hurd: Fix starting static binaries with stack protection enabled

gcc introduces gs:0x14 accesses in most functions, so we need some tcbhead
to be ready very early during initialization.  This configures a static area
which can be referenced by various protected functions, until proper TLS is
set up.

sysdeps/mach/hurd/i386/init-first.c

@@ -42,6 +42,11 @@ extern int __libc_argc attribute_hidden;
 extern char **__libc_argv attribute_hidden;
 extern char **_dl_argv;
 
+#ifndef SHARED
+unsigned short __init1_desc;
+static tcbhead_t __init1_tcbhead;
+#endif
+
 /* Things that want to be run before _hurd_init or much anything else.
    Importantly, these are called before anything tries to use malloc.  */
 DEFINE_HOOK (_hurd_preinit_hook, (void));
@@ -243,6 +248,13 @@ first_init (void)
   /* Initialize data structures so we can do RPCs.  */
   __mach_init ();
 
+#ifndef SHARED
+  /* In the static case, we need to set up TLS early so that the stack
+     protection guard can be read at gs:0x14 by the gcc-generated snippets.  */
+  _hurd_tls_init(&__init1_tcbhead);
+  asm ("movw %%gs,%w0" : "=m" (__init1_desc));
+#endif
+
   RUN_RELHOOK (_hurd_preinit_hook, ());
 }
 
@@ -285,6 +297,7 @@ strong_alias (posixland_init, __libc_init_first);
    This poorly-named function is called by static-start.S,
    which should not exist at all.  */
 void
+inhibit_stack_protector
 _hurd_stack_setup (void)
 {
   intptr_t caller = (intptr_t) __builtin_return_address (0);

sysdeps/mach/hurd/i386/tls.h

@@ -49,7 +49,6 @@ typedef struct
   mach_port_t reply_port;      /* This thread's reply port.  */
   struct hurd_sigstate *_hurd_sigstate;
 } tcbhead_t;
-#endif
 
 /* Return tcbhead_t from a TLS segment descriptor.  */
 # define HURD_DESC_TLS(desc)						      \
@@ -60,10 +59,18 @@ typedef struct
   })
 
 /* Return 1 if TLS is not initialized yet.  */
+#ifndef SHARED
+extern unsigned short __init1_desc;
+#define __HURD_DESC_INITIAL(gs, ds) ((gs) == (ds) || (gs) == __init1_desc)
+#else
+#define __HURD_DESC_INITIAL(gs, ds) ((gs) == (ds))
+#endif
+
 #define __LIBC_NO_TLS()							      \
   ({ unsigned short ds, gs;						      \
      asm ("movw %%ds,%w0; movw %%gs,%w1" : "=q" (ds), "=q" (gs));	      \
-     __builtin_expect (ds == gs, 0); })
+     __builtin_expect(__HURD_DESC_INITIAL(gs, ds), 0); })
+#endif
 
 /* The TCB can have any size and the memory following the address the
    thread pointer points to is unspecified.  Allocate the TCB there.  */