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

fusl/src/unistd/faccessat.c

+#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);
+}