fix over-alignment of TLS, insufficient builtin TLS on 64-bit archs

a conservative estimate of 4*sizeof(size_t) was used as the minimum
alignment for thread-local storage, despite the only requirements
being alignment suitable for struct pthread and void* (which struct
pthread already contains). additional alignment required by the
application or libraries is encoded in their headers and is already
applied.

over-alignment prevented the builtin_tls array from ever being used in
dynamic-linked programs on 64-bit archs, thereby requiring allocation
at startup even in programs with no TLS of their own.

src/env/__init_tls.c

@@ -2,6 +2,7 @@
 #include <limits.h>
 #include <sys/mman.h>
 #include <string.h>
+#include <stddef.h>
 #include "pthread_impl.h"
 #include "libc.h"
 #include "atomic.h"
@@ -22,7 +23,12 @@ int __init_tp(void *p)
 
 #ifndef SHARED
 
-static long long builtin_tls[(sizeof(struct pthread) + 64)/sizeof(long long)];
+static struct builtin_tls {
+	char c;
+	struct pthread pt;
+	void *space[16];
+} builtin_tls[1];
+#define MIN_TLS_ALIGN offsetof(struct builtin_tls, pt)
 
 struct tls_image {
 	void *image;
@@ -86,7 +92,7 @@ void __init_tls(size_t *aux)
 	}
 
 	T.size += (-T.size - (uintptr_t)T.image) & (T.align-1);
-	if (T.align < 4*sizeof(size_t)) T.align = 4*sizeof(size_t);
+	if (T.align < MIN_TLS_ALIGN) T.align = MIN_TLS_ALIGN;
 
 	libc.tls_size = 2*sizeof(void *)+T.size+T.align+sizeof(struct pthread);
 

src/ldso/dynlink.c

@@ -122,6 +122,13 @@ void __init_libc(char **, char *);
 
 const char *__libc_get_version(void);
 
+static struct builtin_tls {
+	char c;
+	struct pthread pt;
+	void *space[16];
+} builtin_tls[1];
+#define MIN_TLS_ALIGN offsetof(struct builtin_tls, pt)
+
 static struct dso *head, *tail, *ldso, *fini_head;
 static char *env_path, *sys_path;
 static unsigned long long gencnt;
@@ -132,10 +139,9 @@ static int noload;
 static jmp_buf *rtld_fail;
 static pthread_rwlock_t lock;
 static struct debug debug;
-static size_t tls_cnt, tls_offset, tls_align = 4*sizeof(size_t);
+static size_t tls_cnt, tls_offset, tls_align = MIN_TLS_ALIGN;
 static size_t static_tls_cnt;
 static pthread_mutex_t init_fini_lock = { ._m_type = PTHREAD_MUTEX_RECURSIVE };
-static long long builtin_tls[(sizeof(struct pthread) + 64)/sizeof(long long)];
 
 struct debug *_dl_debug_addr = &debug;