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Subject[patch 3/5] [kprobes] Tweak to the function return probe design
(The following is a resend from this morning.  The various kernel mailing list
did not seem to get my email, so I am resending the patch series from another
machine.)

The following provides the x86_64 specific changes for the new
return probe design. Note that with this new design, the dependency
on calculating a pointer to the task off the stack pointer no longer
exist (resolving the problem of interruption stacks as pointed out
in the original feedback to this port.)

signed-off-by: Rusty Lynch <Rusty.lynch@intel.com>

arch/x86_64/kernel/kprobes.c | 130 ++++++++++++++++++++++---------------------
1 files changed, 68 insertions(+), 62 deletions(-)

Index: linux-2.6.12-rc6-mm1/arch/x86_64/kernel/kprobes.c
===================================================================
--- linux-2.6.12-rc6-mm1.orig/arch/x86_64/kernel/kprobes.c
+++ linux-2.6.12-rc6-mm1/arch/x86_64/kernel/kprobes.c
@@ -274,48 +274,23 @@ static void prepare_singlestep(struct kp
regs->rip = (unsigned long)p->ainsn.insn;
}

-struct task_struct *arch_get_kprobe_task(void *ptr)
-{
- return ((struct thread_info *) (((unsigned long) ptr) &
- (~(THREAD_SIZE -1))))->task;
-}
-
void arch_prepare_kretprobe(struct kretprobe *rp, struct pt_regs *regs)
{
unsigned long *sara = (unsigned long *)regs->rsp;
- struct kretprobe_instance *ri;
- static void *orig_ret_addr;
+ struct kretprobe_instance *ri;

- /*
- * 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->task = current;
+ ri->ret_addr = (kprobe_opcode_t *) *sara;

- if ((ri = get_free_rp_inst(rp)) != NULL) {
- ri->rp = rp;
- ri->stack_addr = sara;
- ri->ret_addr = orig_ret_addr;
- add_rp_inst(ri);
/* Replace the return addr with trampoline addr */
*sara = (unsigned long) &kretprobe_trampoline;
- } else {
- rp->nmissed++;
- }
-}

-void arch_kprobe_flush_task(struct task_struct *tk)
-{
- struct kretprobe_instance *ri;
- while ((ri = get_rp_inst_tsk(tk)) != NULL) {
- *((unsigned long *)(ri->stack_addr)) =
- (unsigned long) ri->ret_addr;
- recycle_rp_inst(ri);
- }
+ add_rp_inst(ri);
+ } else {
+ rp->nmissed++;
+ }
}

/*
@@ -428,36 +403,58 @@ no_kprobe:
*/
int trampoline_probe_handler(struct kprobe *p, struct pt_regs *regs)
{
- struct task_struct *tsk;
- struct kretprobe_instance *ri;
- struct hlist_head *head;
- struct hlist_node *node;
- unsigned long *sara = (unsigned long *)regs->rsp - 1;
-
- tsk = arch_get_kprobe_task(sara);
- head = kretprobe_inst_table_head(tsk);
-
- hlist_for_each_entry(ri, node, head, hlist) {
- if (ri->stack_addr == sara && ri->rp) {
- if (ri->rp->handler)
- ri->rp->handler(ri, regs);
- }
- }
- return 0;
-}
+ struct kretprobe_instance *ri = NULL;
+ struct hlist_head *head;
+ struct hlist_node *node, *tmp;
+ unsigned long orig_ret_address = 0;

-void trampoline_post_handler(struct kprobe *p, struct pt_regs *regs,
- unsigned long flags)
-{
- struct kretprobe_instance *ri;
- /* RA already popped */
- unsigned long *sara = ((unsigned long *)regs->rsp) - 1;
+ head = kretprobe_inst_table_head(current);
+
+ /*
+ * It is possible to have multiple instances associated with a given
+ * task either because an multiple functions in the call path
+ * have a return probe installed on them, and/or more then one return
+ * return probe was registered for a target function.
+ *
+ * We can handle this because:
+ * - instances are always inserted at the head of the list
+ * - when multiple return probes are registered for the same
+ * function, the first instance's ret_addr will point to the
+ * real return address, and all the rest will point to
+ * kretprobe_trampoline
+ */
+ hlist_for_each_entry_safe(ri, node, tmp, head, hlist) {
+ if (ri->task != current)
+ /* another task is sharing our hash bucket */
+ continue;
+
+ if (ri->rp && ri->rp->handler)
+ ri->rp->handler(ri, regs);

- while ((ri = get_rp_inst(sara))) {
- regs->rip = (unsigned long)ri->ret_addr;
+ orig_ret_address = (unsigned long)ri->ret_addr;
recycle_rp_inst(ri);
+
+ if (orig_ret_address != (unsigned long) &kretprobe_trampoline)
+ /*
+ * This is the real return address. Any other
+ * instances associated with this task are for
+ * other calls deeper on the call stack
+ */
+ break;
}
- regs->eflags &= ~TF_MASK;
+
+ BUG_ON(!orig_ret_address);
+ regs->rip = orig_ret_address;
+
+ unlock_kprobes();
+ preempt_enable_no_resched();
+
+ /*
+ * By returning a non-zero value, we are telling
+ * kprobe_handler() that we have handled unlocking
+ * and re-enabling preemption.
+ */
+ return 1;
}

/*
@@ -550,8 +547,7 @@ int post_kprobe_handler(struct pt_regs *
current_kprobe->post_handler(current_kprobe, regs, 0);
}

- if (current_kprobe->post_handler != trampoline_post_handler)
- resume_execution(current_kprobe, regs);
+ resume_execution(current_kprobe, regs);
regs->eflags |= kprobe_saved_rflags;

/* Restore the original saved kprobes variables and continue. */
@@ -790,3 +786,13 @@ static void free_insn_slot(kprobe_opcode
}
}
}
+
+static struct kprobe trampoline_p = {
+ .addr = (kprobe_opcode_t *) &kretprobe_trampoline,
+ .pre_handler = trampoline_probe_handler
+};
+
+int __init arch_init(void)
+{
+ return register_kprobe(&trampoline_p);
+}
--
-
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