[PATCH] x86_64: Move int 3 handler to debug stack and allow to increase it.

This
- switches the INT3 handler to run on an IST stack (to cope with
  breakpoints set by a kernel debugger on places where the kernel's
  %gs base hasn't been set up, yet); the IST stack used is shared with
  the INT1 handler's
[AK: this also allows setting a kprobe on the interrupt/exception entry
points]
- allows nesting of INT1/INT3 handlers so that one can, with a kernel
  debugger, debug (at least) the user-mode portions of the INT1/INT3
  handling; the nesting isn't actively enabled here since a kernel-
  debugger-free kernel doesn't need it

Signed-Off-By: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 32 insertions, 3 deletions

diff --git a/arch/x86_64/kernel/traps.c b/arch/x86_64/kernel/traps.c
index bd71ddac0dc5..1a9094dab682 100644
--- a/arch/x86_64/kernel/traps.c
+++ b/arch/x86_64/kernel/traps.c
@@ -121,19 +121,31 @@ int printk_address(unsigned long address)
 static unsigned long *in_exception_stack(unsigned cpu, unsigned long stack,
                                         unsigned *usedp, const char **idp)
 {
-        static const char ids[N_EXCEPTION_STACKS][8] = {
+        static char ids[][8] = {
                 [DEBUG_STACK - 1] = "#DB",
                 [NMI_STACK - 1] = "NMI",
                 [DOUBLEFAULT_STACK - 1] = "#DF",
                 [STACKFAULT_STACK - 1] = "#SS",
                 [MCE_STACK - 1] = "#MC",
+#if DEBUG_STKSZ > EXCEPTION_STKSZ
+                [N_EXCEPTION_STACKS ... N_EXCEPTION_STACKS + DEBUG_STKSZ / EXCEPTION_STKSZ - 2] = "#DB[?]"
+#endif
         };
         unsigned k;
 
         for (k = 0; k < N_EXCEPTION_STACKS; k++) {
                 unsigned long end;
 
-                end = per_cpu(init_tss, cpu).ist[k];
+                switch (k + 1) {
+#if DEBUG_STKSZ > EXCEPTION_STKSZ
+                case DEBUG_STACK:
+                        end = cpu_pda[cpu].debugstack + DEBUG_STKSZ;
+                        break;
+#endif
+                default:
+                        end = per_cpu(init_tss, cpu).ist[k];
+                        break;
+                }
                 if (stack >= end)
                         continue;
                 if (stack >= end - EXCEPTION_STKSZ) {
@@ -143,6 +155,22 @@ static unsigned long *in_exception_stack(unsigned cpu, unsigned long stack,
                         *idp = ids[k];
                         return (unsigned long *)end;
                 }
+#if DEBUG_STKSZ > EXCEPTION_STKSZ
+                if (k == DEBUG_STACK - 1 && stack >= end - DEBUG_STKSZ) {
+                        unsigned j = N_EXCEPTION_STACKS - 1;
+
+                        do {
+                                ++j;
+                                end -= EXCEPTION_STKSZ;
+                                ids[j][4] = '1' + (j - N_EXCEPTION_STACKS);
+                        } while (stack < end - EXCEPTION_STKSZ);
+                        if (*usedp & (1U << j))
+                                break;
+                        *usedp |= 1U << j;
+                        *idp = ids[j];
+                        return (unsigned long *)end;
+                }
+#endif
         }
         return NULL;
 }
@@ -613,6 +641,7 @@ asmlinkage void default_do_nmi(struct pt_regs *regs)
                 io_check_error(reason, regs);
 }
 
+/* runs on IST stack. */
 asmlinkage void __kprobes do_int3(struct pt_regs * regs, long error_code)
 {
         if (notify_die(DIE_INT3, "int3", regs, error_code, 3, SIGTRAP) == NOTIFY_STOP) {
@@ -894,7 +923,7 @@ void __init trap_init(void)
         set_intr_gate(0,&divide_error);
         set_intr_gate_ist(1,&debug,DEBUG_STACK);
         set_intr_gate_ist(2,&nmi,NMI_STACK);
-        set_system_gate(3,&int3);
+        set_system_gate_ist(3,&int3,DEBUG_STACK); /* int3 can be called from all */
         set_system_gate(4,&overflow);   /* int4 can be called from all */
         set_intr_gate(5,&bounds);
         set_intr_gate(6,&invalid_op);