bpf: Add bpf_verifier_vlog() and bpf_verifier_log_needed()

The BTF (BPF Type Format) verifier needs to reuse the current
BPF verifier log.  Hence, it requires the following changes:

(1) Expose log_write() in verifier.c for other users.
    Its name is renamed to bpf_verifier_vlog().

(2) The BTF verifier also needs to check
'log->level && log->ubuf && !bpf_verifier_log_full(log);'
independently outside of the current log_write().  It is
because the BTF verifier will do one-check before
making multiple calls to btf_verifier_vlog to log
the details of a type.

Hence, this check is also re-factored to a new function
bpf_verifier_log_needed().  Since it is re-factored,
we can check it before va_start() in the current
bpf_verifier_log_write() and verbose().

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Alexei Starovoitov <ast@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/bpf_verifier.h

@@ -166,6 +166,11 @@ static inline bool bpf_verifier_log_full(const struct bpf_verifier_log *log)
 	return log->len_used >= log->len_total - 1;
 }
 
+static inline bool bpf_verifier_log_needed(const struct bpf_verifier_log *log)
+{
+	return log->level && log->ubuf && !bpf_verifier_log_full(log);
+}
+
 #define BPF_MAX_SUBPROGS 256
 
 /* single container for all structs
@@ -192,6 +197,8 @@ struct bpf_verifier_env {
 	u32 subprog_cnt;
 };
 
+void bpf_verifier_vlog(struct bpf_verifier_log *log, const char *fmt,
+		       va_list args);
 __printf(2, 3) void bpf_verifier_log_write(struct bpf_verifier_env *env,
 					   const char *fmt, ...);
 

kernel/bpf/verifier.c

@@ -168,15 +168,11 @@ struct bpf_call_arg_meta {
 
 static DEFINE_MUTEX(bpf_verifier_lock);
 
-static void log_write(struct bpf_verifier_env *env, const char *fmt,
-		      va_list args)
+void bpf_verifier_vlog(struct bpf_verifier_log *log, const char *fmt,
+		       va_list args)
 {
-	struct bpf_verifier_log *log = &env->log;
 	unsigned int n;
 
-	if (!log->level || !log->ubuf || bpf_verifier_log_full(log))
-		return;
-
 	n = vscnprintf(log->kbuf, BPF_VERIFIER_TMP_LOG_SIZE, fmt, args);
 
 	WARN_ONCE(n >= BPF_VERIFIER_TMP_LOG_SIZE - 1,
@@ -200,18 +196,25 @@ __printf(2, 3) void bpf_verifier_log_write(struct bpf_verifier_env *env,
 {
 	va_list args;
 
+	if (!bpf_verifier_log_needed(&env->log))
+		return;
+
 	va_start(args, fmt);
-	log_write(env, fmt, args);
+	bpf_verifier_vlog(&env->log, fmt, args);
 	va_end(args);
 }
 EXPORT_SYMBOL_GPL(bpf_verifier_log_write);
 
 __printf(2, 3) static void verbose(void *private_data, const char *fmt, ...)
 {
+	struct bpf_verifier_env *env = private_data;
 	va_list args;
 
+	if (!bpf_verifier_log_needed(&env->log))
+		return;
+
 	va_start(args, fmt);
-	log_write(private_data, fmt, args);
+	bpf_verifier_vlog(&env->log, fmt, args);
 	va_end(args);
 }