Use chain-loading for Linux nacl_helper

chrome/nacl.gypi

Index: chrome/nacl.gypi
diff --git a/chrome/nacl.gypi b/chrome/nacl.gypi
index abfab38f0a8b9d44794042964f999cbd131ff71d..42c0035bca71340879ddf62d693453a9cb33d077 100644
--- a/chrome/nacl.gypi
+++ b/chrome/nacl.gypi
@@ -183,9 +183,7 @@
     ['OS=="linux"', {
       'targets': [
         {
-          'target_name': 'nacl_helper.so',
-          # 'executable' will be overridden below when we add the -shared
-          # flag; here it prevents gyp from using the --whole-archive flag
+          'target_name': 'nacl_helper',
           'type': 'executable',
           'include_dirs': [
             '..',
@@ -194,7 +192,7 @@
             'nacl',
           ],
           'sources': [
-            '../chrome/nacl/nacl_helper_linux.cc',
+            'nacl/nacl_helper_linux.cc',
           ],
           'conditions': [
             ['toolkit_uses_gtk == 1', {
@@ -203,36 +201,108 @@
               ],
             }],
           ],
+          'cflags': ['-fPIE'],
           'link_settings': {
-            # NOTE: '-shared' overrides 'executable' above
-            'ldflags': ['-shared',
-                        '-Wl,--version-script=chrome/nacl/nacl_helper_exports.txt',
-                      ],
+            'ldflags': ['-pie'],
           },
         },
         {
-          'target_name': 'nacl_helper_bootstrap',
+          'target_name': 'nacl_helper_bootstrap_munge_phdr',
           'type': 'executable',
-          'dependencies': [
-            'nacl_helper.so',
+          'toolsets': ['host'],
+          'sources': [
+            'nacl/nacl_helper_bootstrap_munge_phdr.c',
+          ],
+          'libraries': [
+            '-lelf',
+          ],
+          # This is an ugly kludge because gyp doesn't actually treat
+          # host_arch=x64 target_arch=ia32 as proper cross compilation.
+          # It still wants to compile the "host" program with -m32 et
+          # al.  Though a program built that way can indeed run on the
+          # x86-64 host, we cannot reliably build this program on such a
+          # host because Ubuntu does not provide the full suite of
+          # x86-32 libraries in packages that can be installed on an
+          # x86-64 host; in particular, libelf is missing.  So here we
+          # use the hack of eliding all the -m* flags from the
+          # compilation lines, getting the command close to what they
+          # would be if gyp were to really build properly for the host.
+          # TODO(bradnelson): Clean up with proper cross support.
+          'conditions': [
+            ['host_arch=="x64"', {
+              'cflags/': [['exclude', '-m.*']],
+              'ldflags/': [['exclude', '-m.*']],
+            }],
+          ],
+        },
+        {
+          'target_name': 'nacl_helper_bootstrap_raw',
+          'type': 'executable',
+          'include_dirs': [
+            '..',
           ],
           'sources': [
-            '../chrome/nacl/nacl_helper_bootstrap_linux.c',
+            'nacl/nacl_helper_bootstrap_linux.c',
+            # We list the linker script here for documentation purposes.
+            # But even this doesn't make gyp treat it as a dependency,
+            # so incremental builds won't relink when the script changes.
+            # TODO(bradnelson): Fix the dependency handling.
+            'nacl/nacl_helper_bootstrap_linux.x',
+          ],
+          'cflags': [
+            # The tiny standalone bootstrap program is incompatible with
+            # -fstack-protector, which might be on by default.  That switch
+            # requires using the standard libc startup code, which we do not.
+            '-fno-stack-protector',
+            # We don't want to compile it PIC (or its cousin PIE), because
+            # it goes at an absolute address anyway, and because any kind
+            # of PIC complicates life for the x86-32 assembly code.  We
+            # append -fno-* flags here instead of using a 'cflags!' stanza
+            # to remove -f* flags, just in case some system's compiler
+            # defaults to using PIC for everything.
+            '-fno-pic', '-fno-PIC',
+            '-fno-pie', '-fno-PIE',
           ],
-          # TODO(bradchen): Delete the -B argument when Gold supports
-          # -Ttext properly. Until then use ld.bfd.
           'link_settings': {
-            'ldflags': ['-B', 'tools/ld_bfd',
-                        # Force text segment at 0x10000 (64KB)
-                        # The max-page-size option is needed on x86-64 linux
-                        # where 4K pages are not the default in the BFD linker.
-                        '-Wl,-Ttext-segment,10000,-z,max-page-size=0x1000',
-                        # reference nacl_helper as a shared library
-                        '<(PRODUCT_DIR)/nacl_helper.so',
-                        '-Wl,-rpath,<(SHARED_LIB_DIR)',
-                      ],
+            'ldflags': [
+              # TODO(bradchen): Delete the -B argument when Gold is verified
+              # to produce good results with our custom linker script.
+              # Until then use ld.bfd.
+              '-B', 'tools/ld_bfd',
+              # This programs is (almost) entirely standalone.  It has
+              # its own startup code, so no crt1.o for it.  It is
+              # statically linked, and on x86 it actually does not use
+              # libc at all.  However, on ARM it needs a few (safe)
+              # things from libc, so we don't use '-nostdlib' here.
+              '-static', '-nostartfiles',
+              # Link with our custom linker script to get out special layout.
+              # TODO(bradnelson): Use some <(foo) instead of chrome/ here.
+              '-Wl,--script=chrome/nacl/nacl_helper_bootstrap_linux.x',
+              # On x86-64, the default page size with some
+              # linkers is 2M rather than the real Linux page
+              # size of 4K.  A larger page size is incompatible
+              # with our custom linker script's special layout.
+              '-Wl,-z,max-page-size=0x1000',
+            ],
           },
         },
+        {
+          'target_name': 'nacl_helper_bootstrap',
+          'dependencies': [
+            'nacl_helper_bootstrap_raw',
+            'nacl_helper_bootstrap_munge_phdr#host',
+          ],
+          'type': 'none',
+          'actions': [{
+              'action_name': 'munge_phdr',
+              'inputs': ['nacl/nacl_helper_bootstrap_munge_phdr.py',
+                         '<(PRODUCT_DIR)/nacl_helper_bootstrap_munge_phdr',
+                         '<(PRODUCT_DIR)/nacl_helper_bootstrap_raw'],
+              'outputs': ['<(PRODUCT_DIR)/nacl_helper_bootstrap'],
+              'message': 'Munging ELF program header',
+              'action': ['python', '<@(_inputs)', '<@(_outputs)']
+            }],
+          }
       ],
     }],
   ],