build: Always set personality to linux32 when cross-building

This means that autodetection during builds works much better.
Fixes https://github.com/flatpak/flatpak/issues/712

app/flatpak-builtins-build.c

@@ -269,7 +269,7 @@ flatpak_builtin_build (int argc, char **argv, GCancellable *cancellable, GError
   argv_array = g_ptr_array_new_with_free_func (g_free);
   g_ptr_array_add (argv_array, g_strdup (flatpak_get_bwrap ()));
 
-  run_flags = FLATPAK_RUN_FLAG_DEVEL | FLATPAK_RUN_FLAG_NO_SESSION_HELPER;
+  run_flags = FLATPAK_RUN_FLAG_DEVEL | FLATPAK_RUN_FLAG_NO_SESSION_HELPER | FLATPAK_RUN_FLAG_SET_PERSONALITY;
   if (custom_usr)
     run_flags |= FLATPAK_RUN_FLAG_WRITABLE_ETC;
 

common/flatpak-run.c

@@ -27,6 +27,7 @@
 #include <sys/utsname.h>
 #include <sys/socket.h>
 #include <sys/ioctl.h>
+#include <sys/personality.h>
 #include <grp.h>
 
 #ifdef ENABLE_SECCOMP
@@ -4133,6 +4134,13 @@ flatpak_run_setup_base_argv (GPtrArray      *argv_array,
   if ((flags & FLATPAK_RUN_FLAG_WRITABLE_ETC) == 0)
     add_monitor_path_args ((flags & FLATPAK_RUN_FLAG_NO_SESSION_HELPER) == 0, argv_array);
 
+  if ((flags & FLATPAK_RUN_FLAG_SET_PERSONALITY) &&
+      flatpak_is_linux32_arch (arch))
+    {
+      g_debug ("Setting personality linux32");
+      personality (PER_LINUX32);
+    }
+
   return TRUE;
 }
 

common/flatpak-run.h

@@ -84,6 +84,7 @@ typedef enum {
   FLATPAK_RUN_FLAG_WRITABLE_ETC       = (1 << 6),
   FLATPAK_RUN_FLAG_NO_SESSION_BUS_PROXY = (1 << 7),
   FLATPAK_RUN_FLAG_NO_SYSTEM_BUS_PROXY = (1 << 8),
+  FLATPAK_RUN_FLAG_SET_PERSONALITY    = (1 << 9),
 } FlatpakRunFlags;
 
 gboolean  flatpak_run_add_extension_args (GPtrArray    *argv_array,

common/flatpak-utils.c

@@ -368,6 +368,22 @@ flatpak_get_arch (void)
 #endif
 }
 
+gboolean
+flatpak_is_linux32_arch (const char *arch)
+{
+  const char *kernel_arch = flatpak_get_kernel_arch ();
+
+  if (strcmp (kernel_arch, "x86_64") == 0 &&
+      strcmp (arch, "i386") == 0)
+    return TRUE;
+
+  if (strcmp (kernel_arch, "aarch64") == 0 &&
+      strcmp (arch, "arm") == 0)
+    return TRUE;
+
+  return FALSE;
+}
+
 /* Get all compatible arches for this host in order of priority */
 const char **
 flatpak_get_arches (void)

common/flatpak-utils.h

@@ -65,6 +65,7 @@ gboolean flatpak_is_in_sandbox (void);
 
 const char * flatpak_get_arch (void);
 const char ** flatpak_get_arches (void);
+gboolean flatpak_is_linux32_arch (const char *arch);
 
 const char ** flatpak_get_gl_drivers (void);
 gboolean flatpak_extension_matches_reason (const char *extension_id,