[PATCH v6 0/3] sched_ext: Harden scx_bpf_cpu_rq()

[Christian Loehle]

scx_bpf_cpu_rq() currently allows accessing struct rq fields without
holding the associated rq.
It is being used by scx_cosmos, scx_flash, scx_lavd, scx_layered, and
scx_tickless. Fortunately it is only ever used to fetch rq->curr.
So provide an alternative scx_bpf_remote_curr() that doesn't expose struct rq
and provide a hardened scx_bpf_cpu_rq_locked() by ensuring we hold the rq lock.
Add a deprecation warning to scx_bpf_cpu_rq() that mentions the two alternatives.

This also simplifies scx code from:

rq = scx_bpf_cpu_rq(cpu);
if (!rq)
	return;
p = rq->curr
/* ... Do something with p */

into:

p = scx_bpf_remote_curr(cpu);
/* ... Do something with p */

Changes since:
v5:
https://lore.kernel.org/lkml/20250901132605.2282650-2-christian.loehle@xxxxxxx/
- Actually expose the RCU pointer in scx_bpf_remote_curr() as such (Andrea)
v4:
https://lore.kernel.org/lkml/20250811212150.85759-1-christian.loehle@xxxxxxx/
- Remove cpu argument from scx_bpf_cpu_rq_locked() as SCX has a unique
locked_rq_state anyway. (Tejun)
- Expose RCU pointer in scx_bpf_remote_curr() (Peter)
v3:
https://lore.kernel.org/lkml/20250805111036.130121-1-christian.loehle@xxxxxxx/
- Don't change scx_bpf_cpu_rq() do not break BPF schedulers without the
grace period. Just add the deprecation warning and do the hardening in
the new scx_bpf_cpu_rq_locked(). (Andrea, Tejun, Jake)
v2:
https://lore.kernel.org/lkml/20250804112743.711816-1-christian.loehle@xxxxxxx/
- Open-code bpf_task_acquire() to avoid the forward declaration (Andrea)
- Rename scx_bpf_task_acquire_remote_curr() to make it more explicit it
behaves like bpf_task_acquire()
v1:
https://lore.kernel.org/lkml/20250801141741.355059-1-christian.loehle@xxxxxxx/
- scx_bpf_cpu_rq() now errors when a not locked rq is requested. (Andrea)
- scx_bpf_remote_curr() calls bpf_task_acquire() which BPF user needs to
release. (Andrea)

Christian Loehle (3):
  sched_ext: Introduce scx_bpf_cpu_rq_locked()
  sched_ext: Introduce scx_bpf_remote_curr()
  sched_ext: deprecation warn for scx_bpf_cpu_rq()

 kernel/sched/ext.c                       | 40 ++++++++++++++++++++++++
 tools/sched_ext/include/scx/common.bpf.h |  2 ++
 2 files changed, 42 insertions(+)

--
2.34.1