Re: [PATCH 00/16] ptrace: cleanups and calling do_cldstop with only siglock

[Peter Zijlstra]

On Fri, May 20, 2022 at 02:32:24PM -0500, Eric W. Biederman wrote:
> Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> writes:
> 
> > On 2022-05-18 17:49:50 [-0500], Eric W. Biederman wrote:
> >> 
> >> For ptrace_stop to work on PREEMT_RT no spinlocks can be taken once
> >> ptrace_freeze_traced has completed successfully.  Which fundamentally
> >> means the lock dance of dropping siglock and grabbing tasklist_lock does
> >> not work on PREEMPT_RT.  So I have worked through what is necessary so
> >> that tasklist_lock does not need to be grabbed in ptrace_stop after
> >> siglock is dropped.
> > …
> > It took me a while to realise that this is a follow-up I somehow assumed
> > that you added a few patches on top. Might have been the yesterday's
> > heat. b4 also refused to download this series because the v4 in this
> > thread looked newer… Anyway. Both series applied:
> >
> > | =============================
> > | WARNING: suspicious RCU usage
> > | 5.18.0-rc7+ #16 Not tainted
> > | -----------------------------
> > | include/linux/ptrace.h:120 suspicious rcu_dereference_check() usage!
> > |
> > | other info that might help us debug this:
> > |
> > | rcu_scheduler_active = 2, debug_locks = 1
> > | 2 locks held by ssdd/1734:
> > |  #0: ffff88800eaa6918 (&sighand->siglock){....}-{2:2}, at: lock_parents_siglocks+0xf0/0x3b0
> > |  #1: ffff88800eaa71d8 (&sighand->siglock/2){....}-{2:2}, at: lock_parents_siglocks+0x115/0x3b0
> > |
> > | stack backtrace:
> > | CPU: 2 PID: 1734 Comm: ssdd Not tainted 5.18.0-rc7+ #16
> > | Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.0-debian-1.16.0-4 04/01/2014
> > | Call Trace:
> > |  <TASK>
> > |  dump_stack_lvl+0x45/0x5a
> > |  unlock_parents_siglocks+0xb6/0xc0
> > |  ptrace_stop+0xb9/0x390
> > |  get_signal+0x51c/0x8d0
> > |  arch_do_signal_or_restart+0x31/0x750
> > |  exit_to_user_mode_prepare+0x157/0x220
> > |  irqentry_exit_to_user_mode+0x5/0x50
> > |  asm_sysvec_apic_timer_interrupt+0x12/0x20
> >
> > That is ptrace_parent() in unlock_parents_siglocks().
> 
> How odd.  I thought I had the appropriate lockdep config options enabled
> in my test build to catch things like this.  I guess not.
> 
> Now I am trying to think how to tell it that holding the appropriate
> iglock makes this ok.

The typical annotation is something like:

	rcu_dereference_protected(foo, lockdep_is_held(&bar))

Except in this case I think the problem is that bar depends on foo in
non-trivial ways. That is, foo is 'task->parent' and bar is
'task->parent->sighand->siglock' or something.

The other option is to use rcu_dereference_raw() in this one instance
and have a comment that explains the situation.