Manually set thread stack sizes.

8KB was empirically determined to work on musl. We can't go lower (musl
supports up to 2KB) because posix_spawn allocates a temporary stack for
vfork (via clone(2)) on its own stack.

This improves our resource consumption and increases the likeliness for
pthread_create to work even when there isn't a lot of free memory.

Using pthread_setattr_default_np isn't possible, because on musl it
doesn't allow us to go below the default thread stack size. This way is
also more portable.

We don't remove the guard pages entirely, because a shell prompt might
be active in attacker controlled directories, and it's a reasonable
protection against any stack overflow exploits that might appear.

ep.c

@@ -22,6 +22,8 @@
 #define PROMPT " ➜ "
 #define JOBS " ✦"
 
+pthread_attr_t *thread_a = 0;
+
 int main(int argc, char **argv)
 {
 	setlocale(LC_ALL, "");
@@ -54,18 +56,37 @@ int main(int argc, char **argv)
 		}
 	}
 
+	pthread_attr_t attr;
+
+	do {
+		if (pthread_attr_init(&attr)) break;
+		/* stack buffer in popen/posix_spawn can get big, so we can't go much below 8KB;
+		 * we subtract 1024 so libc TLS can still fit into a 8KB region instead of
+		 * requiring a 12KB region. This fiddling has no effect if PAGE_SIZE>4KB */
+		if (pthread_attr_setstacksize(&attr, (1 << 13) - 1024)) {
+			pthread_attr_destroy(&attr);
+			break;
+		}
+		/* guarantee at least one memory page as guard */
+		if (pthread_attr_setguardsize(&attr, 1)) {
+			pthread_attr_destroy(&attr);
+			break;
+		}
+		thread_a = &attr;
+	} while(0);
+
 	/* start threads for long(er) running steps */
 	struct threaded_task root_lang_task = { .task = task_launch_root_lang };
 	int git_launched = 1;
 	pthread_t git_handle;
-	if (pthread_create(&git_handle, NULL, git_thread, &root_lang_task)) {
+	if (pthread_create(&git_handle, thread_a, git_thread, &root_lang_task)) {
 		e(INFO, "couldn't create git thread", errno);
 		git_launched = 0;
 	}
 
 	int pwd_lang_launched = 1;
 	pthread_t pwd_lang_handle;
-	if (pthread_create(&pwd_lang_handle, NULL, lang_thread, NULL)) {
+	if (pthread_create(&pwd_lang_handle, thread_a, lang_thread, NULL)) {
 		e(INFO, "couldn't create lang thread", errno);
 		pwd_lang_launched = 0;
 	}

ep.h

@@ -15,6 +15,9 @@ struct threaded_task {
 	enum task_identity task;
 };
 
+/* from ep.c */
+extern pthread_attr_t *thread_a;
+
 /* from out.c */
 extern FILE *out, *outerr;
 enum log_level_value { DEBUG, INFO, WARN, ERROR };

git.c

@@ -94,14 +94,14 @@ void *git_thread(void *arg)
 	 * if we add more stuff to do in repos, launch more threads */
 
 	pthread_t status_handle, root_handle;
-	if (pthread_create(&status_handle, NULL, get_git_status, git_info))
+	if (pthread_create(&status_handle, thread_a, get_git_status, git_info))
 		goto status_create_error;
-	if (pthread_create(&root_handle, NULL, get_git_root, git_info))
+	if (pthread_create(&root_handle, thread_a, get_git_root, git_info))
 		goto root_create_error;
 
 	pthread_join(root_handle, NULL);
 	if (root_lang_task && git_info->git_root) {
-		root_lang_task->launched = !pthread_create(&root_lang_task->handle, NULL, lang_thread, git_info->git_root);
+		root_lang_task->launched = !pthread_create(&root_lang_task->handle, thread_a, lang_thread, git_info->git_root);
 	}
 root_create_error:
 	pthread_join(status_handle, NULL);