x86: Restore compile-time check for shadow stack pointer in longjmp

sysdeps/unix/sysv/linux/x86/longjmp.c

@@ -0,0 +1,39 @@
+/* __libc_siglongjmp for Linux/x86
+   Copyright (C) 2018-2021 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <sysdeps/x86/longjmp.c>
+
+#include <nptl/pthreadP.h>
+#include <jmp_buf-ssp.h>
+
+#ifdef __x86_64__
+# define SHADOW_STACK_POINTER_SIZE 8
+#else
+# define SHADOW_STACK_POINTER_SIZE 4
+#endif
+
+/* Assert that the priv field in struct pthread_unwind_buf has space
+   to store shadow stack pointer.  */
+_Static_assert ((offsetof (struct pthread_unwind_buf, priv)
+                <= SHADOW_STACK_POINTER_OFFSET)
+               && ((offsetof (struct pthread_unwind_buf, priv)
+                    + sizeof (((struct pthread_unwind_buf *) 0)->priv))
+                   >= (SHADOW_STACK_POINTER_OFFSET
+                       + SHADOW_STACK_POINTER_SIZE)),
+               "Shadow stack pointer is not within private storage "
+               "of pthread_unwind_buf.");