Re: kprobe_handler should check pre_handler function

[Ananth N Mavinakayanahalli <ananth at in dot ibm dot com>]

On Mon, Mar 28, 2005 at 04:10:32PM -0500, William Cohen wrote:

Hi Will,

> I found kprobes expects there to be a pre_handler function in the 
> structure. I was writing a probe that only needed a post_handler 
> function, no pre_handler function. The probe was tracking the 
> destinations of indirect calls and jumps, the probe needs to fire after 
> the instruction single steps to get the target address. The probe 
> crashed the machine because arch/i386/kernel/kprobe.c:kprobe_handler() 
> blindly calls p->pre_handler().  There should be a check to verify that 
> the pointer is non-null. There are cases where the pre_handler is not 
> needed and it would make sense to set it to NULL. Thus, a check should 
> be done for pre_handler like post_handler and fault_handler.

You are right. The check for pre_handler is needed and here is a patch
against 2.6.12-rc1-mm3 that does this.

Thanks,
Ananth

Signed-off-by: Ananth N Mavinakayanahalli <ananth@in.ibm.com>


diff -Narup temp/linux-2.6.12-rc1/arch/i386/kernel/kprobes.c linux-2.6.12-rc1/arch/i386/kernel/kprobes.c
--- temp/linux-2.6.12-rc1/arch/i386/kernel/kprobes.c	2005-03-17 20:34:10.000000000 -0500
+++ linux-2.6.12-rc1/arch/i386/kernel/kprobes.c	2005-03-28 17:51:21.000000000 -0500
@@ -159,17 +159,16 @@ static int kprobe_handler(struct pt_regs
 	if (is_IF_modifier(p->opcode))
 		kprobe_saved_eflags &= ~IF_MASK;
 
-	if (p->pre_handler(p, regs)) {
+	if (p->pre_handler && p->pre_handler(p, regs))
 		/* handler has already set things up, so skip ss setup */
 		return 1;
-	}
 
-      ss_probe:
+ss_probe:
 	prepare_singlestep(p, regs);
 	kprobe_status = KPROBE_HIT_SS;
 	return 1;
 
-      no_kprobe:
+no_kprobe:
 	preempt_enable_no_resched();
 	return ret;
 }
diff -Narup temp/linux-2.6.12-rc1/arch/ppc64/kernel/kprobes.c linux-2.6.12-rc1/arch/ppc64/kernel/kprobes.c
--- temp/linux-2.6.12-rc1/arch/ppc64/kernel/kprobes.c	2005-03-28 17:48:56.000000000 -0500
+++ linux-2.6.12-rc1/arch/ppc64/kernel/kprobes.c	2005-03-28 17:51:31.000000000 -0500
@@ -128,10 +128,9 @@ static inline int kprobe_handler(struct 
 	kprobe_status = KPROBE_HIT_ACTIVE;
 	current_kprobe = p;
 	kprobe_saved_msr = regs->msr;
-	if (p->pre_handler(p, regs)) {
+	if (p->pre_handler && p->pre_handler(p, regs))
 		/* handler has already set things up, so skip ss setup */
 		return 1;
-	}
 
 ss_probe:
 	prepare_singlestep(p, regs);
diff -Narup temp/linux-2.6.12-rc1/arch/sparc64/kernel/kprobes.c linux-2.6.12-rc1/arch/sparc64/kernel/kprobes.c
--- temp/linux-2.6.12-rc1/arch/sparc64/kernel/kprobes.c	2005-03-17 20:34:33.000000000 -0500
+++ linux-2.6.12-rc1/arch/sparc64/kernel/kprobes.c	2005-03-28 17:50:55.000000000 -0500
@@ -128,7 +128,7 @@ static int kprobe_handler(struct pt_regs
 
 	kprobe_status = KPROBE_HIT_ACTIVE;
 	current_kprobe = p;
-	if (p->pre_handler(p, regs))
+	if (p->pre_handler && p->pre_handler(p, regs))
 		return 1;
 
 ss_probe:
diff -Narup temp/linux-2.6.12-rc1/arch/x86_64/kernel/kprobes.c linux-2.6.12-rc1/arch/x86_64/kernel/kprobes.c
--- temp/linux-2.6.12-rc1/arch/x86_64/kernel/kprobes.c	2005-03-28 17:48:57.000000000 -0500
+++ linux-2.6.12-rc1/arch/x86_64/kernel/kprobes.c	2005-03-28 17:51:10.000000000 -0500
@@ -293,17 +293,16 @@ int kprobe_handler(struct pt_regs *regs)
 	if (is_IF_modifier(p->ainsn.insn))
 		kprobe_saved_rflags &= ~IF_MASK;
 
-	if (p->pre_handler(p, regs)) {
+	if (p->pre_handler && p->pre_handler(p, regs))
 		/* handler has already set things up, so skip ss setup */
 		return 1;
-	}
 
-      ss_probe:
+ss_probe:
 	prepare_singlestep(p, regs);
 	kprobe_status = KPROBE_HIT_SS;
 	return 1;
 
-      no_kprobe:
+no_kprobe:
 	preempt_enable_no_resched();
 	return ret;
 }