selftest/bpf: Add test for var-offset stack access

Add a higher-level test (C BPF program) for the new functionality -
variable access stack reads and writes.

Signed-off-by: Andrei Matei <andreimatei1@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210207011027.676572-5-andreimatei1@gmail.com

diff --git a/tools/testing/selftests/bpf/prog_tests/stack_var_off.c b/tools/testing/selftests/bpf/prog_tests/stack_var_off.c
new file mode 100644 (file)
index 0000000..2ce9dee
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/stack_var_off.c
@@ -0,0 +1,35 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include "test_stack_var_off.skel.h"
+
+/* Test read and writes to the stack performed with offsets that are not
+ * statically known.
+ */
+void test_stack_var_off(void)
+{
+       int duration = 0;
+       struct test_stack_var_off *skel;
+
+       skel = test_stack_var_off__open_and_load();
+       if (CHECK(!skel, "skel_open", "failed to open skeleton\n"))
+               return;
+
+       /* Give pid to bpf prog so it doesn't trigger for anyone else. */
+       skel->bss->test_pid = getpid();
+       /* Initialize the probe's input. */
+       skel->bss->input[0] = 2;
+       skel->bss->input[1] = 42;  /* This will be returned in probe_res. */
+
+       if (!ASSERT_OK(test_stack_var_off__attach(skel), "skel_attach"))
+               goto cleanup;
+
+       /* Trigger probe. */
+       usleep(1);
+
+       if (CHECK(skel->bss->probe_res != 42, "check_probe_res",
+                 "wrong probe res: %d\n", skel->bss->probe_res))
+               goto cleanup;
+
+cleanup:
+       test_stack_var_off__destroy(skel);
+}

diff --git a/tools/testing/selftests/bpf/progs/test_stack_var_off.c b/tools/testing/selftests/bpf/progs/test_stack_var_off.c
new file mode 100644 (file)
index 0000000..665e6ae
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_stack_var_off.c
@@ -0,0 +1,51 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+int probe_res;
+
+char input[4] = {};
+int test_pid;
+
+SEC("tracepoint/syscalls/sys_enter_nanosleep")
+int probe(void *ctx)
+{
+       /* This BPF program performs variable-offset reads and writes on a
+        * stack-allocated buffer.
+        */
+       char stack_buf[16];
+       unsigned long len;
+       unsigned long last;
+
+       if ((bpf_get_current_pid_tgid() >> 32) != test_pid)
+               return 0;
+
+       /* Copy the input to the stack. */
+       __builtin_memcpy(stack_buf, input, 4);
+
+       /* The first byte in the buffer indicates the length. */
+       len = stack_buf[0] & 0xf;
+       last = (len - 1) & 0xf;
+
+       /* Append something to the buffer. The offset where we write is not
+        * statically known; this is a variable-offset stack write.
+        */
+       stack_buf[len] = 42;
+
+       /* Index into the buffer at an unknown offset. This is a
+        * variable-offset stack read.
+        *
+        * Note that if it wasn't for the preceding variable-offset write, this
+        * read would be rejected because the stack slot cannot be verified as
+        * being initialized. With the preceding variable-offset write, the
+        * stack slot still cannot be verified, but the write inhibits the
+        * respective check on the reasoning that, if there was a
+        * variable-offset to a higher-or-equal spot, we're probably reading
+        * what we just wrote.
+        */
+       probe_res = stack_buf[last];
+       return 0;
+}
+
+char _license[] SEC("license") = "GPL";