[PATCH bpf v2 0/2] bpf: allow UTF-8 literals in bpf_bprintf_prepare()

[PATCH bpf v2 0/2] bpf: allow UTF-8 literals in bpf_bprintf_prepare()

[Yihan Ding]

bpf_bprintf_prepare() currently rejects any non-ASCII byte in format
strings, so helpers such as bpf_trace_printk() fail to emit UTF-8
literal text even when those bytes are not part of a format specifier.

Keep plain text permissive while continuing to parse '%' sequences as
ASCII-only, then extend trace_printk selftests to cover both the valid
UTF-8 literal case and the invalid non-ASCII-after-'%' case.

Changes in v2:
- split the core change and selftest updates into two patches
- drop unnecessary isspace()/ispunct() casts
- add comments to clarify plain-text vs format-specifier handling
- add a negative selftest for non-ASCII bytes inside '%' sequences

Testing:
- Reproduced on x86_64 without the core fix: 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

Yihan Ding (2):
  bpf: allow UTF-8 literals in bpf_bprintf_prepare()
  selftests/bpf: cover UTF-8 trace_printk output

 kernel/bpf/helpers.c                          | 16 +++++++++++-
 .../selftests/bpf/prog_tests/trace_printk.c   | 26 +++++++++++++++----
 .../selftests/bpf/progs/trace_printk.c        |  9 +++++++
 3 files changed, 45 insertions(+), 6 deletions(-)

-- 
2.20.1

[PATCH bpf v2 1/2] bpf: allow UTF-8 literals in bpf_bprintf_prepare()

[Yihan Ding]

bpf_bprintf_prepare() only needs ASCII parsing for conversion
specifiers. Plain text can safely carry bytes >= 0x80, so allow
UTF-8 literals outside '%' sequences while keeping ASCII control
bytes rejected and format specifiers ASCII-only.

This keeps existing parsing rules for format directives unchanged,
while allowing helpers such as bpf_trace_printk() to emit UTF-8
literal text.

Fixes: 48cac3f4a96d ("bpf: Implement formatted output helpers with bstr_printf")
Suggested-by: Paul Chaignon <paul.chaignon@gmail.com>
Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
---
 kernel/bpf/helpers.c | 16 +++++++++++++++-
 1 file changed, 15 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 6eb6c82ed2ee..6319b39c92f9 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 bytes >= 0x80 in plain text so UTF-8 literals can pass
+		 * through unchanged, while still rejecting ASCII control bytes.
+		 */
+		if (isascii(c) && !isprint(c) && !isspace(c)) {
 			err = -EINVAL;
 			goto out;
 		}
@@ -867,6 +873,14 @@ 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++;
+		/*
+		 * The format parser below only understands ASCII conversion
+		 * specifiers and modifiers, so reject non-ASCII after '%'.
+		 */
+		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] == '-' ||
-- 
2.20.1

Re: [PATCH bpf v2 1/2] bpf: allow UTF-8 literals in bpf_bprintf_prepare()

[Paul Chaignon]

On Wed, Apr 15, 2026 at 11:21:25AM +0800, Yihan Ding wrote:
> bpf_bprintf_prepare() only needs ASCII parsing for conversion
> specifiers. Plain text can safely carry bytes >= 0x80, so allow
> UTF-8 literals outside '%' sequences while keeping ASCII control
> bytes rejected and format specifiers ASCII-only.
> 
> This keeps existing parsing rules for format directives unchanged,
> while allowing helpers such as bpf_trace_printk() to emit UTF-8
> literal text.
> 
> Fixes: 48cac3f4a96d ("bpf: Implement formatted output helpers with bstr_printf")
> Suggested-by: Paul Chaignon <paul.chaignon@gmail.com>

I don't think this tag is appropriate here. If you want to give credit
for changes made after reviews, you can do so in the Changelogs of the
cover letter :)

> Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
> ---
>  kernel/bpf/helpers.c | 16 +++++++++++++++-
>  1 file changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 6eb6c82ed2ee..6319b39c92f9 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 bytes >= 0x80 in plain text so UTF-8 literals can pass
> +		 * through unchanged, while still rejecting ASCII control bytes.
> +		 */
> +		if (isascii(c) && !isprint(c) && !isspace(c)) {
>  			err = -EINVAL;
>  			goto out;
>  		}
> @@ -867,6 +873,14 @@ 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++;
> +		/*
> +		 * The format parser below only understands ASCII conversion
> +		 * specifiers and modifiers, so reject non-ASCII after '%'.
> +		 */
> +		if (!isascii((unsigned char)fmt[i])) {
> +			err = -EINVAL;
> +			goto out;
> +		}

Acked-by: Paul Chaignon <paul.chaignon@gmail.com>

>  
>  		/* skip optional "[0 +-][num]" width formatting field */
>  		while (fmt[i] == '0' || fmt[i] == '+'  || fmt[i] == '-' ||
> -- 
> 2.20.1

Re: [PATCH bpf v2 1/2] bpf: allow UTF-8 literals in bpf_bprintf_prepare()

[Paul Chaignon]

On Wed, Apr 15, 2026 at 12:45:02PM +0200, Paul Chaignon wrote:
> On Wed, Apr 15, 2026 at 11:21:25AM +0800, Yihan Ding wrote:
> > bpf_bprintf_prepare() only needs ASCII parsing for conversion
> > specifiers. Plain text can safely carry bytes >= 0x80, so allow
> > UTF-8 literals outside '%' sequences while keeping ASCII control
> > bytes rejected and format specifiers ASCII-only.
> > 
> > This keeps existing parsing rules for format directives unchanged,
> > while allowing helpers such as bpf_trace_printk() to emit UTF-8
> > literal text.
> > 
> > Fixes: 48cac3f4a96d ("bpf: Implement formatted output helpers with bstr_printf")
> > Suggested-by: Paul Chaignon <paul.chaignon@gmail.com>
> 
> I don't think this tag is appropriate here. If you want to give credit
> for changes made after reviews, you can do so in the Changelogs of the
> cover letter :)
> 
> > Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
> > ---
> >  kernel/bpf/helpers.c | 16 +++++++++++++++-
> >  1 file changed, 15 insertions(+), 1 deletion(-)
> > 
> > diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> > index 6eb6c82ed2ee..6319b39c92f9 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 bytes >= 0x80 in plain text so UTF-8 literals can pass
> > +		 * through unchanged, while still rejecting ASCII control bytes.
> > +		 */
> > +		if (isascii(c) && !isprint(c) && !isspace(c)) {
> >  			err = -EINVAL;
> >  			goto out;
> >  		}
> > @@ -867,6 +873,14 @@ 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++;
> > +		/*
> > +		 * The format parser below only understands ASCII conversion
> > +		 * specifiers and modifiers, so reject non-ASCII after '%'.
> > +		 */
> > +		if (!isascii((unsigned char)fmt[i])) {
> > +			err = -EINVAL;
> > +			goto out;
> > +		}
> 
> Acked-by: Paul Chaignon <paul.chaignon@gmail.com>

Actually, this patch will require changes to fixup the existing
test_snprintf_negative() selftest that is currently failing. That fixup
needs to be in this commit to not break selftests during bisections.

> 
> >  
> >  		/* skip optional "[0 +-][num]" width formatting field */
> >  		while (fmt[i] == '0' || fmt[i] == '+'  || fmt[i] == '-' ||
> > -- 
> > 2.20.1

[PATCH bpf v2 2/2] selftests/bpf: cover UTF-8 trace_printk output

[Yihan Ding]

Extend trace_printk coverage to verify that UTF-8 literal text is
emitted successfully and that non-ASCII bytes are still rejected once
parsing is inside a '%' format sequence.

Suggested-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
---
 .../selftests/bpf/prog_tests/trace_printk.c   | 26 +++++++++++++++----
 .../selftests/bpf/progs/trace_printk.c        |  9 +++++++
 2 files changed, 30 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/trace_printk.c b/tools/testing/selftests/bpf/prog_tests/trace_printk.c
index e56e88596d64..40499f01d228 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 +49,24 @@ 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;
+
+	if (!ASSERT_LT(bss->trace_printk_utf8_spec_ret, 0,
+		       "bss->trace_printk_utf8_spec_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..62153d8c5eba 100644
--- a/tools/testing/selftests/bpf/progs/trace_printk.c
+++ b/tools/testing/selftests/bpf/progs/trace_printk.c
@@ -10,13 +10,22 @@ char _license[] SEC("license") = "GPL";
 
 int trace_printk_ret = 0;
 int trace_printk_ran = 0;
+int trace_printk_utf8_spec_ret = 0;
+int trace_printk_utf8_ret = 0;
+int trace_printk_utf8_ran = 0;
 
 const char fmt[] = "Testing,testing %d\n";
+static const char utf8_fmt[] = "中文,测试 %d\n";
+static const char utf8_spec_fmt[] = "%中文\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);
+	trace_printk_utf8_spec_ret = bpf_trace_printk(utf8_spec_fmt,
+						      sizeof(utf8_spec_fmt));
 	return 0;
 }
-- 
2.20.1

Re: [PATCH bpf v2 2/2] selftests/bpf: cover UTF-8 trace_printk output

[Paul Chaignon]

On Wed, Apr 15, 2026 at 11:21:26AM +0800, Yihan Ding wrote:
> Extend trace_printk coverage to verify that UTF-8 literal text is
> emitted successfully and that non-ASCII bytes are still rejected once
> parsing is inside a '%' format sequence.
> 
> Suggested-by: Alan Maguire <alan.maguire@oracle.com>
> Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
> ---
>  .../selftests/bpf/prog_tests/trace_printk.c   | 26 +++++++++++++++----
>  .../selftests/bpf/progs/trace_printk.c        |  9 +++++++
>  2 files changed, 30 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/trace_printk.c b/tools/testing/selftests/bpf/prog_tests/trace_printk.c
> index e56e88596d64..40499f01d228 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 +49,24 @@ 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;
> +
> +	if (!ASSERT_LT(bss->trace_printk_utf8_spec_ret, 0,
> +		       "bss->trace_printk_utf8_spec_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..62153d8c5eba 100644
> --- a/tools/testing/selftests/bpf/progs/trace_printk.c
> +++ b/tools/testing/selftests/bpf/progs/trace_printk.c
> @@ -10,13 +10,22 @@ char _license[] SEC("license") = "GPL";
>  
>  int trace_printk_ret = 0;
>  int trace_printk_ran = 0;
> +int trace_printk_utf8_spec_ret = 0;
> +int trace_printk_utf8_ret = 0;
> +int trace_printk_utf8_ran = 0;
>  
>  const char fmt[] = "Testing,testing %d\n";
> +static const char utf8_fmt[] = "中文,测试 %d\n";
> +static const char utf8_spec_fmt[] = "%中文\n";

What's the purpose of the second string here?

>  
>  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);
> +	trace_printk_utf8_spec_ret = bpf_trace_printk(utf8_spec_fmt,
> +						      sizeof(utf8_spec_fmt));
>  	return 0;
>  }
> -- 
> 2.20.1
>