Merge branch 'Add a snprintf eBPF helper'

Florent Revest says:

====================

We have a usecase where we want to audit symbol names (if available) in
callback registration hooks. (ex: fentry/nf_register_net_hook)

A few months back, I proposed a bpf_kallsyms_lookup series but it was
decided in the reviews that a more generic helper, bpf_snprintf, would
be more useful.

This series implements the helper according to the feedback received in
https://lore.kernel.org/bpf/20201126165748.1748417-1-revest@google.com/T/#u

- A new arg type guarantees the NULL-termination of string arguments and
  lets us pass format strings in only one arg
- A new helper is implemented using that guarantee. Because the format
  string is known at verification time, the format string validation is
  done by the verifier
- To implement a series of tests for bpf_snprintf, the logic for
  marshalling variadic args in a fixed-size array is reworked as per:
https://lore.kernel.org/bpf/20210310015455.1095207-1-revest@chromium.org/T/#u

---
Changes in v5:
- Fixed the bpf_printf_buf_used counter logic in try_get_fmt_tmp_buf
- Added a couple of extra incorrect specifiers tests
- Call test_snprintf_single__destroy unconditionally
- Fixed a C++-style comment

---
Changes in v4:
- Moved bpf_snprintf, bpf_printf_prepare and bpf_printf_cleanup to
  kernel/bpf/helpers.c so that they get built without CONFIG_BPF_EVENTS
- Added negative test cases (various invalid format strings)
- Renamed put_fmt_tmp_buf() as bpf_printf_cleanup()
- Fixed a mistake that caused temporary buffers to be unconditionally
  freed in bpf_printf_prepare
- Fixed a mistake that caused missing 0 character to be ignored
- Fixed a warning about integer to pointer conversion
- Misc cleanups

---
Changes in v3:
- Simplified temporary buffer acquisition with try_get_fmt_tmp_buf()
- Made zero-termination check more consistent
- Allowed NULL output_buffer
- Simplified the BPF_CAST_FMT_ARG macro
- Three new test cases: number padding, simple string with no arg and
  string length extraction only with a NULL output buffer
- Clarified helper's description for edge cases (eg: str_size == 0)
- Lots of cosmetic changes

---
Changes in v2:
- Extracted the format validation/argument sanitization in a generic way
  for all printf-like helpers.
- bpf_snprintf's str_size can now be 0
- bpf_snprintf is now exposed to all BPF program types
- We now preempt_disable when using a per-cpu temporary buffer
- Addressed a few cosmetic changes
====================

Acked-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 125 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/snprintf.c b/tools/testing/selftests/bpf/prog_tests/snprintf.c
new file mode 100644
index 000000000000..a958c22aec75
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/snprintf.c
@@ -0,0 +1,125 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Google LLC. */
+
+#include <test_progs.h>
+#include "test_snprintf.skel.h"
+#include "test_snprintf_single.skel.h"
+
+#define EXP_NUM_OUT  "-8 9 96 -424242 1337 DABBAD00"
+#define EXP_NUM_RET  sizeof(EXP_NUM_OUT)
+
+#define EXP_IP_OUT   "127.000.000.001 0000:0000:0000:0000:0000:0000:0000:0001"
+#define EXP_IP_RET   sizeof(EXP_IP_OUT)
+
+/* The third specifier, %pB, depends on compiler inlining so don't check it */
+#define EXP_SYM_OUT  "schedule schedule+0x0/"
+#define MIN_SYM_RET  sizeof(EXP_SYM_OUT)
+
+/* The third specifier, %p, is a hashed pointer which changes on every reboot */
+#define EXP_ADDR_OUT "0000000000000000 ffff00000add4e55 "
+#define EXP_ADDR_RET sizeof(EXP_ADDR_OUT "unknownhashedptr")
+
+#define EXP_STR_OUT  "str1 longstr"
+#define EXP_STR_RET  sizeof(EXP_STR_OUT)
+
+#define EXP_OVER_OUT "%over"
+#define EXP_OVER_RET 10
+
+#define EXP_PAD_OUT "    4 000"
+#define EXP_PAD_RET 900007
+
+#define EXP_NO_ARG_OUT "simple case"
+#define EXP_NO_ARG_RET 12
+
+#define EXP_NO_BUF_RET 29
+
+void test_snprintf_positive(void)
+{
+	char exp_addr_out[] = EXP_ADDR_OUT;
+	char exp_sym_out[]  = EXP_SYM_OUT;
+	struct test_snprintf *skel;
+
+	skel = test_snprintf__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel_open"))
+		return;
+
+	if (!ASSERT_OK(test_snprintf__attach(skel), "skel_attach"))
+		goto cleanup;
+
+	/* trigger tracepoint */
+	usleep(1);
+
+	ASSERT_STREQ(skel->bss->num_out, EXP_NUM_OUT, "num_out");
+	ASSERT_EQ(skel->bss->num_ret, EXP_NUM_RET, "num_ret");
+
+	ASSERT_STREQ(skel->bss->ip_out, EXP_IP_OUT, "ip_out");
+	ASSERT_EQ(skel->bss->ip_ret, EXP_IP_RET, "ip_ret");
+
+	ASSERT_OK(memcmp(skel->bss->sym_out, exp_sym_out,
+			 sizeof(exp_sym_out) - 1), "sym_out");
+	ASSERT_LT(MIN_SYM_RET, skel->bss->sym_ret, "sym_ret");
+
+	ASSERT_OK(memcmp(skel->bss->addr_out, exp_addr_out,
+			 sizeof(exp_addr_out) - 1), "addr_out");
+	ASSERT_EQ(skel->bss->addr_ret, EXP_ADDR_RET, "addr_ret");
+
+	ASSERT_STREQ(skel->bss->str_out, EXP_STR_OUT, "str_out");
+	ASSERT_EQ(skel->bss->str_ret, EXP_STR_RET, "str_ret");
+
+	ASSERT_STREQ(skel->bss->over_out, EXP_OVER_OUT, "over_out");
+	ASSERT_EQ(skel->bss->over_ret, EXP_OVER_RET, "over_ret");
+
+	ASSERT_STREQ(skel->bss->pad_out, EXP_PAD_OUT, "pad_out");
+	ASSERT_EQ(skel->bss->pad_ret, EXP_PAD_RET, "pad_ret");
+
+	ASSERT_STREQ(skel->bss->noarg_out, EXP_NO_ARG_OUT, "no_arg_out");
+	ASSERT_EQ(skel->bss->noarg_ret, EXP_NO_ARG_RET, "no_arg_ret");
+
+	ASSERT_EQ(skel->bss->nobuf_ret, EXP_NO_BUF_RET, "no_buf_ret");
+
+cleanup:
+	test_snprintf__destroy(skel);
+}
+
+#define min(a, b) ((a) < (b) ? (a) : (b))
+
+/* Loads an eBPF object calling bpf_snprintf with up to 10 characters of fmt */
+static int load_single_snprintf(char *fmt)
+{
+	struct test_snprintf_single *skel;
+	int ret;
+
+	skel = test_snprintf_single__open();
+	if (!skel)
+		return -EINVAL;
+
+	memcpy(skel->rodata->fmt, fmt, min(strlen(fmt) + 1, 10));
+
+	ret = test_snprintf_single__load(skel);
+	test_snprintf_single__destroy(skel);
+
+	return ret;
+}
+
+void test_snprintf_negative(void)
+{
+	ASSERT_OK(load_single_snprintf("valid %d"), "valid usage");
+
+	ASSERT_ERR(load_single_snprintf("0123456789"), "no terminating zero");
+	ASSERT_ERR(load_single_snprintf("%d %d"), "too many specifiers");
+	ASSERT_ERR(load_single_snprintf("%pi5"), "invalid specifier 1");
+	ASSERT_ERR(load_single_snprintf("%a"), "invalid specifier 2");
+	ASSERT_ERR(load_single_snprintf("%"), "invalid specifier 3");
+	ASSERT_ERR(load_single_snprintf("%12345678"), "invalid specifier 4");
+	ASSERT_ERR(load_single_snprintf("%--------"), "invalid specifier 5");
+	ASSERT_ERR(load_single_snprintf("\x80"), "non ascii character");
+	ASSERT_ERR(load_single_snprintf("\x1"), "non printable character");
+}
+
+void test_snprintf(void)
+{
+	if (test__start_subtest("snprintf_positive"))
+		test_snprintf_positive();
+	if (test__start_subtest("snprintf_negative"))
+		test_snprintf_negative();
+}