Fix use of `rb_profile_frames` start parameter

rb_profile_frames was always behaving as if the value given for the
start parameter was 0.

The reason for this was that it would check if (start > 0) { then
continue without updating the control frame pointer or anything other
than decrementing start.

[ruby-core:86147] [Bug #14607]

Co-authored-by: Dylan Thacker-Smith <[email protected]>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63265 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

2 files changed, 5 insertions, 2 deletions

diff --git a/test/-ext-/debug/test_profile_frames.rb b/test/-ext-/debug/test_profile_frames.rb
index 5ea506046e..672c9c4b5a 100644
--- a/test/-ext-/debug/test_profile_frames.rb
+++ b/test/-ext-/debug/test_profile_frames.rb
@@ -119,4 +119,8 @@ class TestProfileFrames < Test::Unit::TestCase
       a
     end;
   end
+
+  def test_start
+    assert_equal Bug::Debug.profile_frames(0, 10).tap(&:shift), Bug::Debug.profile_frames(1, 9)
+  end
 end
diff --git a/vm_backtrace.c b/vm_backtrace.c
index 59340292cf..3a13088855 100644
--- a/vm_backtrace.c
+++ b/vm_backtrace.c
@@ -1276,7 +1276,7 @@ rb_profile_frames(int start, int limit, VALUE *buff, int *lines)
     const rb_control_frame_t *cfp = ec->cfp, *end_cfp = RUBY_VM_END_CONTROL_FRAME(ec);
     const rb_callable_method_entry_t *cme;
 
-    for (i=0; i<limit && cfp != end_cfp;) {
+    for (i=0; i<limit && cfp != end_cfp; cfp = RUBY_VM_PREVIOUS_CONTROL_FRAME(cfp)) {
 	if (cfp->iseq && cfp->pc) {
 	    if (start > 0) {
 		start--;
@@ -1296,7 +1296,6 @@ rb_profile_frames(int start, int limit, VALUE *buff, int *lines)
 
 	    i++;
 	}
-	cfp = RUBY_VM_PREVIOUS_CONTROL_FRAME(cfp);
     }
 
     return i;