Use chain-loading for Linux nacl_helper

chrome/nacl/nacl_helper_bootstrap_munge_phdr.c

+/* 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 trivial program to edit an ELF file in place, making
+ * one crucial modification to a program header.  It's invoked:
+ *      bootstrap_phdr_hacker FILENAME SEGMENT_NUMBER
+ * where SEGMENT_NUMBER is the zero-origin index of the program header
+ * we'll touch.  This is a PT_LOAD with p_filesz of zero.  We change its
+ * p_filesz to match its p_memsz value.
+ */

[Brad Chen, 2011/08/29 23:04:57]

It may be that this should go into tools/ instead of chrome/nacl/. We should
find a chance to ask Brad Nelson or somebody else who might be more aware of
directory conventions.

+
+#define _FILE_OFFSET_BITS 64

[Mark Seaborn, 2011/08/29 22:26:50]

This needs a comment.  Is it really needed?

+#include <errno.h>
+#include <error.h>
+#include <fcntl.h>
+#include <gelf.h>
+#include <libelf.h>

[Mark Seaborn, 2011/08/29 22:26:50]

A new build-time dependency?  If you must depend on this, you should add it to
build/install-build-deps.sh.

However, the file modification is pretty trivial.  You could do this in Python
quite easily instead.

+#include <stdlib.h>
+#include <unistd.h>
+
+int main(int argc, char **argv) {
+  if (argc != 3)
+    error(1, 0, "Usage: %s FILENAME SEGMENT_NUMBER", argv[0]);
+
+  const char *const file = argv[1];
+  const int segment = atoi(argv[2]);
+
+  int fd = open(file, O_RDWR);
+  if (fd < 0)
+    error(2, errno, "Cannot open %s for read/write", file);
+
+  if (elf_version(EV_CURRENT) == EV_NONE)
+    error(2, 0, "elf_version: %s", elf_errmsg(-1));
+
+  Elf *elf = elf_begin(fd, ELF_C_RDWR, NULL);
+  if (elf == NULL)
+    error(2, 0, "elf_begin: %s", elf_errmsg(-1));
+
+  if (elf_flagelf(elf, ELF_C_SET, ELF_F_LAYOUT) == 0)
+    error(2, 0, "elf_flagelf: %s", elf_errmsg(-1));
+
+  GElf_Phdr phdr;
+  GElf_Phdr *ph = gelf_getphdr(elf, segment, &phdr);
+  if (ph == NULL)
+    error(2, 0, "gelf_getphdr: %s", elf_errmsg(-1));
+
+  if (ph->p_type != PT_LOAD)
+    error(3, 0, "Program header %d is %u, not PT_LOAD",
+          segment, (unsigned int) ph->p_type);
+  if (ph->p_filesz != 0)
+    error(3, 0, "Program header %d has nonzero p_filesz", segment);
+
+  ph->p_filesz = ph->p_memsz;
+  if (gelf_update_phdr(elf, segment, ph) == 0)
+    error(2, 0, "gelf_update_phdr: %s", elf_errmsg(-1));
+
+  if (elf_flagphdr(elf, ELF_C_SET, ELF_F_DIRTY) == 0)
+    error(2, 0, "elf_flagphdr: %s", elf_errmsg(-1));
+
+  if (elf_update(elf, ELF_C_WRITE) < 0)
+    error(2, 0, "elf_update: %s", elf_errmsg(-1));
+
+  close(fd);
+
+  return 0;
+}