Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: 5.8-rc*: kernel BUG at kernel/signal.c:1917 | Date | Mon, 20 Jul 2020 15:26:16 +0100 |
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On 20/07/20 14:17, peterz@infradead.org wrote: > On Mon, Jul 20, 2020 at 01:20:26PM +0100, Valentin Schneider wrote: >> On 20/07/20 12:26, peterz@infradead.org wrote: > >> > + /* >> > + * We must re-load prev->state in case ttwu_remote() changed it >> > + * before we acquired rq->lock. >> > + */ >> > + tmp_state = prev->state; >> > + if (unlikely(prev_state != tmp_state)) { >> > + /* >> > + * ptrace_{,un}freeze_traced() think it is cool to change >> > + * ->state around behind our backs between TASK_TRACED and >> > + * __TASK_TRACED. >> > + * >> > + * This is safe because this, as well as any __TASK_TRACED >> > + * wakeups are under siglock. >> > + * >> > + * For any other case, a changed prev_state must be to >> > + * TASK_RUNNING, such that when it blocks, the load has >> > + * happened before the smp_mb(). >> > + * >> > + * Also see the comment with deactivate_task(). >> > + */ >> > + SCHED_WARN_ON(tmp_state && (prev_state & __TASK_TRACED && >> > + !(tmp_state & __TASK_TRACED))); >> > + >> >> IIUC if the state changed and isn't TASK_RUNNING it *has* to have >> __TASK_TRACED, so can't that be >> >> SCHED_WARN_ON(tmp_state && !(tmp_state & __TASK_TRACED)); > > Suppose task->state == TASK_UNINTERRUPTIBLE, and task != current, and > then someone goes and does task->state = __TASK_TRACED. > > That is, your statement is correct given the current code, but we also > want to verify no new code comes along and does something 'creative'. >
That is what I was trying to go for; AFAICT your approach only warns if __TASK_TRACED gets removed between the two loads (IOW it was already there). The way I was seeing it is:
- We only get here if prev_state != tmp_state; IOW we know we raced with something - if (tmp_state), then it wasn't with ttwu_remote() - thus it must only be with ptrace shenanigans, IOW __TASK_TRACED must be there.
Now, what I suggested still doesn't detect what you pointed out, or some crazier thing that sets __TASK_TRACED *and* some other stuff. IIUC the ptrace transformation does TASK_TRACED -> __TASK_TRACED, so we could have it as:
/* TODO: name me */ #define foobar TASK_TRACED ^ __TASK_TRACED
...
/* not TASK_RUNNING; check against allowed transformations SCHED_WARN_ON(tmp_state && ((prev_state ^ tmp_state) & ~foobar));
That said...
> Or is the heat getting to me?
... that may apply to me as well :-)
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