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

[Kees Cook <kees@kernel.org>]

On Mon, Oct 05, 2026 at 06:43:23PM +0200, Alejandro Colomar wrote:
> > Yes, though not often: the sched_ext debug dump builds a nested per-CPU
> > seq_buf from seq_buf_get_buf(), which returns a size of 0 once the dump
> > buffer has overflowed, and it handles that case on purpose (see the
> > "$s may already have overflowed" comment in kernel/sched/ext/ext.c).
>
> Hmmmm, so IIUC, it is a sentinel value, and not really a buffer size?

It's the real size of what's left. seq_buf_get_buf() hands out the
unused tail of the seq_buf. Once that has overflowed there is no tail,
so it returns NULL for the buffer and 0 for the size, and sched_ext
initializes the nested seq_buf with both. So that case does get the NULL;
the 0 is how much room it has.

> Did you consider not holding the size of the buffer, but rather an end
> pointer?  It also allows you to find how much you can write to the
> buffer, but has slightly better semantics: the end doesn't change if you
> advance the position.  'end' can only change through reallocations
> (which I don't know if you do in the kernel).

The size doesn't change either: it is set by seq_buf_init(), and writers
advance "len", an offset from the start of the buffer. seq_buf never
reallocates.

An end pointer wouldn't remove the empty case: buffer == end is the same
condition as size == 0, so the accessors would still need the check. It
also makes overflow harder to record. seq_buf marks it with len == size +
1, and the matching position pointer, end + 1, would be weird.

Changing the representation would also means changes outside
lib/seq_buf.c, since tracing reads seq.len and seq.size directly
(trace_seq treats a zero size as "not yet initialized", for one). I'm
not saying that's a good thing to bypass the API, but rather that such
a change should be its own series.

-Kees

-- 
Kees Cook