Use chain-loading for Linux nacl_helper

chrome/nacl/nacl_helper_bootstrap_linux.x

+/* Copyright (c) 2011 The Chromium Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ *
+ * This is a custom linker script used to build nacl_helper_bootstrap.
+ * It has a very special layout.  This script will only work with input
+ * that is kept extremely minimal.  If there are unexpected input sections
+ * not named here, the result will not be correct.
+ *
+ * We need to use a standalone loader program rather than just using a
+ * dynamically-linked program here because its entire address space will be
+ * taken over for the NaCl untrusted address space.  A normal program would
+ * cause dynamic linker data structures to point to its .dynamic section,
+ * which is no longer available after startup.
+ *
+ * We need this special layout (and the nacl_helper_bootstrap_munge_phdr
+ * step) because simply having bss space large enough to reserve the
+ * address space would cause the kernel loader to think we're using that
+ * much anonymous memory and refuse to execute the program on a machine
+ * with not much memory available.
+ */
+
+/*
+ * Set the entry point to the symbol called _start, which we define in assembly.
+ */
+ENTRY(_start)
+
+/*
+ * This is the address where the program text starts.
+ * We set this as low as we think we can get away with.
+ * The common settings for sysctl vm.mmap_min_addr range from 4k to 64k.
+ */
+TEXT_START = 0x10000;
+
+/*
+ * This is the top of the range we are trying to reserve, which is 1G
+ * for x86-32 and ARM.  For an x86-64 zero-based sandbox, this really
+ * needs to be 36G.
+ */
+RESERVE_TOP = 1 << 30;
+
+/*
+ * We specify the program headers we want explicitly, to get the layout
+ * exactly right and to give the "reserve" segment p_flags of zero, so
+ * that it gets mapped as PROT_NONE.
+ */
+PHDRS {
+  text PT_LOAD FILEHDR PHDRS;
+  reserve PT_LOAD FLAGS(0);
+  stack PT_GNU_STACK FLAGS(6);  /* RW, no E */
+}
+
+/*
+ * Now we lay out the sections across those segments.
+ */
+SECTIONS {
+  /*
+   * Here is the program itself.
+   */
+  .text TEXT_START + SIZEOF_HEADERS : {
+    *(.note.gnu.build-id)
+    *(.text*)
+    *(.rodata*)
+    *(.eh_frame*)
+  } :text
+  etext = .;
+
+  /*
+   * Now we move up to the next p_align increment, and place the dummy
+   * segment there.  The linker emits this segment with the p_vaddr and
+   * p_memsz we want, which reserves the address space.  But the linker
+   * gives it a p_filesz of zero.  We have to edit the phdr after link
+   * time to give it a p_filesz matching its p_memsz.  That way, the
+   * kernel doesn't think we are preallocating a huge amount of memory.
+   * It just maps it from the file, i.e. way off the end of the file,
+   * which is perfect for reserving the address space.
+   */
+  . = ALIGN(CONSTANT(COMMONPAGESIZE));
+  RESERVE_START = .;
+  .reserve : {
+    . = RESERVE_TOP - RESERVE_START;
+  } :reserve
+
+  /*
+   * These are empty input sections the linker generates.
+   * If we don't discard them, they pollute the flags in the output segment.
+   */
+  /DISCARD/ : {
+    *(.iplt)
+    *(.rel*)
+    *(.igot.plt)
+  }
+}