refactor setxid return path to use __syscall_ret

this avoids some spurious negation and duplicated errno logic, and
brings the code in line with the newly-added multithreaded setgroups.

src/unistd/setxid.c

@@ -1,20 +1,19 @@
 #include <unistd.h>
-#include <errno.h>
+#include <signal.h>
 #include "syscall.h"
 #include "libc.h"
-#include "pthread_impl.h"
 
 struct ctx {
 	int id, eid, sid;
-	int nr, err;
+	int nr, ret;
 };
 
 static void do_setxid(void *p)
 {
 	struct ctx *c = p;
-	if (c->err>0) return;
-	int ret = -__syscall(c->nr, c->id, c->eid, c->sid);
-	if (ret && !c->err) {
+	if (c->ret<0) return;
+	int ret = __syscall(c->nr, c->id, c->eid, c->sid);
+	if (ret && !c->ret) {
 		/* If one thread fails to set ids after another has already
 		 * succeeded, forcibly killing the process is the only safe
 		 * thing to do. State is inconsistent and dangerous. Use
@@ -22,18 +21,14 @@ static void do_setxid(void *p)
 		__block_all_sigs(0);
 		__syscall(SYS_kill, __syscall(SYS_getpid), SIGKILL);
 	}
-	c->err = ret;
+	c->ret = ret;
 }
 
 int __setxid(int nr, int id, int eid, int sid)
 {
-	/* err is initially nonzero so that failure of the first thread does not
+	/* ret is initially nonzero so that failure of the first thread does not
 	 * trigger the safety kill above. */
-	struct ctx c = { .nr = nr, .id = id, .eid = eid, .sid = sid, .err = -1 };
+	struct ctx c = { .nr = nr, .id = id, .eid = eid, .sid = sid, .ret = 1 };
 	__synccall(do_setxid, &c);
-	if (c.err) {
-		if (c.err>0) errno = c.err;
-		return -1;
-	}
-	return 0;
+	return __syscall_ret(c.ret);
 }