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

[Paul Chaignon <paul.chaignon@gmail.com>]

On Tue, Apr 14, 2026 at 09:40:01AM +0800, Yihan Ding wrote:
> 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.

Nice! I can finally print proper French from BPF :)

> 
> 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>

Not sure if this should go to bpf or bpf-next. I'll let the maintainers
decide.

> ---
> 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])) {

Why is this change needed? isspace and ispunct already cast to unsigned
char.

>  				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])) {

Same here.

>  				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";

The build is failing because you made these static.

>  
>  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;
>  }

Please put the selftest coverage extensions in a second patch.

pw-bot: cr

> -- 
> 2.20.1
>