add PL_curstackinfo->si_stack_hwm

On debugging builds only, add a mechanism for checking pp function calls
for insufficient stack extending. It works by:

* make the runops loop set a high-water-mark (HWM) variable equal to
  PL_stack_sp just before calling each pp function;

* make EXTEND() etc update this HWM;

* on return from the pp function, panic if PL_stack_sp is > HWM.

This detects whether pp functions are pushing more items onto the stack
than they are requesting space for.

There's a possibility of false positives if the code is doing weird stuff
like direct manipulation of stacks via PL_curstack, SWITCHSTACK() etc.

It's also possible that one pp function "knows" that a previous pp
function will have already grown the stack enough. Currently the only
place in core that seems to do this is pp_enteriter, which allocates 1
stack slot so that pp_iter doesn't have to check each time it returns
&PL_sv_yes/no. To accommodate this, the new macro EXTEND_SKIP() has been
added, that tells perl that it's safely skipping an EXTEND() here.

1 files changed, 18 insertions, 1 deletions

diff --git a/dump.c b/dump.c
index 7cdebfe875..b95092956f 100644
--- a/dump.c
+++ b/dump.c
@@ -2413,16 +2413,30 @@ Perl_sv_dump(pTHX_ SV *sv)
 int
 Perl_runops_debug(pTHX)
 {
+#ifdef DEBUGGING && !defined DEBUGGING_RE_ONLY
+    SSize_t orig_stack_hwm = PL_curstackinfo->si_stack_hwm;
+
+    PL_curstackinfo->si_stack_hwm = PL_stack_sp - PL_stack_base;
+#endif
+
     if (!PL_op) {
 	Perl_ck_warner_d(aTHX_ packWARN(WARN_DEBUGGING), "NULL OP IN RUN");
 	return 0;
     }
-
     DEBUG_l(Perl_deb(aTHX_ "Entering new RUNOPS level\n"));
     do {
 #ifdef PERL_TRACE_OPS
         ++PL_op_exec_cnt[PL_op->op_type];
 #endif
+#ifdef DEBUGGING && !defined DEBUGGING_RE_ONLY
+        if (PL_curstackinfo->si_stack_hwm < PL_stack_sp - PL_stack_base)
+            Perl_croak_nocontext(
+                "panic: previous op failed to extend arg stack: "
+                "base=%p, sp=%p, hwm=%p\n",
+                    PL_stack_base, PL_stack_sp,
+                    PL_stack_base + PL_curstackinfo->si_stack_hwm);
+        PL_curstackinfo->si_stack_hwm = PL_stack_sp - PL_stack_base;
+#endif
 	if (PL_debug) {
             ENTER;
             SAVETMPS;
@@ -2452,6 +2466,9 @@ Perl_runops_debug(pTHX)
     DEBUG_l(Perl_deb(aTHX_ "leaving RUNOPS level\n"));
     PERL_ASYNC_CHECK();
 
+#ifdef DEBUGGING && !defined DEBUGGING_RE_ONLY
+    PL_curstackinfo->si_stack_hwm = orig_stack_hwm;
+#endif
     TAINT_NOT;
     return 0;
 }