[PATCH bpf v2 2/2] selftest/bpf: Test bpf_probe_read_user_str() strips trailing bytes after NUL

[Daniel Xu <dxu@dxuuu.xyz>]

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>
---
 .../bpf/prog_tests/probe_read_user_str.c      | 60 +++++++++++++++++++
 .../bpf/progs/test_probe_read_user_str.c      | 34 +++++++++++
 2 files changed, 94 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_probe_read_user_str.c

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
index 000000000000..597a166e6c8d
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include "test_probe_read_user_str.skel.h"
+
+static const char str[] = "mestring";
+
+void test_probe_read_user_str(void)
+{
+	struct test_probe_read_user_str *skel;
+	int fd, err, duration = 0;
+	char buf[256];
+	ssize_t n;
+
+	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"))
+		goto out;
+
+	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;
+
+	fd = open("/dev/null", O_WRONLY);
+	if (CHECK(fd < 0, "open", "open /dev/null failed: %d\n", fd))
+		goto out;
+
+	/* Give pid to bpf prog so it doesn't read from anyone else */
+	skel->bss->pid = getpid();
+
+	/* Ensure bytes after string are ones */
+	memset(buf, 1, sizeof(buf));
+	memcpy(buf, str, sizeof(str));
+
+	/* Trigger tracepoint */
+	n = write(fd, buf, sizeof(buf));
+	if (CHECK(n != sizeof(buf), "write", "write failed: %ld\n", n))
+		goto fd_out;
+
+	/* Did helper fail? */
+	if (CHECK(skel->bss->ret < 0, "prog ret", "prog returned: %d\n",
+		  skel->bss->ret))
+		goto fd_out;
+
+	/* Check that string was copied correctly */
+	err = memcmp(skel->bss->buf, str, sizeof(str));
+	if (CHECK(err, "memcmp", "prog copied wrong string"))
+		goto fd_out;
+
+	/* Now check that no extra trailing bytes were copied */
+	memset(buf, 0, sizeof(buf));
+	err = memcmp(skel->bss->buf + sizeof(str), buf, sizeof(buf) - sizeof(str));
+	if (CHECK(err, "memcmp", "trailing bytes were not stripped"))
+		goto fd_out;
+
+fd_out:
+	close(fd);
+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
index 000000000000..41c3e296566e
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+#include <sys/types.h>
+
+struct sys_enter_write_args {
+	unsigned long long pad;
+	int syscall_nr;
+	int pad1; /* 4 byte hole */
+	unsigned int fd;
+	int pad2; /* 4 byte hole */
+	const char *buf;
+	size_t count;
+};
+
+pid_t pid = 0;
+int ret = 0;
+char buf[256] = {};
+
+SEC("tracepoint/syscalls/sys_enter_write")
+int on_write(struct sys_enter_write_args *ctx)
+{
+	if (pid != (bpf_get_current_pid_tgid() >> 32))
+		return 0;
+
+	ret = bpf_probe_read_user_str(buf, sizeof(buf), ctx->buf);
+
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
-- 
2.28.0