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Subject[patch 5/5] [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. I also included a fix to the problem that Ananth pointed out.)

The following is a patch provided by Ananth Mavinakayanahalli that implements
the new PPC64 specific parts of the new function return probe design.

NOTE: Since getting Ananth's patch, I changed trampoline_probe_handler()
to consume each of the outstanding return probem instances (feedback
on my original RFC after Ananth cut a patch), and also added the
arch_init() function (adding arch specific initialization.) I have
cross compiled but have not testing this on a PPC64 machine.

--rusty

arch/ppc64/kernel/kprobes.c | 98 ++++++++++++++++++++++++++++++++++++++++++++
arch/ppc64/kernel/process.c | 4 +
include/asm-ppc64/kprobes.h | 3 +
3 files changed, 105 insertions(+)
Index: linux-2.6.12-rc6-mm1/arch/ppc64/kernel/kprobes.c
===================================================================
--- linux-2.6.12-rc6-mm1.orig/arch/ppc64/kernel/kprobes.c
+++ linux-2.6.12-rc6-mm1/arch/ppc64/kernel/kprobes.c
@@ -100,6 +100,23 @@ static inline void restore_previous_kpro
kprobe_saved_msr = kprobe_saved_msr_prev;
}

+void arch_prepare_kretprobe(struct kretprobe *rp, struct pt_regs *regs)
+{
+ struct kretprobe_instance *ri;
+
+ if ((ri = get_free_rp_inst(rp)) != NULL) {
+ ri->rp = rp;
+ ri->task = current;
+ ri->ret_addr = (kprobe_opcode_t *)regs->link;
+
+ /* Replace the return addr with trampoline addr */
+ regs->link = (unsigned long)kretprobe_trampoline;
+ add_rp_inst(ri);
+ } else {
+ rp->nmissed++;
+ }
+}
+
static inline int kprobe_handler(struct pt_regs *regs)
{
struct kprobe *p;
@@ -190,6 +207,77 @@ no_kprobe:
}

/*
+ * Function return probe trampoline:
+ * - init_kprobes() establishes a probepoint here
+ * - When the probed function returns, this probe
+ * causes the handlers to fire
+ */
+void kretprobe_trampoline_holder(void)
+{
+ asm volatile(".global kretprobe_trampoline\n"
+ "kretprobe_trampoline:\n"
+ "nop\n");
+}
+
+/*
+ * Called when the probe at kretprobe trampoline is hit
+ */
+int trampoline_probe_handler(struct kprobe *p, struct pt_regs *regs)
+{
+ struct kretprobe_instance *ri = NULL;
+ struct hlist_head *head;
+ struct hlist_node *node, *tmp;
+ unsigned long orig_ret_address = 0;
+
+ 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);
+
+ 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;
+ }
+
+ BUG_ON(!orig_ret_address);
+ regs->nip = orig_ret_address;
+
+ unlock_kprobes();
+
+ /*
+ * By returning a non-zero value, we are telling
+ * kprobe_handler() that we have handled unlocking
+ * and re-enabling preemption.
+ */
+ return 1;
+}
+
+/*
* Called after single-stepping. p->addr is the address of the
* instruction whose first byte has been replaced by the "breakpoint"
* instruction. To avoid the SMP problems that can occur when we
@@ -329,3 +417,13 @@ int longjmp_break_handler(struct kprobe
memcpy(regs, &jprobe_saved_regs, sizeof(struct pt_regs));
return 1;
}
+
+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);
+}
Index: linux-2.6.12-rc6-mm1/arch/ppc64/kernel/process.c
===================================================================
--- linux-2.6.12-rc6-mm1.orig/arch/ppc64/kernel/process.c
+++ linux-2.6.12-rc6-mm1/arch/ppc64/kernel/process.c
@@ -37,6 +37,7 @@
#include <linux/interrupt.h>
#include <linux/utsname.h>
#include <linux/perfctr.h>
+#include <linux/kprobes.h>

#include <asm/pgtable.h>
#include <asm/uaccess.h>
@@ -310,6 +311,8 @@ void show_regs(struct pt_regs * regs)

void exit_thread(void)
{
+ kprobe_flush_task(current);
+
#ifndef CONFIG_SMP
if (last_task_used_math == current)
last_task_used_math = NULL;
@@ -325,6 +328,7 @@ void flush_thread(void)
{
struct thread_info *t = current_thread_info();

+ kprobe_flush_task(current);
if (t->flags & _TIF_ABI_PENDING)
t->flags ^= (_TIF_ABI_PENDING | _TIF_32BIT);

Index: linux-2.6.12-rc6-mm1/include/asm-ppc64/kprobes.h
===================================================================
--- linux-2.6.12-rc6-mm1.orig/include/asm-ppc64/kprobes.h
+++ linux-2.6.12-rc6-mm1/include/asm-ppc64/kprobes.h
@@ -42,6 +42,9 @@ typedef unsigned int kprobe_opcode_t;

#define JPROBE_ENTRY(pentry) (kprobe_opcode_t *)((func_descr_t *)pentry)

+#define ARCH_SUPPORTS_KRETPROBES
+void kretprobe_trampoline(void);
+
/* Architecture specific copy of original instruction */
struct arch_specific_insn {
/* copy of original instruction */
--
-
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