Re: [PATCH v7 4/6] files: add a replace_fd_files() function

[Jann Horn <jannh@google.com>]

On Thu, Sep 27, 2018 at 5:11 PM Tycho Andersen <tycho@tycho.ws> wrote:
> Similar to fd_install/__fd_install, we want to be able to replace an fd of
> an arbitrary struct files_struct, not just current's. We'll use this in the
> next patch to implement the seccomp ioctl that allows inserting fds into a
> stopped process' context.
[...]
> diff --git a/fs/file.c b/fs/file.c
> index 7ffd6e9d103d..3b3c5aadaadb 100644
> --- a/fs/file.c
> +++ b/fs/file.c
> @@ -850,24 +850,32 @@ __releases(&files->file_lock)
>  }
>
>  int replace_fd(unsigned fd, struct file *file, unsigned flags)
> +{
> +       return replace_fd_task(current, fd, file, flags);
> +}
> +
> +/*
> + * Same warning as __alloc_fd()/__fd_install() here.
> + */
> +int replace_fd_task(struct task_struct *task, unsigned fd,
> +                   struct file *file, unsigned flags)
>  {
>         int err;
> -       struct files_struct *files = current->files;

Why did you remove this? You could just do s/current/task/ instead, right?

>         if (!file)
> -               return __close_fd(files, fd);
> +               return __close_fd(task->files, fd);
>
> -       if (fd >= rlimit(RLIMIT_NOFILE))
> +       if (fd >= task_rlimit(task, RLIMIT_NOFILE))
>                 return -EBADF;
>
> -       spin_lock(&files->file_lock);
> -       err = expand_files(files, fd);
> +       spin_lock(&task->files->file_lock);
> +       err = expand_files(task->files, fd);
>         if (unlikely(err < 0))
>                 goto out_unlock;
> -       return do_dup2(files, file, fd, flags);
> +       return do_dup2(task->files, file, fd, flags);
>
>  out_unlock:
> -       spin_unlock(&files->file_lock);
> +       spin_unlock(&task->files->file_lock);
>         return err;
>  }
>
> diff --git a/include/linux/file.h b/include/linux/file.h
> index 6b2fb032416c..f94277fee038 100644
> --- a/include/linux/file.h
> +++ b/include/linux/file.h
> @@ -11,6 +11,7 @@
>  #include <linux/posix_types.h>
>
>  struct file;
> +struct task_struct;
>
>  extern void fput(struct file *);
>
> @@ -79,6 +80,13 @@ static inline void fdput_pos(struct fd f)
>
>  extern int f_dupfd(unsigned int from, struct file *file, unsigned flags);
>  extern int replace_fd(unsigned fd, struct file *file, unsigned flags);
> +/*
> + * Warning! This is only safe if you know the owner of the files_struct is
> + * stopped outside syscall context. It's a very bad idea to use this unless you
> + * have similar guarantees in your code.
> + */
> +extern int replace_fd_task(struct task_struct *task, unsigned fd,
> +                          struct file *file, unsigned flags);

I think Linux kernel coding style is normally to have comments on the
implementations of functions, not in the headers? Maybe replace the
warning above the implemenation of replace_fd_task() with this
comment.