[PATCH bpf-next v3 1/4] bpf: Add support for verifier warning messages

Kumar Kartikeya Dwivedi
2026-04-18T17:17:06Z

Add a mode where log_level 16 can be used to receive warnings and 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>
---
Documentation/bpf/kfuncs.rst | 4 +++-
include/linux/bpf_verifier.h | 11 +++++++++--
kernel/bpf/log.c | 2 +-
kernel/bpf/verifier.c | 21 +++++++++++++++++++--
4 files changed, 32 insertions(+), 6 deletions(-)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index 75e6c078e0e7..47d60011d8eb 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -353,7 +353,9 @@ marked with KF_DEPRECATED should also have any relevant information
captured in its kernel doc. Such information typically includes the
kfunc's expected remaining lifespan, a recommendation for new
functionality that can replace it if any is available, and possibly a
-rationale for why it is being removed.
+rationale for why it is being removed. When verifier warning logging is
+requested, the verifier will emit a warning when a BPF program uses a
+deprecated kfunc.

Note that while on some occasions, a KF_DEPRECATED kfunc may continue to be
supported and have its KF_DEPRECATED flag removed, it is likely to be far more
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index b148f816f25b..515afcb83ec7 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -689,15 +689,21 @@ struct bpf_verifier_log {
#define BPF_LOG_LEVEL2 2
#define BPF_LOG_STATS 4
#define BPF_LOG_FIXED 8
+#define BPF_LOG_LEVEL_WARN 16
#define BPF_LOG_LEVEL (BPF_LOG_LEVEL1 | BPF_LOG_LEVEL2)
-#define BPF_LOG_MASK (BPF_LOG_LEVEL | BPF_LOG_STATS | BPF_LOG_FIXED)
+#define BPF_LOG_MASK (BPF_LOG_LEVEL | BPF_LOG_STATS | BPF_LOG_FIXED | BPF_LOG_LEVEL_WARN)
#define BPF_LOG_KERNEL (BPF_LOG_MASK + 1) /* kernel internal flag */
#define BPF_LOG_MIN_ALIGNMENT 8U
#define BPF_LOG_ALIGNMENT 40U

static inline bool bpf_verifier_log_needed(const struct bpf_verifier_log *log)
{
- return log && log->level;
+ return log && (log->level & ~BPF_LOG_LEVEL_WARN);
+}
+
+static inline bool bpf_verifier_warn_needed(const struct bpf_verifier_log *log)
+{
+ return log && (log->level & BPF_LOG_LEVEL_WARN);
}

#define BPF_MAX_SUBPROGS 256
@@ -848,6 +854,7 @@ 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 011e4ec25acd..2cf79fac8d43 100644
--- a/kernel/bpf/log.c
+++ b/kernel/bpf/log.c
@@ -154,7 +154,7 @@ void bpf_vlog_reset(struct bpf_verifier_log *log, u64 new_pos)
if (WARN_ON_ONCE(new_pos > log->end_pos))
return;

- if (!bpf_verifier_log_needed(log) || log->level == BPF_LOG_KERNEL)
+ if (!log || log->level == 0 || log->level == BPF_LOG_KERNEL)
return;

/* if position to which we reset is beyond current log window,
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 69d75515ed3f..8de2a4e5f5de 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,
};

@@ -282,6 +283,20 @@ __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;
+
+ if (!bpf_verifier_warn_needed(&env->log))
+ return;
+
+ 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,
@@ -1683,7 +1698,8 @@ static int pop_stack(struct bpf_verifier_env *env, int *prev_insn_idx,
if (err)
return err;
}
- if (pop_log)
+ /* Preserve warning output across branch explorations. */
+ if (pop_log && !(env->warnings && bpf_verifier_warn_needed(&env->log)))
bpf_vlog_reset(&env->log, head->log_pos);
if (insn_idx)
*insn_idx = head->insn_idx;
@@ -18803,7 +18819,8 @@ 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_verifier_warn_needed(&env->log)))
bpf_vlog_reset(&env->log, 0);
free_states(env);
return ret;
--
2.52.0