Hi Steven,

> Date: 2026-10-22 16:20:00-0400
> From: Steven Rostedt <rostedt@goodmis.org>
>
> On Thu, 22 Oct 2026 22:11:27 +0200
> Alejandro Colomar <alx+linux-hardening@kernel.org> wrote:
> 
> > Agree.
> > 
> > But still, a better design would do:
> > 
> > 	static inline void
> > 	seq_buf_set_overflow(struct seq_buf *s)
> > 	{
> > 		s->p = NULL;
> > 	}
> > 
> > This is way more robust, and also easier to understand.  My head is
> > boiling when trying to review this patch set for correctness.  That's
> > not a good sign.
> 
> But that loses all data. We still want access to what was written.

No, it doesn't lose data.  My proposal is to have this:

	struct my_seq_buf {
		// .buffer points to the start of the buffer
		char  *buffer;

		/* .p points to the current position.  This is the last
		 * '\0' that has been written to the buffer.  On
		 * truncation, it is a NULL pointer.
		 */
		char  *p;

		/* .end always holds ARRAY_END(buffer).  It is a pointer
		 * one-past-the-last character in the buffer.
		 */
		char  *end;
	};

This would be initialized as (of course, wrapper in static inline
functions as needed) this:

	char            buf[100];
	struct seq_buf  b;

	b.buffer = buf;
	b.p = buf;
	b.end = ARRAY_END(buf);

Then, you would write to it as:

	b.p = stpecpy(b.p, b.end, "foo");
	b.p = stpecpy(b.p, b.end, "bar");
	...
	b.p = stpecpy(b.p, b.end, "baz");
	if (b.p == NULL)
		goto fail;

Then, at some point, you might want to initialize a new buffer out of
the remainings of it:

	struct seq_buf  b2;

	if (b.p == NULL)
		goto fail;
	b2.start = b.p;
	b2.p = b.p;
	b2.end = b.end;

See how simple this becomes?  (So much that you may doubt the necessity
of the set of APIs at all, but they don't hurt much either.)


Cheers,
Alex

> 
> -- Steve

-- 
<https://www.alejandro-colomar.es>