x86/debug: Change thread.debugreg6 to thread.virtual_dr6

Current usage of thread.debugreg6 is convoluted at best. It starts life as
a copy of the hardware DR6 value, but then various bits are cleared and
set.

Replace this with a new variable thread.virtual_dr6 that is initialized to
0 when DR6 is read and only gains bits, at the same time the actual (on
stack) dr6 value which is read from the hardware only gets bits cleared.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
Link: https://lore.kernel.org/r/20200902133201.415372940@infradead.org

Author: Peter Zijlstra

arch/x86/include/asm/processor.h

@@ -517,7 +517,7 @@ struct thread_struct {
 	/* Save middle states of ptrace breakpoints */
 	struct perf_event	*ptrace_bps[HBP_NUM];
 	/* Debug status used for traps, single steps, etc... */
-	unsigned long           debugreg6;
+	unsigned long           virtual_dr6;
 	/* Keep track of the exact dr7 value set by the user */
 	unsigned long           ptrace_dr7;
 	/* Fault info: */

arch/x86/kernel/hw_breakpoint.c

@@ -454,7 +454,7 @@ void flush_ptrace_hw_breakpoint(struct task_struct *tsk)
 		t->ptrace_bps[i] = NULL;
 	}
 
-	t->debugreg6 = 0;
+	t->virtual_dr6 = 0;
 	t->ptrace_dr7 = 0;
 }
 
@@ -489,8 +489,8 @@ static int hw_breakpoint_handler(struct die_args *args)
 {
 	int i, rc = NOTIFY_STOP;
 	struct perf_event *bp;
-	unsigned long dr6;
 	unsigned long *dr6_p;
+	unsigned long dr6;
 
 	/* The DR6 value is pointed by args->err */
 	dr6_p = (unsigned long *)ERR_PTR(args->err);
@@ -504,12 +504,6 @@ static int hw_breakpoint_handler(struct die_args *args)
 	if ((dr6 & DR_TRAP_BITS) == 0)
 		return NOTIFY_DONE;
 
-	/*
-	 * Reset the DRn bits in the virtualized register value.
-	 * The ptrace trigger routine will add in whatever is needed.
-	 */
-	current->thread.debugreg6 &= ~DR_TRAP_BITS;
-
 	/* Handle all the breakpoints that were triggered */
 	for (i = 0; i < HBP_NUM; ++i) {
 		if (likely(!(dr6 & (DR_TRAP0 << i))))
@@ -554,7 +548,7 @@ static int hw_breakpoint_handler(struct die_args *args)
 	 * breakpoints (to generate signals) and b) when the system has
 	 * taken exception due to multiple causes
 	 */
-	if ((current->thread.debugreg6 & DR_TRAP_BITS) ||
+	if ((current->thread.virtual_dr6 & DR_TRAP_BITS) ||
 	    (dr6 & (~DR_TRAP_BITS)))
 		rc = NOTIFY_DONE;
 

arch/x86/kernel/kgdb.c

@@ -629,9 +629,10 @@ static void kgdb_hw_overflow_handler(struct perf_event *event,
 	struct task_struct *tsk = current;
 	int i;
 
-	for (i = 0; i < 4; i++)
+	for (i = 0; i < 4; i++) {
 		if (breakinfo[i].enabled)
-			tsk->thread.debugreg6 |= (DR_TRAP0 << i);
+			tsk->thread.virtual_dr6 |= (DR_TRAP0 << i);
+	}
 }
 
 void kgdb_arch_late(void)

arch/x86/kernel/ptrace.c

@@ -465,7 +465,7 @@ static void ptrace_triggered(struct perf_event *bp,
 			break;
 	}
 
-	thread->debugreg6 |= (DR_TRAP0 << i);
+	thread->virtual_dr6 |= (DR_TRAP0 << i);
 }
 
 /*
@@ -601,7 +601,7 @@ static unsigned long ptrace_get_debugreg(struct task_struct *tsk, int n)
 		if (bp)
 			val = bp->hw.info.address;
 	} else if (n == 6) {
-		val = thread->debugreg6 ^ DR6_RESERVED; /* Flip back to arch polarity */
+		val = thread->virtual_dr6 ^ DR6_RESERVED; /* Flip back to arch polarity */
 	} else if (n == 7) {
 		val = thread->ptrace_dr7;
 	}
@@ -657,7 +657,7 @@ static int ptrace_set_debugreg(struct task_struct *tsk, int n,
 	if (n < HBP_NUM) {
 		rc = ptrace_set_breakpoint_addr(tsk, n, val);
 	} else if (n == 6) {
-		thread->debugreg6 = val ^ DR6_RESERVED; /* Flip to positive polarity */
+		thread->virtual_dr6 = val ^ DR6_RESERVED; /* Flip to positive polarity */
 		rc = 0;
 	} else if (n == 7) {
 		rc = ptrace_write_dr7(tsk, val);

arch/x86/kernel/traps.c

@@ -748,6 +748,12 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
 	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;
+
 	/*
 	 * The SDM says "The processor clears the BTF flag when it
 	 * generates a debug exception."  Clear TIF_BLOCKSTEP to keep
@@ -785,17 +791,16 @@ static __always_inline unsigned long debug_read_clear_dr6(void)
 
 static bool notify_debug(struct pt_regs *regs, unsigned long *dr6)
 {
-	struct task_struct *tsk = current;
-
-	/* Store the virtualized DR6 value */
-	tsk->thread.debugreg6 = *dr6;
-
+	/*
+	 * Notifiers will clear bits in @dr6 to indicate the event has been
+	 * consumed - hw_breakpoint_handler(), single_stop_cont().
+	 *
+	 * Notifiers will set bits in @virtual_dr6 to indicate the desire
+	 * for signals - ptrace_triggered(), kgdb_hw_overflow_handler().
+	 */
 	if (notify_die(DIE_DEBUG, "debug", regs, (long)dr6, 0, SIGTRAP) == NOTIFY_STOP)
 		return true;
 
-	/* Reload the DR6 value, the notifier might have changed it */
-	*dr6 = tsk->thread.debugreg6;
-
 	return false;
 }
 
@@ -853,7 +858,7 @@ static __always_inline void exc_debug_kernel(struct pt_regs *regs,
 	 * A known way to trigger this is through QEMU's GDB stub,
 	 * which leaks #DB into the guest and causes IST recursion.
 	 */
-	if (WARN_ON_ONCE(current->thread.debugreg6 & DR_STEP))
+	if (WARN_ON_ONCE(dr6 & DR_STEP))
 		regs->flags &= ~X86_EFLAGS_TF;
 out:
 	instrumentation_end();
@@ -903,6 +908,8 @@ static __always_inline void exc_debug_user(struct pt_regs *regs,
 		goto out_irq;
 	}
 
+	/* Add the virtual_dr6 bits for signals. */
+	dr6 |= current->thread.virtual_dr6;
 	if (dr6 & (DR_STEP | DR_TRAP_BITS) || icebp)
 		send_sigtrap(regs, 0, get_si_code(dr6));