mmap64: always use unsigned 64bit offsets

The current code will sometimes cast the offset to an unsigned long.
On 32bit systems, that doesn't make much sense since we always know
that the off64_t type is 64bits.  So drop the ifdef logic, cast it
to an unsigned 64bit value, and then do the shift.

If the system is 32bits, and the address is still too large, then
there's really nothing we could do about it anyways, so the extra
ifdef logic wasn't helping.

URL: https://bugs.busybox.net/show_bug.cgi?id=3853
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

libc/sysdeps/linux/common/mmap64.c

@@ -59,13 +59,14 @@ __ptr_t mmap64(__ptr_t addr, size_t len, int prot, int flags, int fd, __off64_t
 		return MAP_FAILED;
 	}
 
-#  ifdef __USE_FILE_OFFSET64
-	return __syscall_mmap2(addr, len, prot, flags,
-	                       fd, ((__u_quad_t) offset >> MMAP2_PAGE_SHIFT));
-#  else
-	return __syscall_mmap2(addr, len, prot, flags,
-	                       fd, ((__u_long) offset >> MMAP2_PAGE_SHIFT));
-#  endif
+	/*
+	 * We know __off64_t is always a signed 64-bit type, but need things
+	 * to be unsigned before doing the shift.  If it isn't, we might
+	 * sign extend things and pass in the wrong value.  So cast it to
+	 * an unsigned 64-bit value before doing the shift.
+	 */
+	return __syscall_mmap2(addr, len, prot, flags, fd,
+	                       ((uint64_t)offset >> MMAP2_PAGE_SHIFT));
 }
 
 # endif