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

[Kees Cook <kees@kernel.org>]

On Sat, Oct 03, 2026 at 06:36:49PM +0300, Andy Shevchenko wrote:
> 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 "";
> 
> ?

It could, but I'm vaguely nervous about the difference between
	s->buffer[0] == '\0'
and
	.data ""

i.e. we only force the return of seq_buf_str() to be _not_ just
s->buffer when s->buffer is weirdly impossible (due to size == 0).
I'd rather not make all 0-len strings return the .data segment's const
"" string...

-- 
Kees Cook