x86/traps: Enable DEBUG_STACK after cpu_init() for TRAP_DB/BP

commit b4d8327024637cb2a1f7910dcb5d0ad7a096f473 upstream.

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@huawei.com>
Reviewed-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Cc: <dave.hansen@linux.intel.com>
Cc: <lizefan@huawei.com>
Cc: <luto@amacapital.net>
Cc: <oleg@redhat.com>
Link: http://lkml.kernel.org/r/1424929779-13174-1-git-send-email-wangnan0@huawei.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index edbd30a..55e5f64 100644 (file)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -718,9 +718,17 @@ 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 it is impossible to hit an invalid stack.
+        * Using the original stack works well enough at this early
+        * stage. 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);
        set_intr_gate(X86_TRAP_PF, &page_fault);
        load_idt(&idt_descr);
 }
@@ -781,5 +789,14 @@ void __init trap_init(void)
         */
        cpu_init();
 
+       /*
+        * X86_TRAP_DB and X86_TRAP_BP have been set
+        * in early_trap_init(). However, DEBUG_STACK works only after
+        * cpu_init() loads 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();
 }