selftest/bpf: Test bpf_probe_read_user_str() strips trailing bytes after NUL

Previously, bpf_probe_read_user_str() could potentially overcopy the
trailing bytes after the NUL due to how do_strncpy_from_user() does the
copy in long-sized strides. The issue has been fixed in the previous
commit.

This commit adds a selftest that ensures we don't regress
bpf_probe_read_user_str() again.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/4d977508fab4ec5b7b574b85bdf8b398868b6ee9.1605642949.git.dxu@dxuuu.xyz

diff --git a/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c b/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
new file mode 100644 (file)
index 0000000..e419298
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
@@ -0,0 +1,71 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include "test_probe_read_user_str.skel.h"
+
+static const char str1[] = "mestring";
+static const char str2[] = "mestringalittlebigger";
+static const char str3[] = "mestringblubblubblubblubblub";
+
+static int test_one_str(struct test_probe_read_user_str *skel, const char *str,
+                       size_t len)
+{
+       int err, duration = 0;
+       char buf[256];
+
+       /* Ensure bytes after string are ones */
+       memset(buf, 1, sizeof(buf));
+       memcpy(buf, str, len);
+
+       /* Give prog our userspace pointer */
+       skel->bss->user_ptr = buf;
+
+       /* Trigger tracepoint */
+       usleep(1);
+
+       /* Did helper fail? */
+       if (CHECK(skel->bss->ret < 0, "prog_ret", "prog returned: %ld\n",
+                 skel->bss->ret))
+               return 1;
+
+       /* Check that string was copied correctly */
+       err = memcmp(skel->bss->buf, str, len);
+       if (CHECK(err, "memcmp", "prog copied wrong string"))
+               return 1;
+
+       /* Now check that no extra trailing bytes were copied */
+       memset(buf, 0, sizeof(buf));
+       err = memcmp(skel->bss->buf + len, buf, sizeof(buf) - len);
+       if (CHECK(err, "memcmp", "trailing bytes were not stripped"))
+               return 1;
+
+       return 0;
+}
+
+void test_probe_read_user_str(void)
+{
+       struct test_probe_read_user_str *skel;
+       int err, duration = 0;
+
+       skel = test_probe_read_user_str__open_and_load();
+       if (CHECK(!skel, "test_probe_read_user_str__open_and_load",
+                 "skeleton open and load failed\n"))
+               return;
+
+       /* Give pid to bpf prog so it doesn't read from anyone else */
+       skel->bss->pid = getpid();
+
+       err = test_probe_read_user_str__attach(skel);
+       if (CHECK(err, "test_probe_read_user_str__attach",
+                 "skeleton attach failed: %d\n", err))
+               goto out;
+
+       if (test_one_str(skel, str1, sizeof(str1)))
+               goto out;
+       if (test_one_str(skel, str2, sizeof(str2)))
+               goto out;
+       if (test_one_str(skel, str3, sizeof(str3)))
+               goto out;
+
+out:
+       test_probe_read_user_str__destroy(skel);
+}

diff --git a/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
new file mode 100644 (file)
index 0000000..3ae398b
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+#include <sys/types.h>
+
+pid_t pid = 0;
+long ret = 0;
+void *user_ptr = 0;
+char buf[256] = {};
+
+SEC("tracepoint/syscalls/sys_enter_nanosleep")
+int on_write(void *ctx)
+{
+       if (pid != (bpf_get_current_pid_tgid() >> 32))
+               return 0;
+
+       ret = bpf_probe_read_user_str(buf, sizeof(buf), user_ptr);
+
+       return 0;
+}
+
+char _license[] SEC("license") = "GPL";