[RFC bpf-next 2/2] selftests/bpf: add a kallsyms iter subtest

[Alan Maguire <alan.maguire@oracle.com>]

add subtest verifying BPF kallsyms iter behaviour.  The BPF kallsyms
iter program shows an example of dumping a format different to
kallsyms.  It adds KIND and MAX_SIZE fields which represent the
kind of symbol (core kernel, module, ftrace, bpf, or kprobe) and
the maximum size the symbol can be.  The latter is calculated from
the difference between current symbol value and the next symbol
value.

The key benefit for this iterator will likely be supporting in-kernel
data-gathering rather than dumping symbol details to userspace and
parsing the results.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
---
 tools/testing/selftests/bpf/prog_tests/bpf_iter.c  | 16 +++++
 .../selftests/bpf/progs/bpf_iter_kallsyms.c        | 71 ++++++++++++++++++++++
 2 files changed, 87 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/bpf_iter_kallsyms.c

diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
index 7ff5fa9..1197a58 100644
--- a/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
+++ b/tools/testing/selftests/bpf/prog_tests/bpf_iter.c
@@ -27,6 +27,7 @@
 #include "bpf_iter_test_kern5.skel.h"
 #include "bpf_iter_test_kern6.skel.h"
 #include "bpf_iter_bpf_link.skel.h"
+#include "bpf_iter_kallsyms.skel.h"
 
 static int duration;
 
@@ -1120,6 +1121,19 @@ static void test_link_iter(void)
 	bpf_iter_bpf_link__destroy(skel);
 }
 
+static void test_kallsyms_iter(void)
+{
+	struct bpf_iter_kallsyms *skel;
+
+	skel = bpf_iter_kallsyms__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "bpf_iter_kallsyms__open_and_load"))
+		return;
+
+	do_dummy_read(skel->progs.dump_kallsyms);
+
+	bpf_iter_kallsyms__destroy(skel);
+}
+
 #define CMP_BUFFER_SIZE 1024
 static char task_vma_output[CMP_BUFFER_SIZE];
 static char proc_maps_output[CMP_BUFFER_SIZE];
@@ -1267,4 +1281,6 @@ void test_bpf_iter(void)
 		test_buf_neg_offset();
 	if (test__start_subtest("link-iter"))
 		test_link_iter();
+	if (test__start_subtest("kallsyms"))
+		test_kallsyms_iter();
 }
diff --git a/tools/testing/selftests/bpf/progs/bpf_iter_kallsyms.c b/tools/testing/selftests/bpf/progs/bpf_iter_kallsyms.c
new file mode 100644
index 0000000..20c8d85
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/bpf_iter_kallsyms.c
@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022, Oracle and/or its affiliates. */
+#include "bpf_iter.h"
+#include <bpf/bpf_helpers.h>
+
+char _license[] SEC("license") = "GPL";
+
+unsigned long last_sym_value = 0;
+
+static inline char tolower(char c)
+{
+	if (c >= 'A' && c <= 'Z')
+		c += ('a' - 'A');
+	return c;
+}
+
+static inline char toupper(char c)
+{
+	if (c >= 'a' && c <= 'z')
+		c -= ('a' - 'A');
+	return c;
+}
+
+/* Dump symbols with max size; the latter is calculated by caching symbol N value
+ * and when iterating on symbol N+1, we can print max size of symbol N via
+ * address of N+1 - address of N.
+ */
+SEC("iter/kallsyms")
+int dump_kallsyms(struct bpf_iter__kallsyms *ctx)
+{
+	struct seq_file *seq = ctx->meta->seq;
+	struct kallsym_iter *iter = ctx->kallsym_iter;
+	__u32 seq_num = ctx->meta->seq_num;
+	char type;
+	int ret;
+
+	if (!iter)
+		return 0;
+
+	if (seq_num == 0) {
+		BPF_SEQ_PRINTF(seq, "ADDR TYPE NAME MODULE_NAME KIND MAX_SIZE\n");
+		return 0;
+	}
+	if (last_sym_value)
+		BPF_SEQ_PRINTF(seq, "0x%x\n", iter->value - last_sym_value);
+	else
+		BPF_SEQ_PRINTF(seq, "\n");
+
+	last_sym_value = iter->value;
+
+	type = iter->type;
+
+	if (iter->module_name[0]) {
+		type = iter->exported ? toupper(type) : tolower(type);
+		BPF_SEQ_PRINTF(seq, "0x%llx %c %s [ %s ] ",
+			       iter->value, type, iter->name, iter->module_name);
+	} else {
+		BPF_SEQ_PRINTF(seq, "0x%llx %c %s ", iter->value, type, iter->name);
+	}
+	if (!iter->pos_arch_end || iter->pos_arch_end > iter->pos)
+		BPF_SEQ_PRINTF(seq, "CORE ");
+	else if (!iter->pos_mod_end || iter->pos_mod_end > iter->pos)
+		BPF_SEQ_PRINTF(seq, "MOD ");
+	else if (!iter->pos_ftrace_mod_end || iter->pos_ftrace_mod_end > iter->pos)
+		BPF_SEQ_PRINTF(seq, "FTRACE_MOD ");
+	else if (!iter->pos_bpf_end || iter->pos_bpf_end > iter->pos)
+		BPF_SEQ_PRINTF(seq, "BPF ");
+	else
+		BPF_SEQ_PRINTF(seq, "KPROBE ");
+	return 0;
+}
-- 
1.8.3.1