Fix GH-15980: Signed integer overflow in main/streams/streams.c

We need to avoid signed integer overflows which are undefined behavior.
We catch that, and set `offset` to `ZEND_LONG_MAX` (which is also the
largest value of `zend_off_t` on all platforms).  Of course, after such
a seek a stream is no longer readable, but that matches the current
behavior for offsets near `ZEND_LONG_MAX`.

Closes GH-15989.

Author: cmb69

NEWS

@@ -29,6 +29,8 @@ PHP                                                                        NEWS
 - Streams:
   . Fixed bugs GH-15908 and GH-15026 (leak / assertion failure in streams.c).
     (nielsdos)
+  . Fixed bug GH-15980 (Signed integer overflow in main/streams/streams.c).
+    (cmb)
 
 - TSRM:
   . Prevent closing of unrelated handles. (cmb)

ext/standard/tests/streams/gh15980.phpt

@@ -0,0 +1,11 @@
+--TEST--
+GH-15980 (Signed integer overflow in main/streams/streams.c)
+--FILE--
+<?php
+$s = fopen(__FILE__, "r");
+fseek($s, 1);
+fseek($s, PHP_INT_MAX, SEEK_CUR);
+var_dump(ftell($s) > 1);
+?>
+--EXPECT--
+bool(true)

main/streams/streams.c

@@ -1354,8 +1354,13 @@ PHPAPI int _php_stream_seek(php_stream *stream, zend_off_t offset, int whence)
 
 		switch(whence) {
 			case SEEK_CUR:
-				offset = stream->position + offset;
-				whence = SEEK_SET;
+				ZEND_ASSERT(stream->position >= 0);
+				if (UNEXPECTED(offset > ZEND_LONG_MAX - stream->position)) {
+					offset = ZEND_LONG_MAX;
+				} else {
+					offset = stream->position + offset;
+				}
+ 				whence = SEEK_SET;
 				break;
 		}
 		ret = stream->ops->seek(stream, offset, whence, &stream->position);