Re: [PATCH bpf v3 1/2] bpf: allow UTF-8 literals in bpf_bprintf_prepare()

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

On Thu, Apr 16, 2026 at 08:01:41PM +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.
> 
> Update test_snprintf_negative() in the same commit so selftests keep
> matching the new plain-text vs format-specifier split during bisection.
> 
> Fixes: 48cac3f4a96d ("bpf: Implement formatted output helpers with bstr_printf")
> Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
> ---
>  kernel/bpf/helpers.c                            | 17 ++++++++++++++++-
>  .../testing/selftests/bpf/prog_tests/snprintf.c |  3 ++-
>  2 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 6eb6c82ed2ee..d51f1b612f1d 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];

I'm a bit unsure this extra variable is worth it, but it's probably not
worth sending a v4 just for that.

> +
> +		/*
> +		 * 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 +873,15 @@ 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++;
> +		c = fmt[i];
> +		/*
> +		 * The format parser below only understands ASCII conversion
> +		 * specifiers and modifiers, so reject non-ASCII after '%'.
> +		 */
> +		if (!isascii(c)) {
> +			err = -EINVAL;
> +			goto out;
> +		}
>  
>  		/* skip optional "[0 +-][num]" width formatting field */
>  		while (fmt[i] == '0' || fmt[i] == '+'  || fmt[i] == '-' ||
> diff --git a/tools/testing/selftests/bpf/prog_tests/snprintf.c b/tools/testing/selftests/bpf/prog_tests/snprintf.c
> index 594441acb707..4e4a82d54f79 100644
> --- a/tools/testing/selftests/bpf/prog_tests/snprintf.c
> +++ b/tools/testing/selftests/bpf/prog_tests/snprintf.c
> @@ -114,7 +114,8 @@ static void test_snprintf_negative(void)
>  	ASSERT_ERR(load_single_snprintf("%--------"), "invalid specifier 5");
>  	ASSERT_ERR(load_single_snprintf("%lc"), "invalid specifier 6");
>  	ASSERT_ERR(load_single_snprintf("%llc"), "invalid specifier 7");
> -	ASSERT_ERR(load_single_snprintf("\x80"), "non ascii character");
> +	ASSERT_OK(load_single_snprintf("\x80"), "non ascii plain text");
> +	ASSERT_ERR(load_single_snprintf("%\x80"), "non ascii in specifier");

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

>  	ASSERT_ERR(load_single_snprintf("\x1"), "non printable character");
>  	ASSERT_ERR(load_single_snprintf("%p%"), "invalid specifier 8");
>  	ASSERT_ERR(load_single_snprintf("%s%"), "invalid specifier 9");
> -- 
> 2.20.1
>