linux-kernel - [PATCH] x86, traps: Enable DEBUG_STACK after cpu_init() for TRAP

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Date:	Thu, 26 Feb 2015 11:57:56 +0800
From:	Wang Nan <wangnan0@...wei.com>
To:	<mingo@...hat.com>, <hpa@...or.com>, <tglx@...utronix.de>,
	<x86@...nel.org>, <luto@...capital.net>, <oleg@...hat.com>,
	<masami.hiramatsu.pt@...achi.com>, <dave.hansen@...ux.intel.com>,
	<rostedt@...dmis.org>
CC:	<linux-kernel@...r.kernel.org>, <lizefan@...wei.com>
Subject: [PATCH] x86, traps: Enable DEBUG_STACK after cpu_init() for TRAP_DB/BP.

Before this patch early_trap_init() installs DEBUG_STACK for X86_TRAP_BP
and X86_TRAP_DB. However, DEBUG_STACK doesn't work correctly until
cpu_init() <-- trap_init().

This patch passes 0 to set_intr_gate_ist() and
set_system_intr_gate_ist() instead of DEBUG_STACK to let it use same
stack as kernel, and installs DEBUG_STACK for them in trap_init().

As core runs at ring 0 between early_trap_init() and trap_init(), there
is no chance to get a bad stack before trap_init().

As NMI is also enabled in trap_init(), we don't need to care about
is_debug_stack() and related things used in arch/x86/kernel/nmi.c.

Signed-off-by: Wang Nan <wangnan0@...wei.com>
---
 arch/x86/kernel/traps.c | 20 ++++++++++++++++++--
 1 file changed, 18 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 9d2073e..a9b8640 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -925,9 +925,16 @@ dotraplinkage void do_iret_error(struct pt_regs *regs, long error_code)
 /* Set of traps needed for early debugging. */
 void __init early_trap_init(void)
 {
-	set_intr_gate_ist(X86_TRAP_DB, &debug, DEBUG_STACK);
+	/*
+	 * Don't set ist to DEBUG_STACK as it doesn't work until TSS is
+	 * ready in cpu_init() <-- trap_init(). Before trap_init(), CPU
+	 * runs at ring 0 so there should be impossible to hit a invalid
+	 * stack. Use original stack is enough. DEBUG_STACK will be
+	 * equipped after cpu_init() in trap_init().
+	 */
+	set_intr_gate_ist(X86_TRAP_DB, &debug, 0);
 	/* int3 can be called from all */
-	set_system_intr_gate_ist(X86_TRAP_BP, &int3, DEBUG_STACK);
+	set_system_intr_gate_ist(X86_TRAP_BP, &int3, 0);
 #ifdef CONFIG_X86_32
 	set_intr_gate(X86_TRAP_PF, page_fault);
 #endif
@@ -1005,6 +1012,15 @@ void __init trap_init(void)
 	 */
 	cpu_init();
 
+	/*
+	 * X86_TRAP_DB and X86_TRAP_BP have been setup
+	 * in early_trap_init(). However, DEBUG_STACK works only after
+	 * cpu_init() load TSS. See comments in early_trap_init().
+	 */
+	set_intr_gate_ist(X86_TRAP_DB, &debug, DEBUG_STACK);
+	/* int3 can be called from all */
+	set_system_intr_gate_ist(X86_TRAP_BP, &int3, DEBUG_STACK);
+
 	x86_init.irqs.trap_init();
 
 #ifdef CONFIG_X86_64
-- 
1.8.4

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