Re: [PATCH v2 bpf-next 2/2] selftests/bpf: Add verifier test exercising jit PROBE_MEM logic

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On 12/16/22 10:31 AM, Dave Marchevsky wrote:
This patch adds a test exercising logic that was fixed / improved in
the previous patch in the series, as well as general sanity checking for
jit's PROBE_MEM logic which should've been unaffected by the previous
patch.

The added verifier test does the following:
   * Acquire a referenced kptr to struct prog_test_ref_kfunc using
     existing net/bpf/test_run.c kfunc
     * Helper returns ptr to a specific prog_test_ref_kfunc whose first
       two fields - both ints - have been prepopulated w/ vals 42 and
       108, respectively
   * kptr_xchg the acquired ptr into an arraymap
   * Do a direct map_value load of the just-added ptr
     * Goal of all this setup is to get an unreferenced kptr pointing to
       struct with ints of known value, which is the result of this step
   * Using unreferenced kptr obtained in previous step, do loads of
     prog_test_ref_kfunc.a (offset 0) and .b (offset 4)
   * Then incr the kptr by 8 and load prog_test_ref_kfunc.a again (this
     time at offset -8)
   * Add all the loaded ints together and return

Before the PROBE_MEM fixes in previous patch, the loads at offset 0 and
4 would succeed, while the load at offset -8 would incorrectly fail
runtime check emitted by the JIT and 0 out dst reg as a result. This
confirmed by retval of 150 for this test before previous patch - since
second .a read is 0'd out - and a retval of 192 with the fixed logic.

The test exercises the two optimizations to fixed logic added in last
patch as well:
   * First load, with insn "r8 = *(u32 *)(r9 + 0)" exercises "insn->off
     is 0, no need to add / sub from src_reg" optimization
   * Third load, with insn "r9 = *(u32 *)(r9 - 8)" exercises "src_reg ==
     dst_reg, no need to restore src_reg after load" optimization

Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>

Ack with one nit below.

Acked-by: Yonghong Song <yhs@xxxxxx>

---
v1 -> v2: lore.kernel.org/bpf/20221213182726.325137-2-davemarchevsky@xxxxxx
   * Rewrite the test to be a "normal" C prog in selftests/bpf/progs. Result
     is a much easier-to-understand test with assembly used only for the 3
     loads. (Yonghong)

  .../selftests/bpf/prog_tests/jit_probe_mem.c  | 28 +++++++++
  .../selftests/bpf/progs/jit_probe_mem.c       | 61 +++++++++++++++++++
  2 files changed, 89 insertions(+)
  create mode 100644 tools/testing/selftests/bpf/prog_tests/jit_probe_mem.c
  create mode 100644 tools/testing/selftests/bpf/progs/jit_probe_mem.c

diff --git a/tools/testing/selftests/bpf/prog_tests/jit_probe_mem.c b/tools/testing/selftests/bpf/prog_tests/jit_probe_mem.c
new file mode 100644
index 000000000000..5639428607e6
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/jit_probe_mem.c
@@ -0,0 +1,28 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
+#include <test_progs.h>
+#include <network_helpers.h>
+
+#include "jit_probe_mem.skel.h"
+
+void test_jit_probe_mem(void)
+{
+	LIBBPF_OPTS(bpf_test_run_opts, opts,
+		.data_in = &pkt_v4,
+		.data_size_in = sizeof(pkt_v4),
+		.repeat = 1,
+	);
+	struct jit_probe_mem *skel;
+	int ret;
+
+	skel = jit_probe_mem__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "jit_probe_mem__open_and_load"))
+		return;
+
+	ret = bpf_prog_test_run_opts(bpf_program__fd(skel->progs.test_jit_probe_mem), &opts);
+	ASSERT_OK(ret, "jit_probe_mem ret");
+	ASSERT_OK(opts.retval, "jit_probe_mem opts.retval");
+	ASSERT_EQ(skel->data->total_sum, 192, "jit_probe_mem total_sum");
+
+	jit_probe_mem__destroy(skel);
+}
diff --git a/tools/testing/selftests/bpf/progs/jit_probe_mem.c b/tools/testing/selftests/bpf/progs/jit_probe_mem.c
new file mode 100644
index 000000000000..3bb8af4df837
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/jit_probe_mem.c
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2022 Meta Platforms, Inc. and affiliates. */
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+#include <bpf/bpf_helpers.h>
+
+static struct prog_test_ref_kfunc __kptr_ref *v;
+long total_sum = -1;
+
+extern struct prog_test_ref_kfunc *bpf_kfunc_call_test_acquire(unsigned long *sp) __ksym;
+extern void bpf_kfunc_call_test_release(struct prog_test_ref_kfunc *p) __ksym;
+
+SEC("tc")
+int test_jit_probe_mem(struct __sk_buff *ctx)
+{
+	struct prog_test_ref_kfunc *p;
+	unsigned long zero = 0, sum;
+
+	p = bpf_kfunc_call_test_acquire(&zero);
+	if (!p)
+		return 1;
+
+	p = bpf_kptr_xchg(&v, p);
+	if (p)
+		goto release_out;
+
+	/* Direct map value access of kptr, should be PTR_UNTRUSTED */
+	p = v;
+	if (!p)
+		return 1;
+
+	asm volatile (
+		"r9 = %[p];\n"
+		"%[sum] = 0;\n"
+
+		/* r8 = p->a */
+		"r8 = *(u32 *)(r9 + 0);\n"
+		"%[sum] += r8;\n"
+
+		/* r8 = p->b */
+		"r8 = *(u32 *)(r9 + 4);\n"
+		"%[sum] += r8;\n"
+
+		"r9 += 8;\n"
+		/* r9 = p->a */
+		"r9 = *(u32 *)(r9 - 8);\n"
+		"%[sum] += r9;\n"

All these '\n' are not necessary.

+
+		: [sum] "=r"(sum)
+		: [p] "r"(p)
+		: "r8", "r9"
+	);
+
+	total_sum = sum;
+	return 0;
+release_out:
+	bpf_kfunc_call_test_release(p);
+	return 1;
+}
+
+char _license[] SEC("license") = "GPL";



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