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@xxxxxxxxx>

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@xxxxxxxxxxxxx>
> ---
>  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@xxxxxxxxx>

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