	return 0;
 }
 
-- 
2.39.0