[PATCH v2 bpf-next 21/21] selftests/bpf: C based test for sysctl and strtoX

[Andrey Ignatov]

Add C based test for a few bpf_sysctl_* helpers and bpf_strtoul.

Make sure that sysctl can be identified by name and that multiple
integers can be parsed from sysctl value with bpf_strtoul.

net/ipv4/tcp_mem is chosen as a testing sysctl, it contains 3 unsigned
longs, they all are parsed and compared (val[0] < val[1] < val[2]).

One verifier limitation is being worked around in the test: variable
stack access. Since such an access is unsupported, offset, returned by
bpf_strtoul while parsing current ulong, can't be used directly when
specifying 'buf' pointer to the next bpf_strtoul. That's why instead of
'base + offset' where offset non-const tnum, the test uses brute force
search to find static number, that matches with 'offset', and use
'base + static' to access the stack.

Example of output:
  # ./test_sysctl
  ...
  Test case: C prog: deny all writes .. [PASS]
  Test case: C prog: deny access by name .. [PASS]
  Test case: C prog: read tcp_mem .. [PASS]
  Summary: 39 PASSED, 0 FAILED

Signed-off-by: Andrey Ignatov <rdna@xxxxxx>
---
 .../selftests/bpf/progs/test_sysctl_prog.c    | 89 +++++++++++++++++++
 tools/testing/selftests/bpf/test_sysctl.c     | 57 +++++++++++-
 2 files changed, 145 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/bpf/progs/test_sysctl_prog.c

diff --git a/tools/testing/selftests/bpf/progs/test_sysctl_prog.c b/tools/testing/selftests/bpf/progs/test_sysctl_prog.c
new file mode 100644
index 000000000000..29c02b6a0979
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_sysctl_prog.c
@@ -0,0 +1,89 @@
+// SPDX-License-Identifier: GPL-2.0
+// Copyright (c) 2019 Facebook
+
+#include <stdint.h>
+#include <string.h>
+
+#include <linux/stddef.h>
+#include <linux/bpf.h>
+
+#include "bpf_helpers.h"
+
+/* Min/max supported lengths of a string with unsigned long in base 10. */
+#define MIN_ULONG_STR_LEN 2
+#define MAX_ULONG_STR_LEN 15
+
+static __always_inline int is_tcp_mem(struct bpf_sysctl *ctx)
+{
+	char tcp_mem_name[] = "net/ipv4/tcp_mem";
+	unsigned char i;
+	char name[64];
+	int ret;
+
+	memset(name, 0, sizeof(name));
+	ret = bpf_sysctl_get_name(ctx, name, sizeof(name), 0);
+	if (ret < 0 || ret != sizeof(tcp_mem_name) - 1)
+		return 0;
+
+#pragma clang loop unroll(full)
+	for (i = 0; i < sizeof(tcp_mem_name); ++i)
+		if (name[i] != tcp_mem_name[i])
+			return 0;
+
+	return 1;
+}
+
+SEC("cgroup/sysctl")
+int sysctl_tcp_mem(struct bpf_sysctl *ctx)
+{
+	unsigned long mem_min = 0, mem_pressure = 0, mem_max = 0;
+	unsigned char i, off;
+	char value[64];
+	int ret;
+
+	if (ctx->write)
+		return 0;
+
+	if (!is_tcp_mem(ctx))
+		return 0;
+
+	ret = bpf_sysctl_get_current_value(ctx, value, sizeof(value));
+	if (ret < 0 || ret >= sizeof(value))
+		return 0;
+
+	ret = bpf_strtoul(value, sizeof(value), 0, &mem_min);
+	if (ret <= 0 || ret > MAX_ULONG_STR_LEN)
+		return 0;
+	off = ret;
+	ret = -1;
+
+	/* Make verifier happy, prevent "invalid variable stack read R1
+	 * var_off=(0x0; 0xf)" by using static offset for value.
+	 */
+#pragma clang loop unroll(full)
+	for (i = MIN_ULONG_STR_LEN; i <= MAX_ULONG_STR_LEN; ++i)
+		if (off == i) {
+			ret = bpf_strtoul(value + i, sizeof(value) - i, 0,
+					  &mem_pressure);
+			break;
+		}
+	if (ret <= 0 || ret > MAX_ULONG_STR_LEN)
+		return 0;
+	off += ret;
+	ret = -1;
+
+	/* Same here. */
+#pragma clang loop unroll(full)
+	for (i = MIN_ULONG_STR_LEN * 2; i <= MAX_ULONG_STR_LEN * 2 + 2; ++i)
+		if (off == i) {
+			ret = bpf_strtoul(value + i, sizeof(value) - i, 0,
+					  &mem_max);
+			break;
+		}
+	if (ret <= 0 || ret > MAX_ULONG_STR_LEN)
+		return 0;
+
+	return mem_min < mem_pressure && mem_pressure < mem_max;
+}
+
+char _license[] SEC("license") = "GPL";
diff --git a/tools/testing/selftests/bpf/test_sysctl.c b/tools/testing/selftests/bpf/test_sysctl.c
index 885675480af9..a3bebd7c68dd 100644
--- a/tools/testing/selftests/bpf/test_sysctl.c
+++ b/tools/testing/selftests/bpf/test_sysctl.c
@@ -11,6 +11,7 @@
 #include <linux/filter.h>
 
 #include <bpf/bpf.h>
+#include <bpf/libbpf.h>
 
 #include "bpf_rlimit.h"
 #include "bpf_util.h"
@@ -26,6 +27,7 @@ struct sysctl_test {
 	const char *descr;
 	size_t fixup_value_insn;
 	struct bpf_insn	insns[MAX_INSNS];
+	const char *prog_file;
 	enum bpf_attach_type attach_type;
 	const char *sysctl;
 	int open_flags;
@@ -1302,6 +1304,31 @@ static struct sysctl_test tests[] = {
 		.open_flags = O_RDONLY,
 		.result = SUCCESS,
 	},
+	{
+		"C prog: deny all writes",
+		.prog_file = "./test_sysctl_prog.o",
+		.attach_type = BPF_CGROUP_SYSCTL,
+		.sysctl = "net/ipv4/tcp_mem",
+		.open_flags = O_WRONLY,
+		.newval = "123 456 789",
+		.result = OP_EPERM,
+	},
+	{
+		"C prog: deny access by name",
+		.prog_file = "./test_sysctl_prog.o",
+		.attach_type = BPF_CGROUP_SYSCTL,
+		.sysctl = "net/ipv4/route/mtu_expires",
+		.open_flags = O_RDONLY,
+		.result = OP_EPERM,
+	},
+	{
+		"C prog: read tcp_mem",
+		.prog_file = "./test_sysctl_prog.o",
+		.attach_type = BPF_CGROUP_SYSCTL,
+		.sysctl = "net/ipv4/tcp_mem",
+		.open_flags = O_RDONLY,
+		.result = SUCCESS,
+	},
 };
 
 static size_t probe_prog_length(const struct bpf_insn *fp)
@@ -1335,7 +1362,8 @@ static int fixup_sysctl_value(const char *buf, size_t buf_len,
 	return 0;
 }
 
-static int load_sysctl_prog(struct sysctl_test *test, const char *sysctl_path)
+static int load_sysctl_prog_insns(struct sysctl_test *test,
+				  const char *sysctl_path)
 {
 	struct bpf_insn *prog = test->insns;
 	struct bpf_load_program_attr attr;
@@ -1377,6 +1405,33 @@ static int load_sysctl_prog(struct sysctl_test *test, const char *sysctl_path)
 	return ret;
 }
 
+static int load_sysctl_prog_file(struct sysctl_test *test)
+{
+	struct bpf_prog_load_attr attr;
+	struct bpf_object *obj;
+	int prog_fd;
+
+	memset(&attr, 0, sizeof(struct bpf_prog_load_attr));
+	attr.file = test->prog_file;
+	attr.prog_type = BPF_PROG_TYPE_CGROUP_SYSCTL;
+
+	if (bpf_prog_load_xattr(&attr, &obj, &prog_fd)) {
+		if (test->result != LOAD_REJECT)
+			log_err(">>> Loading program (%s) error.\n",
+				test->prog_file);
+		return -1;
+	}
+
+	return prog_fd;
+}
+
+static int load_sysctl_prog(struct sysctl_test *test, const char *sysctl_path)
+{
+		return test->prog_file
+			? load_sysctl_prog_file(test)
+			: load_sysctl_prog_insns(test, sysctl_path);
+}
+
 static int access_sysctl(const char *sysctl_path,
 			 const struct sysctl_test *test)
 {
-- 
2.17.1