nacl_helper_bootstrap: Reserve correct address space size for ARM, none for x86-64

The address space reservation has only been thoroughly used and tested on
x86-32.  It's also necessary on ARM, but it needs a different exact amount
reserved.  For x86-64, the reserved space is not used, so it's a waste of
address space to consume a gigabyte for it.

BUG= none
TEST= nacl_integration

R=mseaborn@chromium.org,bradchen@google.com

Review URL: http://codereview.chromium.org/8588046

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110739 0039d316-1c4b-4281-b951-d872f2087c98

chrome/nacl.gypi

@@ -127,6 +127,27 @@
           ],
         }],
         ['OS=="linux" and coverage==0', {
+          'conditions': [
+            ['target_arch=="x64"', {
+              'variables': {
+                # No extra reservation.
+                'nacl_reserve_top': [],
+              }
+            }],
+            ['target_arch=="ia32"', {
+              'variables': {
+                # 1G address space.
+                'nacl_reserve_top': ['--defsym', 'RESERVE_TOP=0x40000000'],
+              }
+            }],
+            ['target_arch=="arm"', {
+              'variables': {
+                # 1G address space, plus 4K guard area above because
+                # immediate offsets are 12 bits.
+                'nacl_reserve_top': ['--defsym', 'RESERVE_TOP=0x40001000'],
+              }
+            }],
+          ],
           'targets': [
             {
               'target_name': 'nacl_helper',
@@ -274,8 +295,8 @@
                     ['target_arch=="arm"', {
                       'variables': {
                         'linker_emulation': 'armelf_linux_eabi',
-                        # ARM requires linking against libc due to ABI dependencies on
-                        # memset
+                        # ARM requires linking against libc due to ABI
+                        # dependencies on memset.
                         'bootstrap_extra_lib' : "${SYSROOT}/usr/lib/libc.a",
                       }
                     }],
@@ -283,13 +304,16 @@
                   'action': ['../tools/ld_bfd/ld',
                              '-m', '<(linker_emulation)',
                              '--build-id',
-                             # This program is (almost) entirely standalone.  It
-                             # has its own startup code, so no crt1.o for it.  It is
-                             # statically linked, and on x86 it does not use
-                             # libc at all.  However, on ARM it needs a few (safe)
-                             # things from libc.
+                             # This program is (almost) entirely
+                             # standalone.  It has its own startup code, so
+                             # no crt1.o for it.  It is statically linked,
+                             # and on x86 it does not use libc at all.
+                             # However, on ARM it needs a few (safe) things
+                             # from libc.
                              '-static',
-                             # Link with custom linker script for special layout.
+                             # Link with custom linker script for special
+                             # layout.  The script uses the symbol RESERVE_TOP.
+                             '<@(nacl_reserve_top)',
                              '--script=<(linker_script)',
                              '-o', '<@(_outputs)',
                              # On x86-64, the default page size with some

chrome/nacl/nacl_helper_bootstrap_linux.x

@@ -33,11 +33,11 @@ ENTRY(_start)
 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.
+ * The symbol RESERVE_TOP is the top of the range we are trying to reserve.
+ * This is set via --defsym on the linker command line, because the correct
+ * value differs for each machine.  It's not defined at all if we do not
+ * actually need any space reserved for this configuration.
  */
-RESERVE_TOP = 1 << 30;
 
 /*
  * We specify the program headers we want explicitly, to get the layout
@@ -109,7 +109,7 @@ SECTIONS {
   . = ALIGN(CONSTANT(COMMONPAGESIZE));
   RESERVE_START = .;
   .reserve : {
-    . = RESERVE_TOP - RESERVE_START;
+    . += DEFINED(RESERVE_TOP) ? (RESERVE_TOP - RESERVE_START) : 0;
   } :reserve
 
   /*