[PATCH v4 05/11] seq_buf: Add seq_buf_strlen()

[Andy Shevchenko]

On Fri, Oct 02, 2026 at 08:59:10PM -0700, Kees Cook wrote:
> Several strlcat() call sites being converted to seq_buf need behavior
> seq_buf doesn't currently provide. The return from seq_buf_used() is
> not the length of the string in a seq_buf. Once the buffer is full or
> has overflowed it returns the buffer size, which counts the byte that
> seq_buf_str() replaces with the NUL, so a caller that needs the string
> and its length has to call seq_buf_str() and then walk the string with
> strlen().
> 
> Move the termination out of seq_buf_str() into a helper that returns
> where it put the NUL, and add seq_buf_strlen(), which terminates the
> buffer in the same way and returns that offset.
> 
> As discussed in review, don't add WARN_ON() for seq_buf_strlen() and
> drop it from seq_buf_str().
> 
> Add tests comparing seq_buf_strlen() against strlen() of seq_buf_str()
> for empty, appended, truncated, exactly full, and overflowed buffers,
> checking that seq_buf_strlen() alone terminates a full buffer, and
> checking that a zero-sized seq_buf reports an empty string from both
> accessors without touching the buffer.
> 
> Tests passed under qemu on ARCH=x86_64 with GCC 16.2.0 and CONFIG_KASAN=y,
> and on big-endian ARCH=s390 with GCC s390x-linux-gnu 16.2.0.

...

>  static inline const char *seq_buf_str(struct seq_buf *s)
>  {
> -	if (WARN_ON(s->size == 0))
> +	if (s->size == 0)
>  		return "";
>  
> -	if (seq_buf_buffer_left(s))
> -		s->buffer[s->len] = 0;
> -	else
> -		s->buffer[s->size - 1] = 0;
> +	__seq_buf_terminate(s);
>  
>  	return s->buffer;
>  }

Looking at this again, can't it be rewritten now using _strlen()?

	if (seq_buf_strlen(s))
		return s->buffer;

	return "";

?

...

> +static inline size_t seq_buf_strlen(struct seq_buf *s)
> +{
> +	if (s->size == 0)
> +		return 0;
> +
> +	return __seq_buf_terminate(s);
> +}

(Left for the context to the above.)

-- 
With Best Regards,
Andy Shevchenko