From a08fa6fd157fd0d61da7f20f07b939fbc302c2c6 Mon Sep 17 00:00:00 2001
From: Hugo van der Sanden <hv@crypt.org>
Date: Wed, 5 Oct 2016 12:56:05 +0100
Subject: [PATCH] [perl #129377] don't read past start of string for unmatched
 backref
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Ported to 5.24.1:

commit 2dfc11ec3af312f4fa3eb244077c79dbb5fc2d85
Author: Hugo van der Sanden <hv@crypt.org>
Date:   Wed Oct 5 12:56:05 2016 +0100

    [perl #129377] don't read past start of string for unmatched backref

    We can have (start, end) == (0, -1) for an unmatched backref, we must
    check for that.

Signed-off-by: Petr Písař <ppisar@redhat.com>
---
 regexec.c  | 10 ++++++----
 t/re/pat.t | 16 +++++++++++++++-
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/regexec.c b/regexec.c
index a5d5db4..a7bc0c3 100644
--- a/regexec.c
+++ b/regexec.c
@@ -5179,6 +5179,7 @@ S_regmatch(pTHX_ regmatch_info *reginfo, char *startpos, regnode *prog)
     regnode *next;
     U32 n = 0;	/* general value; init to avoid compiler warning */
     SSize_t ln = 0; /* len or last;  init to avoid compiler warning */
+    SSize_t endref = 0; /* offset of end of backref when ln is start */
     char *locinput = startpos;
     char *pushinput; /* where to continue after a PUSH */
     I32 nextchr;   /* is always set to UCHARAT(locinput), or -1 at EOS */
@@ -6489,10 +6490,11 @@ S_regmatch(pTHX_ regmatch_info *reginfo, char *startpos, regnode *prog)
 
 	  do_nref_ref_common:
 	    ln = rex->offs[n].start;
+	    endref = rex->offs[n].end;
 	    reginfo->poscache_iter = reginfo->poscache_maxiter; /* Void cache */
-	    if (rex->lastparen < n || ln == -1)
+	    if (rex->lastparen < n || ln == -1 || endref == -1)
 		sayNO;			/* Do not match unless seen CLOSEn. */
-	    if (ln == rex->offs[n].end)
+	    if (ln == endref)
 		break;
 
 	    s = reginfo->strbeg + ln;
@@ -6506,7 +6508,7 @@ S_regmatch(pTHX_ regmatch_info *reginfo, char *startpos, regnode *prog)
                     * not going off the end given by reginfo->strend, and
                     * returns in <limit> upon success, how much of the
                     * current input was matched */
-		if (! foldEQ_utf8_flags(s, NULL, rex->offs[n].end - ln, utf8_target,
+		if (! foldEQ_utf8_flags(s, NULL, endref - ln, utf8_target,
 				    locinput, &limit, 0, utf8_target, utf8_fold_flags))
 		{
 		    sayNO;
@@ -6521,7 +6523,7 @@ S_regmatch(pTHX_ regmatch_info *reginfo, char *startpos, regnode *prog)
 		(type == REF ||
 		 UCHARAT(s) != fold_array[nextchr]))
 		sayNO;
-	    ln = rex->offs[n].end - ln;
+	    ln = endref - ln;
 	    if (locinput + ln > reginfo->strend)
 		sayNO;
 	    if (ln > 1 && (type == REF
diff --git a/t/re/pat.t b/t/re/pat.t
index 4aa77cf..749edd0 100644
--- a/t/re/pat.t
+++ b/t/re/pat.t
@@ -23,7 +23,7 @@ BEGIN {
     skip_all_without_unicode_tables();
 }
 
-plan tests => 791;  # Update this when adding/deleting tests.
+plan tests => 792;  # Update this when adding/deleting tests.
 
 run_tests() unless caller;
 
@@ -1765,6 +1765,20 @@ EOP
             utf8::upgrade($str);
             ok( $str =~ m{^(a|a\x{e4})$}, "fix [perl #129950] - utf8 case" );
         }
+    {
+	# [perl #129377] backref to an unmatched capture should not cause
+	# reading before start of string.
+	SKIP: {
+	    skip "no re-debug under miniperl" if is_miniperl;
+	    my $prog = <<'EOP';
+use re qw(Debug EXECUTE);
+"x" =~ m{ () y | () \1 }x;
+EOP
+	    fresh_perl_like($prog, qr{
+		\A (?! .* ^ \s+ - )
+	    }msx, { stderr => 1 }, "Offsets in debug output are not negative");
+	}
+    }
 } # End of sub run_tests
 
 1;
-- 
2.7.4