vfs: Harden task_conv() to return EFAULT when cpath argument is null

This patch also reverts the commit a0dcf85.

Signed-off-by: Waldemar Kozaczuk <jwkozaczuk@gmail.com>
Message-Id: <20190519204728.16752-1-jwkozaczuk@gmail.com>

fs/vfs/main.cc

@@ -120,11 +120,6 @@ int open(const char *pathname, int flags, ...)
     int fd, error;
     int acc;
 
-    if (pathname == nullptr) {
-        error = EFAULT;
-        goto out_errno;
-    }
-
     acc = 0;
     switch (flags & O_ACCMODE) {
     case O_RDONLY:

fs/vfs/vfs_syscalls.cc

@@ -117,10 +117,6 @@ sys_open(char *path, int flags, mode_t mode, struct file **fpp)
 	DPRINTF(VFSDB_SYSCALL, ("sys_open: path=%s flags=%x mode=%x\n",
 				path, flags, mode));
 
-	if (path == nullptr) {
-		return (EFAULT);
-	}
-
 	flags = fflags(flags);
 	if (flags & O_CREAT) {
 		error = namei(path, &dp);

fs/vfs/vfs_task.cc

@@ -146,6 +146,10 @@ task_conv(struct task *t, const char *cpath, int acc, char *full)
 {
 	int rc;
 
+	if (cpath == nullptr) {
+		return (EFAULT);
+	}
+
 	rc = path_conv(t->t_cwd, cpath, full);
 	if (rc != 0) {
 		return (rc);