[PATCH bpf] bpf: allow UTF-8 literals in bpf_bprintf_prepare()

[Yihan Ding <dingyihan@uniontech.com>]

bpf_bprintf_prepare() currently rejects any non-ASCII byte in the format
string, such as UTF-8 Chinese text.

All BPF formatted output helpers that go through bpf_bprintf_prepare(),
such as bpf_trace_printk(), bpf_seq_printf() and bpf_snprintf(), only
need ASCII parsing for conversion specifiers. Plain text does not need
that restriction, but today any byte >= 0x80 makes the format fail
validation.

As a result, UTF-8 text literals are rejected even when they are not part
of a format specifier. In practice, an ASCII-only bpf_trace_printk()
format works, while the same format with UTF-8 literal text produces no
trace output.

Allow non-ASCII bytes in plain text while keeping the existing control
character checks and keeping format specifiers ASCII-only. This preserves
the current parsing rules for '%' sequences and allows valid UTF-8 text
to be emitted.

Extend the trace_printk selftest accordingly by emitting both ASCII and
UTF-8 strings and verifying that both appear in the trace output.

Fixes: 48cac3f4a96d ("bpf: Implement formatted output helpers with bstr_printf")
Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
---
Testing:
- Reproduced on x86_64 without this patch: ASCII trace output works, while
   UTF-8 literal text in bpf_trace_printk() is rejected and produces no trace
   output.
- Verified with tools/testing/selftests/bpf: ./test_progs -t trace_printk

 kernel/bpf/helpers.c                          | 21 +++++++++++++-----
 .../selftests/bpf/prog_tests/trace_printk.c   | 22 ++++++++++++++-----
 .../selftests/bpf/progs/trace_printk.c        |  5 +++++
 3 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 6eb6c82ed2ee..e2f103297e4a 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -845,7 +845,13 @@ int bpf_bprintf_prepare(const char *fmt, u32 fmt_size, const u64 *raw_args,
 		data->buf = buffers->buf;
 
 	for (i = 0; i < fmt_size; i++) {
-		if ((!isprint(fmt[i]) && !isspace(fmt[i])) || !isascii(fmt[i])) {
+		unsigned char c = fmt[i];
+
+		/*
+		 * Permit non-ASCII bytes in plain text so UTF-8 messages can be
+		 * emitted, while keeping format specifiers ASCII-only.
+		 */
+		if (isascii(c) && !isprint(c) && !isspace(c)) {
 			err = -EINVAL;
 			goto out;
 		}
@@ -867,6 +873,10 @@ int bpf_bprintf_prepare(const char *fmt, u32 fmt_size, const u64 *raw_args,
 		 * always access fmt[i + 1], in the worst case it will be a 0
 		 */
 		i++;
+		if (!isascii((unsigned char)fmt[i])) {
+			err = -EINVAL;
+			goto out;
+		}
 
 		/* skip optional "[0 +-][num]" width formatting field */
 		while (fmt[i] == '0' || fmt[i] == '+'  || fmt[i] == '-' ||
@@ -881,8 +891,9 @@ int bpf_bprintf_prepare(const char *fmt, u32 fmt_size, const u64 *raw_args,
 		if (fmt[i] == 'p') {
 			sizeof_cur_arg = sizeof(long);
 
-			if (fmt[i + 1] == 0 || isspace(fmt[i + 1]) ||
-			    ispunct(fmt[i + 1])) {
+			if (fmt[i + 1] == 0 ||
+			    isspace((unsigned char)fmt[i + 1]) ||
+			    ispunct((unsigned char)fmt[i + 1])) {
 				if (tmp_buf)
 					cur_arg = raw_args[num_spec];
 				goto nocopy_fmt;
@@ -958,8 +969,8 @@ int bpf_bprintf_prepare(const char *fmt, u32 fmt_size, const u64 *raw_args,
 			fmt_ptype = fmt[i];
 fmt_str:
 			if (fmt[i + 1] != 0 &&
-			    !isspace(fmt[i + 1]) &&
-			    !ispunct(fmt[i + 1])) {
+			    !isspace((unsigned char)fmt[i + 1]) &&
+			    !ispunct((unsigned char)fmt[i + 1])) {
 				err = -EINVAL;
 				goto out;
 			}
diff --git a/tools/testing/selftests/bpf/prog_tests/trace_printk.c b/tools/testing/selftests/bpf/prog_tests/trace_printk.c
index e56e88596d64..f7b03dc4eaf4 100644
--- a/tools/testing/selftests/bpf/prog_tests/trace_printk.c
+++ b/tools/testing/selftests/bpf/prog_tests/trace_printk.c
@@ -6,18 +6,21 @@
 #include "trace_printk.lskel.h"
 
 #define SEARCHMSG	"testing,testing"
+#define SEARCHMSG_UTF8	"中文,测试"
 
 static void trace_pipe_cb(const char *str, void *data)
 {
 	if (strstr(str, SEARCHMSG) != NULL)
-		(*(int *)data)++;
+		((int *)data)[0]++;
+	if (strstr(str, SEARCHMSG_UTF8) != NULL)
+		((int *)data)[1]++;
 }
 
 void serial_test_trace_printk(void)
 {
 	struct trace_printk_lskel__bss *bss;
 	struct trace_printk_lskel *skel;
-	int err = 0, found = 0;
+	int err = 0, found[2] = {};
 
 	skel = trace_printk_lskel__open();
 	if (!ASSERT_OK_PTR(skel, "trace_printk__open"))
@@ -46,11 +49,20 @@ void serial_test_trace_printk(void)
 	if (!ASSERT_GT(bss->trace_printk_ret, 0, "bss->trace_printk_ret"))
 		goto cleanup;
 
-	/* verify our search string is in the trace buffer */
-	ASSERT_OK(read_trace_pipe_iter(trace_pipe_cb, &found, 1000),
+	if (!ASSERT_GT(bss->trace_printk_utf8_ran, 0, "bss->trace_printk_utf8_ran"))
+		goto cleanup;
+
+	if (!ASSERT_GT(bss->trace_printk_utf8_ret, 0, "bss->trace_printk_utf8_ret"))
+		goto cleanup;
+
+	/* verify our search strings are in the trace buffer */
+	ASSERT_OK(read_trace_pipe_iter(trace_pipe_cb, found, 1000),
 		 "read_trace_pipe_iter");
 
-	if (!ASSERT_EQ(found, bss->trace_printk_ran, "found"))
+	if (!ASSERT_EQ(found[0], bss->trace_printk_ran, "found"))
+		goto cleanup;
+
+	if (!ASSERT_EQ(found[1], bss->trace_printk_utf8_ran, "found_utf8"))
 		goto cleanup;
 
 cleanup:
diff --git a/tools/testing/selftests/bpf/progs/trace_printk.c b/tools/testing/selftests/bpf/progs/trace_printk.c
index 6695478c2b25..97afe8b149b0 100644
--- a/tools/testing/selftests/bpf/progs/trace_printk.c
+++ b/tools/testing/selftests/bpf/progs/trace_printk.c
@@ -10,13 +10,18 @@ char _license[] SEC("license") = "GPL";
 
 int trace_printk_ret = 0;
 int trace_printk_ran = 0;
+int trace_printk_utf8_ret = 0;
+int trace_printk_utf8_ran = 0;
 
-const char fmt[] = "Testing,testing %d\n";
+static const char fmt[] = "Testing,testing %d\n";
+static const char utf8_fmt[] = "中文,测试 %d\n";
 
 SEC("fentry/" SYS_PREFIX "sys_nanosleep")
 int sys_enter(void *ctx)
 {
 	trace_printk_ret = bpf_trace_printk(fmt, sizeof(fmt),
 					    ++trace_printk_ran);
+	trace_printk_utf8_ret = bpf_trace_printk(utf8_fmt, sizeof(utf8_fmt),
+						 ++trace_printk_utf8_ran);
 	return 0;
 }
-- 
2.20.1