Relax conditions such that we allow log_level = 0 to have an associated
log buffer. Add a warn() macro that emits messages to log buffer without
any restrictions, aggregate the warnings emitted, and then use it to
decide whether we reset the log or not.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
---
 include/linux/bpf_verifier.h |  1 +
 kernel/bpf/log.c             |  6 ++----
 kernel/bpf/verifier.c        | 17 +++++++++++++++--
 3 files changed, 18 insertions(+), 6 deletions(-)

diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 36bfd96d4563..c7b34236a4b5 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -795,6 +795,7 @@ struct bpf_verifier_env {
 	bool bypass_spec_v4;
 	bool seen_direct_write;
 	bool seen_exception;
+	bool warnings;
 	struct bpf_insn_aux_data *insn_aux_data; /* array of per-insn state */
 	const struct bpf_line_info *prev_linfo;
 	struct bpf_verifier_log log;
diff --git a/kernel/bpf/log.c b/kernel/bpf/log.c
index 37d72b052192..c67fbbbd5768 100644
--- a/kernel/bpf/log.c
+++ b/kernel/bpf/log.c
@@ -18,13 +18,11 @@ static bool bpf_verifier_log_attr_valid(const struct bpf_verifier_log *log)
 	/* ubuf and len_total should both be specified (or not) together */
 	if (!!log->ubuf != !!log->len_total)
 		return false;
-	/* log buf without log_level is meaningless */
-	if (log->ubuf && log->level == 0)
-		return false;
 	if (log->level & ~BPF_LOG_MASK)
 		return false;
 	if (log->len_total > UINT_MAX >> 2)
 		return false;
+	/* log->ubuf may be set for log->level = 0 to get warning messages. */
 	return true;
 }
 
@@ -229,7 +227,7 @@ int bpf_vlog_finalize(struct bpf_verifier_log *log, u32 *log_size_actual)
 	int err;
 
 	*log_size_actual = 0;
-	if (!log || log->level == 0 || log->level == BPF_LOG_KERNEL)
+	if (!log || (!log->ubuf && log->level == 0) || log->level == BPF_LOG_KERNEL)
 		return 0;
 
 	if (!log->ubuf)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 594260c1f382..86d77d5f7f83 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -47,6 +47,7 @@ static const struct bpf_verifier_ops * const bpf_verifier_ops[] = {
 enum bpf_features {
 	BPF_FEAT_RDONLY_CAST_TO_VOID = 0,
 	BPF_FEAT_STREAMS	     = 1,
+	BPF_FEAT_VERIFIER_WARNINGS   = 2,
 	__MAX_BPF_FEAT,
 };
 
@@ -368,6 +369,17 @@ __printf(2, 3) static void verbose(void *private_data, const char *fmt, ...)
 	va_end(args);
 }
 
+__printf(2, 3) static void warn(void *private_data, const char *fmt, ...)
+{
+	struct bpf_verifier_env *env = private_data;
+	va_list args;
+
+	va_start(args, fmt);
+	bpf_verifier_vlog(&env->log, fmt, args);
+	va_end(args);
+	env->warnings = true;
+}
+
 static void verbose_invalid_scalar(struct bpf_verifier_env *env,
 				   struct bpf_reg_state *reg,
 				   struct bpf_retval_range range, const char *ctx,
@@ -2128,7 +2140,8 @@ static int pop_stack(struct bpf_verifier_env *env, int *prev_insn_idx,
 		if (err)
 			return err;
 	}
-	if (pop_log)
+	/* Preserve warning-only output across branch explorations. */
+	if (pop_log && !(env->warnings && env->log.level == 0))
 		bpf_vlog_reset(&env->log, head->log_pos);
 	if (insn_idx)
 		*insn_idx = head->insn_idx;
@@ -25211,7 +25224,7 @@ static int do_check_common(struct bpf_verifier_env *env, int subprog)
 
 	ret = do_check(env);
 out:
-	if (!ret && pop_log)
+	if (!ret && pop_log && !env->warnings)
 		bpf_vlog_reset(&env->log, 0);
 	free_states(env);
 	return ret;
-- 
2.52.0