selftests/bpf: Add verifier test exercising jit PROBE_MEM logic

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 <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: Yonghong Song <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
+// 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);
+}

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

···
+// 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];"
+		"%[sum] = 0;"
+
+		/* r8 = p->a */
+		"r8 = *(u32 *)(r9 + 0);"
+		"%[sum] += r8;"
+
+		/* r8 = p->b */
+		"r8 = *(u32 *)(r9 + 4);"
+		"%[sum] += r8;"
+
+		"r9 += 8;"
+		/* r9 = p->a */
+		"r9 = *(u32 *)(r9 - 8);"
+		"%[sum] += r9;"
+
+		: [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";