Re: [PATCH] arm64: don't preempt_disable in do_debug_exception

From: Will Deacon <will@kernel.org>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>,
	linux-arm-kernel@lists.infradead.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Naresh Kamboju <naresh.kamboju@linaro.org>,
	stable@vger.kernel.org, Masami Hiramatsu <mhiramat@kernel.org>,
	"Paul E . McKenney" <paulmck@linux.ibm.com>,
	James Morse <james.morse@arm.com>
Subject: Re: [PATCH] arm64: don't preempt_disable in do_debug_exception
Date: Fri, 26 Jun 2020 10:55:54 +0100	[thread overview]
Message-ID: <20200626095551.GA9312@willie-the-truck> (raw)
In-Reply-To: <20200623165557.GA12767@C02TD0UTHF1T.local>

On Tue, Jun 23, 2020 at 05:55:57PM +0100, Mark Rutland wrote:
> On Tue, Jun 23, 2020 at 04:59:01PM +0100, Will Deacon wrote:
> > On Thu, Jun 18, 2020 at 01:29:29PM -0400, Paul Gortmaker wrote:
> > >  BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:975
> > >  in_atomic(): 1, irqs_disabled(): 0, pid: 35658, name: gdbtest
> > >  Preemption disabled at:
> > >  [<ffff000010081578>] do_debug_exception+0x38/0x1a4
> > >  Call trace:
> > >  dump_backtrace+0x0/0x138
> > >  show_stack+0x24/0x30
> > >  dump_stack+0x94/0xbc
> > >  ___might_sleep+0x13c/0x168
> > >  rt_spin_lock+0x40/0x80
> > >  do_force_sig_info+0x30/0xe0
> > >  force_sig_fault+0x64/0x90
> > >  arm64_force_sig_fault+0x50/0x80
> > >  send_user_sigtrap+0x50/0x80
> > >  brk_handler+0x98/0xc8
> > >  do_debug_exception+0x70/0x1a4
> > >  el0_dbg+0x18/0x20
> > > 
> > > The reproducer was basically an automated gdb test that set a breakpoint
> > > on a simple "hello world" program and then quit gdb once the breakpoint
> > > was hit - i.e. "(gdb) A debugging session is active.  Quit anyway? "
> > 
> > Hmm, the debug exception handler path was definitely written with the
> > expectation that preemption is disabled, so this is unfortunate. For
> > exceptions from kernelspace, we need to keep that guarantee as we implement
> > things like BUG() using this path. For exceptions from userspace, it's
> > plausible that we could re-enable preemption, but then we should also
> > re-enable interrupts and debug exceptions too because we don't
> > context-switch pstate in switch_to() and we would end up with holes in our
> > kernel debug coverage (and these might be fatal if e.g. single step doesn't
> > work in a kprobe OOL buffer). However, that then means that any common code
> > when handling user and kernel debug exceptions needs to be re-entrant,
> > which it probably isn't at the moment (I haven't checked).
> 
> I'm pretty certain existing code is not reentrant, and regardless it's
> going to be a mess to reason about this generally if we have to undo our
> strict exception nesting rules.

Are these rules written down somewhere? I'll need to update them if we
get this working for preempt-rt (and we should try to do that).

> I reckon we need to treat this like an NMI instead -- is that plausible?

I don't think so. It's very much a synchronous exception, and delivering a
signal to the exceptional context doesn't feel like an NMI to me. There's
also a fair amount of code that can run in debug context (hw_breakpoint,
kprobes, uprobes, kasan) which might not be happy to suddenly be in an
NMI-like environment. Furthermore, the masking rules are different depending
on what triggers the exception.

One of the things I've started looking at is ripping out our dodgy
hw_breakpoint code so that kernel debug exceptions are easier to reason
about. Specifically, I think we end up with something like:

- On taking a non-debug exception from EL0, unmask D as soon as we can.

- On taking a debug exception from EL0, unmask {D,I} and invoke user
  handlers. I think this always means SIGTRAP, apart from uprobes.
  This will mean making those paths preemptible, as I don't think they
  are right now (e.g. traversing the callback hooks uses an RCU-protected
  list).

- On taking a non-debug, non-fatal synchronous exception from EL1, unmask
  D as soon as we can (i.e. we step into these exceptions). Fatal exceptions
  can obviously leave D masked.

- On taking an interrupt from EL1, stash MDSCR_EL1.SS in a pcpu variable and
  clear the register bit if it was set. Then unmask only D and leave I set. On
  return from the exception, set D and restore MDSCR_EL1.SS. If we decide to
  reschedule, unmask D (i.e. we only step into interrupts if we need a
  reschedule. Alternatively, we could skip the reschedule if we were
  stepping.)

- On taking a debug exception from EL1, leave {D,I} set. Watchpoints on
  uaccess are silently stepped over.

Thoughts? We could probably simplify this if we could state that stepping an
instruction in kernel space could only ever be interrupted by an interrupt.
That's probably true for kprobes, but relying on it feels like it might bite
us later on.

Will