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

Tycho Andersen
2018-09-27T18:04:23Z

On Thu, Sep 27, 2018 at 06:49:02PM +0200, Jann Horn wrote:
> 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?

No reason, probably just flailing around trying to figure out what
exactly I wanted. I'll make the change, thanks.

> > 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.

Will do.

Cheers,

Tycho