Re: [PATCH v3 0/6] Tracing vs CR2

[Eiichi Tsukata]

On 2019/07/19 5:27, Andy Lutomirski wrote:
> Hi all-
> 
> I suspect that a bunch of the bugs you're all finding boil down to:
> 
>  - Nested debug exceptions could corrupt the outer exception's DR6.
>  - Nested debug exceptions in which *both* exceptions came from the
> kernel were probably all kinds of buggy
>  - Data breakpoints in bad places in the kernel were bad news
> 
> Could you give this not-quite-finished series a try?
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/
> 

Though I'm still trying to find out other cases(other areas which could
be buggy if we set hw breakpoints), as far as I tested, there is
no problem so far.

If I understand correctly, the call trace and the dr6 value will be: 

====

debug() // dr6: 0xffff4ff0, user_mode: 1
  TRACE_IRQS_OFF
    arch_stack_user_walk()
      debug()  // dr6: 0xffff4ff1 == 0xffff4ff0 | 0xffff0ff1 ... (*)
        do_debug()
          WARN_ON_ONCE
  do_debug() // dr6: 0xffff0ff0(cleared in the above do_debug())

(*) :
>   * The Intel SDM says:
>   *
>   *   Certain debug exceptions may clear bits 0-3. The remaining
>   *   contents of the DR6 register are never cleared by the
>   *   processor. To avoid confusion in identifying debug
>   *   exceptions, debug handlers should clear the register before
>   *   returning to the interrupted task.

====

Note: printk() in do_debug() can cause infinite loop(printk() -> 
irq_disable() -> do_debug() -> printk() ...), so printk_deferred()
was preferable.

Thanks

Eiichi