Re: Simplifying or removing DEBUG_STACK?

[Ingo Molnar]

* Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:

> Hi all-
> 
> On x86_64, we use IST for #BP and #DB.  On x86_32, we don't.
> 
> We started using IST for #BP in:
> 
> b556b35e98ad [PATCH] x86_64: Move int 3 handler to debug stack and
> allow to increase it.
> 
> and we started using IST for #DB even earlier in:
> 
> 7abe2c67299e [PATCH] x86-64 merge for 2.6.4
> 
> This has some unpleasant side effects these days.  Primarily, it
> requires a bunch of ugly code to avoid recursive use of the debug
> stack when, say, an NMI interrupts do_int3 or do_debug and either hits
> a kprobe int3 or a #DB if it inadvertently touches a userspace
> watchpoint.  See TRACE_IRQS_OFF_DEBUG for another bit wart in that
> code.
> 
> Here are all of the reasons I can come up with for using IST:
> 
> 1. SYSENTER with TF set will immediately (or after one instruction --
> I'm not quite sure) cause #DB.  This is easy to handle -- we can just
> set up a sysenter stack just like x86_32.
> 
> 2. #DB needs paranoid gsbase handling (due to SYSENTER if nothing
> else).  However, there's no real reason that IST and paranoid gsbase
> handling need to be tied together.
> 
> 3. Stack usage.  Almost anything can hit a kprobe and any uaccess
> operation can hit a watchpoint.  I'm not sure how much of a problem
> this is.  If it is a real problem, we could use something more like
> the irqstack mechanism instead of IST.

This might have been an issue back when we still tried to fit things 
into 8K kernel stacks (4K on 32-bit). These days we have ~15K kernel 
stacks on 64-bit:

  arch/x86/include/asm/page_64_types.h:#define THREAD_SIZE_ORDER  (2 + KASAN_STACK_ORDER)

and we also have irq stacks that dramatically reduce asynchronous 
stack nesting effects.

> 4. kgdb.  kgdb doesn't appear to respect the kprobe blacklist at 
> all, so kdbg would blow up if it tried to breakpoint early or late 
> in syscall handling.  (Hmm.  I bet kdbg also blows up if you use it 
> to put a breakpoint early in do_int3.)

Yes, my answer to kernel debuggers is: "Don't do it then, or implement 
support for it more cleanly than this hackery."

> Thoughts?
> 
> Even if it turns out that we can't get rid of IST for #DB and #BP, I 
> bet we could simplify matters by rigging up the all of the IST 
> entries to switch IST off for #DB and #BP immediately upon entry and 
> to leave them off until immediately before returning, thereby 
> simplifying the logic quite a bit.  I think this would be a pure 
> performance win -- the only patch here in which performance matters 
> is NMI AFAICT, and the NMI code already does that, albeit rather 
> deeply buried.

I'd suggest we try get rid of it and restart with a clean 
implementation.

Thanks,

	Ingo
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