[PATCH 09/10] uprobes: Change prepare_uretprobe() to (try to) flush the dead frames

[Oleg Nesterov <oleg@redhat.com>]

Change prepare_uretprobe() to flush the !arch_uretprobe_is_alive()
return_instance's. This is not needed correctness-wise, but can help
to avoid the failure caused by MAX_URETPROBE_DEPTH.

Note: in this case arch_uretprobe_is_alive() can be false positive,
the stack can grow after longjmp(). Unfortunately, the kernel can't
100% solve this problem, but see the next patch.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/events/uprobes.c |   13 +++++++++++++
 1 files changed, 13 insertions(+), 0 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 0dd7ff7..b6433fb 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1522,6 +1522,16 @@ static unsigned long get_trampoline_vaddr(void)
 	return trampoline_vaddr;
 }
 
+static void cleanup_return_instances(struct uprobe_task *utask, struct pt_regs *regs)
+{
+	struct return_instance *ri = utask->return_instances;
+	while (ri && !arch_uretprobe_is_alive(&ri->auret, regs)) {
+		ri = free_ret_instance(ri);
+		utask->depth--;
+	}
+	utask->return_instances = ri;
+}
+
 static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
 {
 	struct return_instance *ri;
@@ -1576,6 +1586,9 @@ static void prepare_uretprobe(struct uprobe *uprobe, struct pt_regs *regs)
 	ri->orig_ret_vaddr = orig_ret_vaddr;
 	ri->chained = chained;
 
+	if (utask->depth) /* drop the entries invalidated by longjmp() */
+		cleanup_return_instances(utask, regs);
+
 	utask->depth++;
 	ri->next = utask->return_instances;
 	utask->return_instances = ri;
-- 
1.5.5.1