move thread pointer setup to beginning of dynamic linker stage 3

this allows the dynamic linker itself to run with a valid thread
pointer, which is a prerequisite for stack protector on archs where
the ssp canary is stored in TLS. it will also allow us to remove some
remaining runtime checks for whether the thread pointer is valid.

as long as the application and its libraries do not require additional
size or alignment, this early thread pointer will be kept and reused
at runtime. otherwise, a new static TLS block is allocated after
library loading has finished and the thread pointer is switched over.

src/ldso/dynlink.c

@@ -1159,7 +1159,15 @@ _Noreturn void __dls3(size_t *sp)
 	char **argv = (void *)(sp+1);
 	char **argv_orig = argv;
 	char **envp = argv+argc+1;
-	void *initial_tls;
+
+	/* Setup early thread pointer in builtin_tls for ldso/libc itself to
+	 * use during dynamic linking. If possible it will also serve as the
+	 * thread pointer at runtime. */
+	libc.tls_size = sizeof builtin_tls;
+	if (__init_tp(__copy_tls((void *)builtin_tls)) < 0) {
+		dprintf(2, "%s: Thread-local storage not supported by kernel.\n", argv[0]);
+		_exit(127);
+	}
 
 	/* Find aux vector just past environ[] */
 	for (i=argc+1; argv[i]; i++)
@@ -1336,19 +1344,26 @@ _Noreturn void __dls3(size_t *sp)
 	reloc_all(&app);
 
 	update_tls_size();
-	if (libc.tls_size > sizeof builtin_tls) {
-		initial_tls = calloc(libc.tls_size, 1);
+	if (libc.tls_size > sizeof builtin_tls || tls_align > MIN_TLS_ALIGN) {
+		void *initial_tls = calloc(libc.tls_size, 1);
 		if (!initial_tls) {
 			dprintf(2, "%s: Error getting %zu bytes thread-local storage: %m\n",
 				argv[0], libc.tls_size);
 			_exit(127);
 		}
+		if (__init_tp(__copy_tls(initial_tls)) < 0) {
+			dprintf(2, "%s: Failed to switch to new thread pointer.\n", argv[0]);
+			_exit(127);
+		}
 	} else {
-		initial_tls = builtin_tls;
-	}
-	if (__init_tp(__copy_tls(initial_tls)) < 0 && tls_cnt) {
-		dprintf(2, "%s: Thread-local storage not supported by kernel.\n", argv[0]);
-		_exit(127);
+		size_t tmp_tls_size = libc.tls_size;
+		pthread_t self = __pthread_self();
+		/* Temporarily set the tls size to the full size of
+		 * builtin_tls so that __copy_tls will use the same layout
+		 * as it did for before. Then check, just to be safe. */
+		libc.tls_size = sizeof builtin_tls;
+		if (__copy_tls((void*)builtin_tls) != self) a_crash();
+		libc.tls_size = tmp_tls_size;
 	}
 	static_tls_cnt = tls_cnt;