Add a "fork" of musl as //fusl.

fusl/src/unistd/faccessat.c

Index: fusl/src/unistd/faccessat.c
diff --git a/fusl/src/unistd/faccessat.c b/fusl/src/unistd/faccessat.c
new file mode 100644
index 0000000000000000000000000000000000000000..33478959cb0edeaf08ed3860ecac04a1d5c488f3
--- /dev/null
+++ b/fusl/src/unistd/faccessat.c
@@ -0,0 +1,63 @@
+#include <unistd.h>
+#include <fcntl.h>
+#include <errno.h>
+#include <sys/wait.h>
+#include "syscall.h"
+#include "pthread_impl.h"
+
+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
+};
+
+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;
+}
+
+int faccessat(int fd, const char *filename, int amode, int flag)
+{
+	if (!flag || (flag==AT_EACCESS && getuid()==geteuid() && getgid()==getegid()))
+		return syscall(SYS_faccessat, fd, filename, amode, flag);
+
+	if (flag != AT_EACCESS)
+		return __syscall_ret(-EINVAL);
+
+	char stack[1024];
+	sigset_t set;
+	pid_t pid;
+	int ret = -EBUSY;
+	struct ctx c = { .fd = fd, .filename = filename, .amode = amode };
+
+	__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)];
+	}
+
+	__restore_sigs(&set);
+
+	return __syscall_ret(ret);
+}