sv_gets: use a SSize_t append character index

This used an I32, which could overflow when reading from a large
source.

https://www.perlmonks.org/?node_id=11161665

MANIFEST

@@ -5972,6 +5972,7 @@ t/bigmem/stack_over.t			Check handling of stack overflows with 32-bit MARK on 64
 t/bigmem/str.t				Test string primitives with large strings
 t/bigmem/subst.t			Test s/// with large strings
 t/bigmem/subst2.t			Test s//EXPR/e with large strings
+t/bigmem/sv_gets.t			Test reading a lot with sv_gets()
 t/bigmem/vec.t				Check vec() handles large offsets
 t/charset_tools.pl			To aid in portable testing across platforms with different character sets
 t/class/accessor.t			See if accessor methods work

embed.fnc

@@ -3175,7 +3175,7 @@ ATdpx	|SV *	|sv_get_backrefs|NN SV * const sv
 Adip	|void	|SvGETMAGIC	|NN SV *sv
 Adp	|char * |sv_gets	|NN SV * const sv			\
 				|NN PerlIO * const fp			\
-				|I32 append
+				|SSize_t append
 Cdp	|char * |sv_grow	|NN SV * const sv			\
 				|STRLEN newlen
 Cdp	|char * |sv_grow_fresh	|NN SV * const sv			\

sv.c

@@ -8679,7 +8679,7 @@ Perl_sv_collxfrm_flags(pTHX_ SV *const sv, STRLEN *const nxp, const I32 flags)
 #endif /* USE_LOCALE_COLLATE */
 
 static char *
-S_sv_gets_append_to_utf8(pTHX_ SV *const sv, PerlIO *const fp, I32 append)
+S_sv_gets_append_to_utf8(pTHX_ SV *const sv, PerlIO *const fp, SSize_t append)
 {
     SV * const tsv = newSV_type(SVt_NULL);
     ENTER;
@@ -8693,7 +8693,7 @@ S_sv_gets_append_to_utf8(pTHX_ SV *const sv, PerlIO *const fp, I32 append)
 }
 
 static char *
-S_sv_gets_read_record(pTHX_ SV *const sv, PerlIO *const fp, I32 append)
+S_sv_gets_read_record(pTHX_ SV *const sv, PerlIO *const fp, SSize_t append)
 {
     SSize_t bytesread;
     const STRLEN recsize = SvUV(SvRV(PL_rs)); /* RsRECORD() guarantees > 0. */
@@ -8821,7 +8821,7 @@ in the SV (typically, C<SvCUR(sv)> is a suitable choice).
 */
 
 char *
-Perl_sv_gets(pTHX_ SV *const sv, PerlIO *const fp, I32 append)
+Perl_sv_gets(pTHX_ SV *const sv, PerlIO *const fp, SSize_t append)
 {
     const char *rsptr;
     STRLEN rslen;
@@ -8847,7 +8847,8 @@ Perl_sv_gets(pTHX_ SV *const sv, PerlIO *const fp, I32 append)
         if (PerlIO_isutf8(fp)) {
             if (!SvUTF8(sv)) {
                 sv_utf8_upgrade_nomg(sv);
-                sv_pos_u2b(sv,&append,0);
+                append = (SSize_t)sv_pos_u2b_flags(sv, (STRLEN)append, NULL,
+                                                   SV_GMAGIC|SV_CONST_RETURN);
             }
         } else if (SvUTF8(sv)) {
             return S_sv_gets_append_to_utf8(aTHX_ sv, fp, append);
@@ -8961,11 +8962,11 @@ Perl_sv_gets(pTHX_ SV *const sv, PerlIO *const fp, I32 append)
     STDCHAR *ptr;       /* pointer into fp's read-ahead buffer */
     STRLEN bpx;         /* length of the data in the target sv
                            used to fix pointers after a SvGROW */
-    I32 shortbuffered;  /* If the pv buffer is shorter than the amount
-                           of data left in the read-ahead buffer.
-                           If 0 then the pv buffer can hold the full
-                           amount left, otherwise this is the amount it
-                           can hold. */
+    SSize_t shortbuffered;/* If the pv buffer is shorter than the amount
+                             of data left in the read-ahead buffer.
+                             If 0 then the pv buffer can hold the full
+                             amount left, otherwise this is the amount it
+                             can hold. */
 
     /* Here is some breathtakingly efficient cheating */
 
@@ -9028,11 +9029,11 @@ Perl_sv_gets(pTHX_ SV *const sv, PerlIO *const fp, I32 append)
     cnt = PerlIO_get_cnt(fp);
 
     /* make sure we have the room */
-    if ((I32)(SvLEN(sv) - append) <= cnt + 1) {
+    if ((SSize_t)(SvLEN(sv) - append) <= cnt + 1) {
         /* Not room for all of it
            if we are looking for a separator and room for some
          */
-        if (rslen && cnt > 80 && (I32)SvLEN(sv) > append) {
+        if (rslen && cnt > 80 && SvLEN(sv) > (STRLEN)append) {
             /* just process what we have room for */
             shortbuffered = cnt - SvLEN(sv) + append + 1;
             cnt -= shortbuffered;

t/bigmem/sv_gets.t

@@ -0,0 +1,21 @@
+#!perl
+BEGIN {
+    chdir 't' if -d 't';
+    @INC = "../lib";
+}
+
+use strict;
+require './test.pl';
+use Config qw(%Config);
+
+# 2 for the child, 2 for the parent
+$ENV{PERL_TEST_MEMORY} >= 4
+    or skip_all("Need ~4Gb for this test");
+$Config{ptrsize} >= 8
+    or skip_all("Need 64-bit pointers for this test");
+
+# https://www.perlmonks.org/?node_id=11161665
+my $x = `$^X -e "print q/x/ x 0x80000000"`;
+is(length $x, 0x80000000, "check entire input read");
+
+done_testing();