implement pthread_getattr_np

this function is mainly (purely?) for obtaining stack address
information, but we also provide the detach state since it's easy to
do anyway.

src/internal/pthread_impl.h

@@ -23,6 +23,8 @@ struct pthread {
 	int detached;
 	unsigned char *map_base;
 	size_t map_size;
+	void *stack;
+	size_t stack_size;
 	void *start_arg;
 	void *(*start)(void *);
 	void *result;

src/thread/pthread_create.c

@@ -101,7 +101,7 @@ int pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict attrp
 	int ret;
 	size_t size, guard;
 	struct pthread *self = pthread_self(), *new;
-	unsigned char *map = 0, *stack = 0, *tsd = 0;
+	unsigned char *map = 0, *stack = 0, *tsd = 0, *stack_limit;
 	unsigned flags = 0x7d8f00;
 	int do_sched = 0;
 	pthread_attr_t attr = {0};
@@ -123,6 +123,7 @@ int pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict attrp
 		size_t need = libc.tls_size + __pthread_tsd_size;
 		size = attr._a_stacksize + DEFAULT_STACK_SIZE;
 		stack = (void *)(attr._a_stackaddr & -16);
+		stack_limit = attr._a_stackaddr - size;
 		/* Use application-provided stack for TLS only when
 		 * it does not take more than ~12% or 2k of the
 		 * application's stack space. */
@@ -152,12 +153,17 @@ int pthread_create(pthread_t *restrict res, const pthread_attr_t *restrict attrp
 			if (map == MAP_FAILED) goto fail;
 		}
 		tsd = map + size - __pthread_tsd_size;
-		if (!stack) stack = tsd - libc.tls_size;
+		if (!stack) {
+			stack = tsd - libc.tls_size;
+			stack_limit = map + guard;
+		}
 	}
 
 	new = __copy_tls(tsd - libc.tls_size);
 	new->map_base = map;
 	new->map_size = size;
+	new->stack = stack;
+	new->stack_size = stack - stack_limit;
 	new->pid = self->pid;
 	new->errno_ptr = &new->errno_val;
 	new->start = entry;

src/thread/pthread_getattr_np.c

+#include "pthread_impl.h"
+#include <sys/mman.h>
+
+int pthread_getattr_np(pthread_t t, pthread_attr_t *a)
+{
+	*a = (pthread_attr_t){0};
+	a->_a_detach = !!t->detached;
+	if (t->stack) {
+		a->_a_stackaddr = (uintptr_t)t->stack;
+		a->_a_stacksize = t->stack_size - DEFAULT_STACK_SIZE;
+	} else {
+		char *p = (void *)libc.auxv;
+		size_t l = PAGE_SIZE;
+		p += -(uintptr_t)p & PAGE_SIZE-1;
+		a->_a_stackaddr = (uintptr_t)p;
+		while (!posix_madvise(p-l-PAGE_SIZE, PAGE_SIZE, POSIX_MADV_NORMAL))
+			l += PAGE_SIZE;
+		a->_a_stacksize = l - DEFAULT_STACK_SIZE;
+	}
+	return 0;
+}