Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no longer reported in dr6

[Peter Zijlstra]

On Mon, Oct 26, 2020 at 07:33:08AM -0700, Kyle Huey wrote:
> After resuming a ptracee with PTRACE_SINGLESTEP, in the following
> ptrace stop retrieving the dr6 value for the tracee gets a value that
> does not include DR_STEP (it is in fact always DR6_RESERVED). I
> bisected this to the 13cb73490f475f8e7669f9288be0bcfa85399b1f merge. I
> did not bisect further.
> 
> I don't see any handling to ever set DR_STEP in virtual_dr6, so I
> think this code is just broken.
> 
> Sorry for not testing this when I was CCd on the original patch series :)

Urgh, now I have to try and remember how all that worked again ...

I suspect it's either one (or both) of the last two:

  f4956cf83ed1 ("x86/debug: Support negative polarity DR6 bits")
  d53d9bc0cf78 ("x86/debug: Change thread.debugreg6 to thread.virtual_dr6")


Just to clarify, the sequence is something like:

 - tracer: ptrace(PTRACE_SINGLESTEP)
 - tracee: #DB, DR6 contains DR_STEP
 - tracer: ptrace_get_debugreg(6)

?

You're right that that would be broken, let me try and figure out what
the best way would be 'fix' that.

Also, can you confirm that pthread_set_debugreg(6) should not do
anything useful?