[PATCH 09/13] RFC ONLY - powerpc,kgdb: Introduce low level trap catching

[Jason Wessel <jason.wessel@windriver.com>]

This is a RFC patch.  The work to possibly merge kdb and kgdb is being
evaluated and this patch is considered only a proof of concept or
prototype.

The only way the debugger can handle a trap in inside rcu_lock,
notify_die, or atomic_notifier_call_chain without a recursive fault is
to have a low level "first opportunity handler" low level
program_check_exception() handler.

The other change here is to make sure that kgdb_handle_exception() is
called with correct parameters when catching an oops, because kdb
needs to know if the entry was an oops, single step, or breakpoint
exception.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
---
 arch/powerpc/kernel/kgdb.c  |    7 +++++--
 arch/powerpc/kernel/traps.c |    7 +++++++
 lib/Kconfig.kgdb            |    2 +-
 3 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kernel/kgdb.c b/arch/powerpc/kernel/kgdb.c
index e8207a3..d0ef25c 100644
--- a/arch/powerpc/kernel/kgdb.c
+++ b/arch/powerpc/kernel/kgdb.c
@@ -20,6 +20,7 @@
 #include <linux/smp.h>
 #include <linux/signal.h>
 #include <linux/ptrace.h>
+#include <linux/kdebug.h>
 #include <asm/current.h>
 #include <asm/processor.h>
 #include <asm/machdep.h>
@@ -115,7 +116,9 @@ void kgdb_roundup_cpus(unsigned long flags)
 /* KGDB functions to use existing PowerPC64 hooks. */
 static int kgdb_debugger(struct pt_regs *regs)
 {
-	return kgdb_handle_exception(0, computeSignal(TRAP(regs)), 0, regs);
+	if (kgdb_handle_exception(1, computeSignal(TRAP(regs)), DIE_OOPS, regs))
+		return 0;
+	return 1;
 }
 
 static int kgdb_handle_breakpoint(struct pt_regs *regs)
@@ -123,7 +126,7 @@ static int kgdb_handle_breakpoint(struct pt_regs *regs)
 	if (user_mode(regs))
 		return 0;
 
-	if (kgdb_handle_exception(0, SIGTRAP, 0, regs) != 0)
+	if (kgdb_handle_exception(1, SIGTRAP, 0, regs) != 0)
 		return 0;
 
 	if (*(u32 *) (regs->nip) == *(u32 *) (&arch_kgdb_ops.gdb_bpt_instr))
diff --git a/arch/powerpc/kernel/traps.c b/arch/powerpc/kernel/traps.c
index 678fbff..ae409df 100644
--- a/arch/powerpc/kernel/traps.c
+++ b/arch/powerpc/kernel/traps.c
@@ -812,12 +812,19 @@ void __kprobes program_check_exception(struct pt_regs *regs)
 		return;
 	}
 	if (reason & REASON_TRAP) {
+
+#ifdef CONFIG_KGDB_LOW_LEVEL_TRAP
+		if (debugger_bpt(regs))
+			return;
+#endif /* CONFIG_KGDB_LOW_LEVEL_TRAP */
 		/* trap exception */
 		if (notify_die(DIE_BPT, "breakpoint", regs, 5, 5, SIGTRAP)
 				== NOTIFY_STOP)
 			return;
+#ifndef CONFIG_KGDB_LOW_LEVEL_TRAP
 		if (debugger_bpt(regs))
 			return;
+#endif /* ! CONFIG_KGDB_LOW_LEVEL_TRAP */
 
 		if (!(regs->msr & MSR_PR) &&  /* not user-mode */
 		    report_bug(regs->nip, regs) == BUG_TRAP_TYPE_WARN) {
diff --git a/lib/Kconfig.kgdb b/lib/Kconfig.kgdb
index 617f797..a2f63be 100644
--- a/lib/Kconfig.kgdb
+++ b/lib/Kconfig.kgdb
@@ -59,7 +59,7 @@ config KGDB_TESTS_BOOT_STRING
 
 config KGDB_LOW_LEVEL_TRAP
        bool "KGDB: Allow debugging with traps in notifiers"
-       depends on X86
+       depends on X86 || PPC
        default n
        help
          This will add an extra call back to kgdb for the breakpoint
-- 
1.6.3.rc0.1.gf800