selftests/bpf: Test bpf_iter buffer access with negative offset

Commit afbf21dc ("bpf: Support readonly/readwrite buffers
in verifier") added readonly/readwrite buffer support which
is currently used by bpf_iter tracing programs. It has
a bug with incorrect parameter ordering which later fixed
by Commit f6dfbe31 ("bpf: Fix swapped arguments in calls
to check_buffer_access").

This patch added a test case with a negative offset access
which will trigger the error path.

Without Commit f6dfbe31, running the test case in the patch,
the error message looks like:
   R1_w=rdwr_buf(id=0,off=0,imm=0) R10=fp0
  ; value_sum += *(__u32 *)(value - 4);
  2: (61) r1 = *(u32 *)(r1 -4)
  R1 invalid (null) buffer access: off=-4, size=4

With the above commit, the error message looks like:
   R1_w=rdwr_buf(id=0,off=0,imm=0) R10=fp0
  ; value_sum += *(__u32 *)(value - 4);
  2: (61) r1 = *(u32 *)(r1 -4)
  R1 invalid rdwr buffer access: off=-4, size=4
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200728221801.1090406-1-yhs@fb.com

tools/testing/selftests/bpf/prog_tests/bpf_iter.c

@@ -21,6 +21,7 @@
 #include "bpf_iter_bpf_percpu_array_map.skel.h"
 #include "bpf_iter_bpf_sk_storage_map.skel.h"
 #include "bpf_iter_test_kern5.skel.h"
+#include "bpf_iter_test_kern6.skel.h"
 
 static int duration;
 
@@ -885,6 +886,16 @@ static void test_rdonly_buf_out_of_bound(void)
 	bpf_iter_test_kern5__destroy(skel);
 }
 
+static void test_buf_neg_offset(void)
+{
+	struct bpf_iter_test_kern6 *skel;
+
+	skel = bpf_iter_test_kern6__open_and_load();
+	if (CHECK(skel, "bpf_iter_test_kern6__open_and_load",
+		  "skeleton open_and_load unexpected success\n"))
+		bpf_iter_test_kern6__destroy(skel);
+}
+
 void test_bpf_iter(void)
 {
 	if (test__start_subtest("btf_id_or_null"))
@@ -933,4 +944,6 @@ void test_bpf_iter(void)
 		test_bpf_sk_storage_map();
 	if (test__start_subtest("rdonly-buf-out-of-bound"))
 		test_rdonly_buf_out_of_bound();
+	if (test__start_subtest("buf-neg-offset"))
+		test_buf_neg_offset();
 }

tools/testing/selftests/bpf/progs/bpf_iter_test_kern6.c

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2020 Facebook */
+#include "bpf_iter.h"
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+__u32 value_sum = 0;
+
+SEC("iter/bpf_map_elem")
+int dump_bpf_hash_map(struct bpf_iter__bpf_map_elem *ctx)
+{
+	void *value = ctx->value;
+
+	if (value == (void *)0)
+		return 0;
+
+	/* negative offset, verifier failure. */
+	value_sum += *(__u32 *)(value - 4);
+	return 0;
+}