[PATCH bpf-next 1/2] bpf: Fix verifier log for async callback return values

[PATCH bpf-next 1/2] bpf: Fix verifier log for async callback return values

[David Vernet]

The verifier, as part of check_return_code(), verifies that async
callbacks such as from e.g. timers, will return 0. It does this by
correctly checking that R0->var_off is in tnum_const(0), which
effectively checks that it's in a range of 0. If this condition fails,
however, it prints an error message which says that the value should
have been in (0x0; 0x1). This results in possibly confusing output such
as the following in which an async callback returns 1:

At async callback the register R0 has value (0x1; 0x0) should have been
in (0x0; 0x1)

The fix is easy -- we should just pass the tnum_const(0) as the correct
range to verbose_invalid_scalar(), which will then print the following:

At async callback the register R0 has value (0x1; 0x0) should have been
in (0x0; 0x0)

Fixes: bfc6bb74e4f1 ("bpf: Implement verifier support for validation of async callbacks.")
Signed-off-by: David Vernet <void@manifault.com>
---
 kernel/bpf/verifier.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index eed7350e15f4..9093fb74c88e 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -14729,7 +14729,7 @@ static int check_return_code(struct bpf_verifier_env *env, int regno)
 	struct tnum enforce_attach_type_range = tnum_unknown;
 	const struct bpf_prog *prog = env->prog;
 	struct bpf_reg_state *reg;
-	struct tnum range = tnum_range(0, 1);
+	struct tnum range = tnum_range(0, 1), const_0 = tnum_const(0);
 	enum bpf_prog_type prog_type = resolve_prog_type(env->prog);
 	int err;
 	struct bpf_func_state *frame = env->cur_state->frame[0];
@@ -14777,8 +14777,8 @@ static int check_return_code(struct bpf_verifier_env *env, int regno)
 			return -EINVAL;
 		}
 
-		if (!tnum_in(tnum_const(0), reg->var_off)) {
-			verbose_invalid_scalar(env, reg, &range, "async callback", "R0");
+		if (!tnum_in(const_0, reg->var_off)) {
+			verbose_invalid_scalar(env, reg, &const_0, "async callback", "R0");
 			return -EINVAL;
 		}
 		return 0;
-- 
2.41.0

[PATCH bpf-next 2/2] bpf/selftests: Add testcase for async callback return value failure

[David Vernet]

A previous commit updated the verifier to print an accurate failure
message for when someone specifies a nonzero return value from an async
callback. This adds a testcase for validating that the verifier emits
the correct message in such a case.

Signed-off-by: David Vernet <void@manifault.com>
---
 .../testing/selftests/bpf/prog_tests/timer.c  |  6 ++-
 .../selftests/bpf/progs/timer_failure.c       | 47 +++++++++++++++++++
 2 files changed, 51 insertions(+), 2 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/progs/timer_failure.c

diff --git a/tools/testing/selftests/bpf/prog_tests/timer.c b/tools/testing/selftests/bpf/prog_tests/timer.c
index d8bc838445ec..760ad96b4be0 100644
--- a/tools/testing/selftests/bpf/prog_tests/timer.c
+++ b/tools/testing/selftests/bpf/prog_tests/timer.c
@@ -2,6 +2,7 @@
 /* Copyright (c) 2021 Facebook */
 #include <test_progs.h>
 #include "timer.skel.h"
+#include "timer_failure.skel.h"
 
 static int timer(struct timer *timer_skel)
 {
@@ -53,10 +54,11 @@ void serial_test_timer(void)
 
 	timer_skel = timer__open_and_load();
 	if (!ASSERT_OK_PTR(timer_skel, "timer_skel_load"))
-		goto cleanup;
+		return;
 
 	err = timer(timer_skel);
 	ASSERT_OK(err, "timer");
-cleanup:
 	timer__destroy(timer_skel);
+
+	RUN_TESTS(timer_failure);
 }
diff --git a/tools/testing/selftests/bpf/progs/timer_failure.c b/tools/testing/selftests/bpf/progs/timer_failure.c
new file mode 100644
index 000000000000..226d33b5a05c
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/timer_failure.c
@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include <linux/bpf.h>
+#include <time.h>
+#include <errno.h>
+#include <bpf/bpf_helpers.h>
+#include "bpf_misc.h"
+#include "bpf_tcp_helpers.h"
+
+char _license[] SEC("license") = "GPL";
+
+struct elem {
+	struct bpf_timer t;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, struct elem);
+} timer_map SEC(".maps");
+
+static int timer_cb_ret1(void *map, int *key, struct bpf_timer *timer)
+{
+	if (bpf_get_smp_processor_id() % 2)
+		return 1;
+	else
+		return 0;
+}
+
+SEC("fentry/bpf_fentry_test1")
+__failure __msg("should have been in (0x0; 0x0)")
+int BPF_PROG2(test_ret_1, int, a)
+{
+	int key = 0;
+	struct bpf_timer *timer;
+
+	timer = bpf_map_lookup_elem(&timer_map, &key);
+	if (timer) {
+		bpf_timer_init(timer, &timer_map, CLOCK_BOOTTIME);
+		bpf_timer_set_callback(timer, timer_cb_ret1);
+		bpf_timer_start(timer, 1000, 0);
+	}
+
+	return 0;
+}
-- 
2.41.0

Re: [PATCH bpf-next 1/2] bpf: Fix verifier log for async callback return values

[patchwork-bot+netdevbpf]

Hello:

This series was applied to bpf/bpf.git (master)
by Daniel Borkmann <daniel@iogearbox.net>:

On Mon,  9 Oct 2023 11:14:13 -0500 you wrote:
> The verifier, as part of check_return_code(), verifies that async
> callbacks such as from e.g. timers, will return 0. It does this by
> correctly checking that R0->var_off is in tnum_const(0), which
> effectively checks that it's in a range of 0. If this condition fails,
> however, it prints an error message which says that the value should
> have been in (0x0; 0x1). This results in possibly confusing output such
> as the following in which an async callback returns 1:
> 
> [...]

Here is the summary with links:
  - [bpf-next,1/2] bpf: Fix verifier log for async callback return values
    https://git.kernel.org/bpf/bpf/c/829955981c55
  - [bpf-next,2/2] bpf/selftests: Add testcase for async callback return value failure
    https://git.kernel.org/bpf/bpf/c/57ddeb86b311

You are awesome, thank you!
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