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Subject[PATCH 2/3] x86/debug: Only clear/set ->virtual_dr6 for userspace #DB
The ->virtual_dr6 is the value used by ptrace_{get,set}_debugreg(6). A
kernel #DB clearing it could mean spurious malfunction of ptrace()
expectations.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/kernel/traps.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -793,12 +793,6 @@ static __always_inline unsigned long deb
set_debugreg(DR6_RESERVED, 6);
dr6 ^= DR6_RESERVED; /* Flip to positive polarity */

- /*
- * Clear the virtual DR6 value, ptrace routines will set bits here for
- * things we want signals for.
- */
- current->thread.virtual_dr6 = 0;
-
return dr6;
}

@@ -943,6 +937,12 @@ static __always_inline void exc_debug_us
instrumentation_begin();

/*
+ * Clear the virtual DR6 value, ptrace() routines will set bits here
+ * for things it wants signals for.
+ */
+ current->thread.virtual_dr6 = 0;
+
+ /*
* The SDM says "The processor clears the BTF flag when it
* generates a debug exception." Clear TIF_BLOCKSTEP to keep
* TIF_BLOCKSTEP in sync with the hardware BTF flag.

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