uprobes: Change handle_trampoline() to find the next chain beforehand

No functional changes, preparation.

Add the new helper, find_next_ret_chain(), which finds the first
!chained entry and returns its ->next. Yes, it is suboptimal. We
probably want to turn ->chained into ->start_of_this_chain
pointer and avoid another loop. But this needs the boring
changes in dup_utask(), so lets do this later.

Change the main loop in handle_trampoline() to unwind the stack
until ri is equal to the pointer returned by this new helper.

Tested-by: Pratyush Anand <panand@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Acked-by: Anton Arapov <arapov@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20150721134013.GA4755@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 4c941fe..98e4d97 100644 (file)
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -1766,11 +1766,22 @@ handle_uretprobe_chain(struct return_instance *ri, struct pt_regs *regs)
        up_read(&uprobe->register_rwsem);
 }
 
+static struct return_instance *find_next_ret_chain(struct return_instance *ri)
+{
+       bool chained;
+
+       do {
+               chained = ri->chained;
+               ri = ri->next;  /* can't be NULL if chained */
+       } while (chained);
+
+       return ri;
+}
+
 static void handle_trampoline(struct pt_regs *regs)
 {
        struct uprobe_task *utask;
-       struct return_instance *ri;
-       bool chained;
+       struct return_instance *ri, *next;
 
        utask = current->utask;
        if (!utask)
@@ -1780,24 +1791,18 @@ static void handle_trampoline(struct pt_regs *regs)
        if (!ri)
                goto sigill;
 
+       next = find_next_ret_chain(ri);
        /*
         * TODO: we should throw out return_instance's invalidated by
         * longjmp(), currently we assume that the probed function always
         * returns.
         */
        instruction_pointer_set(regs, ri->orig_ret_vaddr);
-
-       for (;;) {
+       do {
                handle_uretprobe_chain(ri, regs);
-
-               chained = ri->chained;
                ri = free_ret_instance(ri);
                utask->depth--;
-
-               if (!chained)
-                       break;
-               BUG_ON(!ri);
-       }
+       } while (ri != next);
 
        utask->return_instances = ri;
        return;