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
