revert regression in faccessat AT_EACCESS robustness

commit f9fb20b4 switched from using a
pipe for the result to conveying it via the child process exit status.
Alexander Monakov pointed out that the latter could fail if the
application is not expecting faccessat to produce a child and performs
a wait operation with __WCLONE or __WALL, and that it is not clear
whether it's guaranteed to work when SIGCHLD's disposition has been
set to SIG_IGN.

in addition, that commit introduced a bug that caused EACCES to be
produced instead of EBUSY due to an exit path that was overlooked when
the error channel was changed, and introduced a spurious retry loop
around the wait operation.

src/unistd/faccessat.c

 #include <unistd.h>
 #include <fcntl.h>
-#include <errno.h>
 #include <sys/wait.h>
 #include "syscall.h"
 #include "pthread_impl.h"
@@ -9,26 +8,19 @@ struct ctx {
 	int fd;
 	const char *filename;
 	int amode;
-};
-
-static const int errors[] = {
-	0, -EACCES, -ELOOP, -ENAMETOOLONG, -ENOENT, -ENOTDIR,
-	-EROFS, -EBADF, -EINVAL, -ETXTBSY,
-	-EFAULT, -EIO, -ENOMEM,
-	-EBUSY
+	int p;
 };
 
 static int checker(void *p)
 {
 	struct ctx *c = p;
 	int ret;
-	int i;
 	if (__syscall(SYS_setregid, __syscall(SYS_getegid), -1)
 	    || __syscall(SYS_setreuid, __syscall(SYS_geteuid), -1))
 		__syscall(SYS_exit, 1);
 	ret = __syscall(SYS_faccessat, c->fd, c->filename, c->amode, 0);
-	for (i=0; i < sizeof errors/sizeof *errors - 1 && ret!=errors[i]; i++);
-	return i;
+	__syscall(SYS_write, c->p, &ret, sizeof ret);
+	return 0;
 }
 
 int faccessat(int fd, const char *filename, int amode, int flag)
@@ -42,20 +34,21 @@ int faccessat(int fd, const char *filename, int amode, int flag)
 	char stack[1024];
 	sigset_t set;
 	pid_t pid;
-	int ret = -EBUSY;
-	struct ctx c = { .fd = fd, .filename = filename, .amode = amode };
+	int status;
+	int ret, p[2];
+
+	if (pipe2(p, O_CLOEXEC)) return __syscall_ret(-EBUSY);
+	struct ctx c = { .fd = fd, .filename = filename, .amode = amode, .p = p[1] };
 
 	__block_all_sigs(&set);
 	
 	pid = __clone(checker, stack+sizeof stack, 0, &c);
-	if (pid > 0) {
-		int status;
-		do {
-			__syscall(SYS_wait4, pid, &status, __WCLONE, 0);
-		} while (!WIFEXITED(status) && !WIFSIGNALED(status));
-		if (WIFEXITED(status))
-			ret = errors[WEXITSTATUS(status)];
-	}
+	__syscall(SYS_close, p[1]);
+
+	if (pid<0 || __syscall(SYS_read, p[0], &ret, sizeof ret) != sizeof(ret))
+		ret = -EBUSY;
+	__syscall(SYS_close, p[0]);
+	__syscall(SYS_wait4, pid, &status, __WCLONE, 0);
 
 	__restore_sigs(&set);