Re: [PATCH] [RFC] vm: add a syscall to map a process memory into a pipe

[Andrei Vagin <avagin@virtuozzo.com>]

On Thu, Aug 10, 2017 at 09:42:44PM +0200, Jann Horn wrote:
> On Thu, Aug 10, 2017 at 8:46 PM, Andrei Vagin <avagin@openvz.org> wrote:
> > It is a hybrid of process_vm_readv() and vmsplice().
> >
> > vmsplice can map memory from a current address space into a pipe.
> > process_vm_readv can read memory of another process.
> [...]
> > +/*
> > + * Map pages from a specified task into a pipe
> > + */
> > +static int remote_single_vec_to_pipe(struct task_struct *task,
> > +                       struct mm_struct *mm,
> > +                       const struct iovec *rvec,
> > +                       struct pipe_inode_info *pipe,
> > +                       unsigned int flags,
> > +                       size_t *total)
> > +{
> > +       struct pipe_buffer buf = {
> > +               .ops = &user_page_pipe_buf_ops,
> > +               .flags = flags
> > +       };
> [...]
> > +       while (nr_pages) {
> [...]
> > +               /*
> > +                * Get the pages we're interested in.  We must
> > +                * access remotely because task/mm might not
> > +                * current/current->mm
> > +                */
> > +               down_read(&mm->mmap_sem);
> > +               pages = get_user_pages_remote(task, mm, pa, pages, flags,
> > +                                             process_pages, NULL, &locked);
> 
> This fifth "flags" argument of get_user_pages_remote() should contain
> GUP flags (FOLL_*), but it looks like you're actually passing in 0 or
> PIPE_BUF_FLAG_GIFT, which will be interpreted as FOLL_GET?
> (See the snippets quoted below.) This looks like a bug.
> 
> Maybe use a more meaningful variable name than "flags".

Good catch. I will fix and rename the variable. get_user_pages_remote
has to be called with zero flags here. Thank you.

> 
> > +static ssize_t remote_iovec_to_pipe(struct task_struct *task,
> > +                       struct mm_struct *mm,
> > +                       const struct iovec *rvec,
> > +                       unsigned long riovcnt,
> > +                       struct pipe_inode_info *pipe,
> > +                       unsigned int flags)
> > +{
> [...]
> > +               ret = remote_single_vec_to_pipe(
> > +                               task, mm, &rvec[i], pipe, flags, &total);
> [...]
> > +}
> > +
> > +static long process_vmsplice_to_pipe(struct task_struct *task,
> > +                               struct mm_struct *mm, struct file *file,
> > +                               const struct iovec __user *uiov,
> > +                               unsigned long nr_segs, unsigned int flags)
> > +{
> [...]
> > +       unsigned int buf_flag = 0;
> [...]
> > +       if (flags & SPLICE_F_GIFT)
> > +               buf_flag = PIPE_BUF_FLAG_GIFT;
> [...]
> > +       if (!ret)
> > +               ret = remote_iovec_to_pipe(task, mm, iov,
> > +                                               nr_segs, pipe, buf_flag);
> [...]
> > +}