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

[Peter Zijlstra]

On Mon, Oct 26, 2020 at 04:55:21PM +0100, Peter Zijlstra wrote:
> 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?


Does something like this make sense?


diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 3c70fb34028b..0e7641ac19a8 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -799,6 +799,13 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
 	 */
 	current->thread.virtual_dr6 = 0;
 
+	/*
+	 * If PTRACE requested SINGLE(BLOCK)STEP, make sure to reflect that in
+	 * the ptrace visible DR6 copy.
+	 */
+	if (test_thread_flag(TIF_BLOCKSTEP) || test_thread_flag(TIF_SINGLESTEP))
+		current->thread.virtual_dr6 |= dr6 & DR_STEP;
+
 	/*
 	 * The SDM says "The processor clears the BTF flag when it
 	 * generates a debug exception."  Clear TIF_BLOCKSTEP to keep