[perl #120998] avoid caller() crashing on eval '' stack frames

Starting from v5.17.3-150-g19bcb54e caller() on an eval frame would
end up calling Perl_sv_grow() with newlen = 0xFFFFFFFF on 32-bit
systems.

This eventually started segfaulting with v5.19.0-442-gcbcb2a1 which
added code to round up allocations to the nearest 0x100, setting
newlen to 0, faulting when sv_setpvn() attempted to copy its source
string into the zero space provided.

1 files changed, 10 insertions, 3 deletions

diff --git a/pp_ctl.c b/pp_ctl.c
index e13e4504de..380a7fe7f2 100644
--- a/pp_ctl.c
+++ b/pp_ctl.c
@@ -1847,9 +1847,16 @@ PP(pp_caller)
     if (CxTYPE(cx) == CXt_EVAL) {
 	/* eval STRING */
 	if (CxOLD_OP_TYPE(cx) == OP_ENTEREVAL) {
-	    PUSHs(newSVpvn_flags(SvPVX(cx->blk_eval.cur_text),
-				 SvCUR(cx->blk_eval.cur_text)-2,
-				 SvUTF8(cx->blk_eval.cur_text)|SVs_TEMP));
+            SV *cur_text = cx->blk_eval.cur_text;
+            if (SvCUR(cur_text) >= 2) {
+                PUSHs(newSVpvn_flags(SvPVX(cur_text), SvCUR(cur_text)-2,
+                                     SvUTF8(cur_text)|SVs_TEMP));
+            }
+            else {
+                /* I think this is will always be "", but be sure */
+                PUSHs(sv_2mortal(newSVsv(cur_text)));
+            }
+
 	    PUSHs(&PL_sv_no);
 	}
 	/* require */