Messages in this thread | | | Date | Mon, 26 Oct 2020 16:55:21 +0100 | From | Peter Zijlstra <> | Subject | Re: [REGRESSION] x86/debug: After PTRACE_SINGLESTEP DR_STEP is no longer reported in dr6 |
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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?
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