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SubjectRe: Simplifying or removing DEBUG_STACK?

* Andy Lutomirski <luto@amacapital.net> 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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