Re: x86-64: int3 no longer causes SIGTRAP in 2.6.10

[Andi Kleen]

Juho Snellman <jsnell@xxxxxx> writes:

> 2.6.10 changed the behaviour of the int3 instruction on x86-64. It
> used to result in a SIGTRAP, now it's a SIGSEGV in both native and
> 32-bit legacy modes. This was apparently caused by the kprobe port,
> specifically this part:
>
> --- a/arch/x86_64/kernel/traps.c        2004-12-24 13:36:17 -08:00
> +++ b/arch/x86_64/kernel/traps.c        2004-12-24 13:36:17 -08:00
> @@ -862,8 +910,8 @@
>         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);       /* int3-5 can be called from all */
> -       set_system_gate(4,&overflow);
> +       set_intr_gate(3,&int3);
> +       set_system_gate(4,&overflow);   /* int4-5 can be called from all */
>
> Was effectively disabling int3 a conscious decision, or just an
> unintended side-effect? This breaks at least Steel Bank Common Lisp

It's a bug. Thanks for the report.

I'm not sure why it was even changed. Prasanna? 

I think it should be just changed back. If kprobes cannot 
deal with traps for user space it needs to be fixed. e.g.
by adding a user space check in kprobe_handler().

-Andi

Like this patch.

Index: linux/arch/x86_64/kernel/traps.c
===================================================================
--- linux.orig/arch/x86_64/kernel/traps.c	2005-01-17 10:34:24.%N +0100
+++ linux/arch/x86_64/kernel/traps.c	2005-01-18 02:42:02.%N +0100
@@ -908,7 +908,7 @@
 	set_intr_gate(0,&divide_error);
 	set_intr_gate_ist(1,&debug,DEBUG_STACK);
 	set_intr_gate_ist(2,&nmi,NMI_STACK);
-	set_intr_gate(3,&int3);
+	set_system_gate(3,&int3);
 	set_system_gate(4,&overflow);	/* int4-5 can be called from all */
 	set_system_gate(5,&bounds);
 	set_intr_gate(6,&invalid_op);
Index: linux/arch/x86_64/kernel/kprobes.c
===================================================================
--- linux.orig/arch/x86_64/kernel/kprobes.c	2005-01-04 12:12:39.%N +0100
+++ linux/arch/x86_64/kernel/kprobes.c	2005-01-18 02:46:05.%N +0100
@@ -297,6 +297,8 @@
 	struct die_args *args = (struct die_args *)data;
 	switch (val) {
 	case DIE_INT3:
+		if (args->regs->cs & 3)
+			return NOTIFY_DONE;
 		if (kprobe_handler(args->regs))
 			return NOTIFY_STOP;
 		break;


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