[RFC][PATCH] retprobes - remove orig_ret_addr usage

[Ananth N Mavinakayanahalli <amavin at redhat dot com>]

Hi,

Here is a patch against 2.6.12-rc6-mm1 that removes usage of the static
void *orig_ret_addr. We now populate the ret_addr from the previous
kretprobe_instance in cases where the return address on stack has been
changed to kretprobe_trampoline().

Changes only done to i386. x86_64 may be similar. Rusty?

Ananth
Populate ri->ret_addr from the previous kretprobe_instance in cases where
the return address has been replaced to kretprobe_trampoline().

Signed-off-by: Ananth N Mavinakayanahalli <amavin@redhat.com>


 arch/i386/kernel/kprobes.c |   31 +++++++++++++++++--------------
 1 files changed, 17 insertions(+), 14 deletions(-)

Index: linux-2.6.12-rc6/arch/i386/kernel/kprobes.c
===================================================================
--- linux-2.6.12-rc6.orig/arch/i386/kernel/kprobes.c	2005-06-08 09:20:16.000000000 -0400
+++ linux-2.6.12-rc6/arch/i386/kernel/kprobes.c	2005-06-08 09:27:19.000000000 -0400
@@ -137,25 +137,28 @@ void arch_prepare_kretprobe(struct kretp
 {
 	unsigned long *sara = (unsigned long *)&regs->esp;
 	struct kretprobe_instance *ri;
-	static void *orig_ret_addr;
-
-	/*
-	 * Save the return address when the return probe hits
-	 * the first time, and use it to populate the (krprobe
-	 * instance)->ret_addr for subsequent return probes at
-	 * the same addrress since stack address would have
-	 * the kretprobe_trampoline by then.
-	 */
-	if (((void*) *sara) != kretprobe_trampoline)
-		orig_ret_addr = (void*) *sara;
 
 	if ((ri = get_free_rp_inst(rp)) != NULL) {
 		ri->rp = rp;
 		ri->stack_addr = sara;
-		ri->ret_addr = orig_ret_addr;
+		if (likely((void *)*sara != kretprobe_trampoline)) {
+			/*
+			 * First time hit - replace the return addr
+			 * with trampoline addr
+			 */
+			ri->ret_addr = (void *)*sara;
+			*sara = (unsigned long) &kretprobe_trampoline;
+		} else {
+			/*
+			 * return address on stack has been replaced
+			 * in the case of multiple retprobes - populate
+			 * the correct ret_addr from the previous ri
+			 */
+			struct kretprobe_instance *old_ri = get_rp_inst(sara);
+			BUG_ON(!old_ri);
+			ri->ret_addr = old_ri->ret_addr;
+		}
 		add_rp_inst(ri);
-		/* Replace the return addr with trampoline addr */
-		*sara = (unsigned long) &kretprobe_trampoline;
 	} else {
 		rp->nmissed++;
 	}