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

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 97143d8..d8a82e6 100644 (file)
--- a/arch/x86/include/asm/processor.h
+++ b/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: */

diff --git a/arch/x86/kernel/hw_breakpoint.c b/arch/x86/kernel/hw_breakpoint.c
index d17a1da..03aa33b 100644 (file)
--- a/arch/x86/kernel/hw_breakpoint.c
+++ b/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;
 

diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
index c2f02f3..ff7878d 100644 (file)
--- a/arch/x86/kernel/kgdb.c
+++ b/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)

diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
index 5f98289..bedca01 100644 (file)
--- a/arch/x86/kernel/ptrace.c
+++ b/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);

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 114515b..df9c655 100644 (file)
--- a/arch/x86/kernel/traps.c
+++ b/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));