don't call Perl_fbm_instr() with negative length

RT #131575

re_intuit_start() could calculate a maximum end position less than the
current start position. This used to get rejected by fbm_intr(), until
v5.23.3-110-g147f21b, which made fbm_intr() faster and removed unnecessary
checks.

This commits fixes re_intuit_start(), and adds an assert to  fbm_intr().

Author: iabyn

regexec.c

@@ -126,13 +126,16 @@ static const char* const non_utf8_target_but_utf8_required
                     (U8*)(off >= 0 ? reginfo->strend : reginfo->strbeg)) \
 	    : (U8*)(pos + off))
 
-#define HOPBACKc(pos, off) \
-	(char*)(reginfo->is_utf8_target \
-	    ? reghopmaybe3((U8*)pos, (SSize_t)0-off, (U8*)(reginfo->strbeg)) \
-	    : (pos - off >= reginfo->strbeg)	\
-		? (U8*)pos - off		\
+/* like HOPMAYBE3 but backwards. lim must be +ve. Returns NULL on overshoot */
+#define HOPBACK3(pos, off, lim) \
+	(reginfo->is_utf8_target                          \
+	    ? reghopmaybe3((U8*)pos, (SSize_t)0-off, (U8*)(lim)) \
+	    : (pos - off >= lim)	                         \
+		? (U8*)pos - off		                 \
 		: NULL)
 
+#define HOPBACKc(pos, off) ((char*)HOPBACK3(pos, off, reginfo->strbeg))
+
 #define HOP3(pos,off,lim) (reginfo->is_utf8_target  ? reghop3((U8*)(pos), off, (U8*)(lim)) : (U8*)(pos + off))
 #define HOP3c(pos,off,lim) ((char*)HOP3(pos,off,lim))
 
@@ -884,7 +887,9 @@ Perl_re_intuit_start(pTHX_
                 (IV)prog->check_end_shift);
         });
 
-        end_point = HOP3(strend, -end_shift, strbeg);
+        end_point = HOPBACK3(strend, end_shift, rx_origin);
+        if (!end_point)
+            goto fail_finish;
         start_point = HOPMAYBE3(rx_origin, start_shift, end_point);
         if (!start_point)
             goto fail_finish;

t/re/pat.t

@@ -23,7 +23,7 @@ BEGIN {
     skip_all('no re module') unless defined &DynaLoader::boot_DynaLoader;
     skip_all_without_unicode_tables();
 
-plan tests => 836;  # Update this when adding/deleting tests.
+plan tests => 837;  # Update this when adding/deleting tests.
 
 run_tests() unless caller;
 
@@ -1906,6 +1906,17 @@ EOP
         # [perl #129281] buffer write overflow, detected by ASAN, valgrind
         fresh_perl_is('/0(?0)|^*0(?0)|^*(^*())0|/', '', {}, "don't bump whilem_c too much");
     }
+
+    {
+        # RT #131575 intuit skipping back from the end to find the highest
+        # possible start point, was potentially hopping back beyond pos()
+        # and crashing by calling fbm_instr with a negative length
+
+        my $text = "=t=\x{5000}";
+        pos($text) = 3;
+        ok(scalar($text !~ m{(~*=[a-z]=)}g), "RT #131575");
+    }
+
 } # End of sub run_tests
 
 1;

util.c

@@ -816,6 +816,8 @@ Perl_fbm_instr(pTHX_ unsigned char *big, unsigned char *bigend, SV *littlestr, U
 
     PERL_ARGS_ASSERT_FBM_INSTR;
 
+    assert(bigend >= big);
+
     if ((STRLEN)(bigend - big) < littlelen) {
 	if (     tail
 	     && ((STRLEN)(bigend - big) == littlelen - 1)