Re: [RFC PATCH v3 05/10] mm/mshare: Add ioctl support

[Jann Horn]

On Wed, Sep 4, 2024 at 1:22 AM Anthony Yznaga <anthony.yznaga@xxxxxxxxxx> wrote:
> Reserve a range of ioctls for msharefs and add the first two ioctls
> to get and set the start address and size of an mshare region.
[...]
> +static long
> +msharefs_set_size(struct mm_struct *mm, struct mshare_data *m_data,
> +                       struct mshare_info *minfo)
> +{
> +       unsigned long end = minfo->start + minfo->size;
> +
> +       /*
> +        * Validate alignment for start address, and size
> +        */
> +       if ((minfo->start | end) & (PGDIR_SIZE - 1)) {
> +               spin_unlock(&m_data->m_lock);
> +               return -EINVAL;
> +       }
> +
> +       mm->mmap_base = minfo->start;
> +       mm->task_size = minfo->size;
> +       if (!mm->task_size)
> +               mm->task_size--;
> +
> +       m_data->minfo.start = mm->mmap_base;
> +       m_data->minfo.size = mm->task_size;
> +       spin_unlock(&m_data->m_lock);
> +
> +       return 0;
> +}
> +
> +static long
> +msharefs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
> +{
> +       struct mshare_data *m_data = filp->private_data;
> +       struct mm_struct *mm = m_data->mm;
> +       struct mshare_info minfo;
> +
> +       switch (cmd) {
> +       case MSHAREFS_GET_SIZE:
> +               spin_lock(&m_data->m_lock);
> +               minfo = m_data->minfo;
> +               spin_unlock(&m_data->m_lock);
> +
> +               if (copy_to_user((void __user *)arg, &minfo, sizeof(minfo)))
> +                       return -EFAULT;
> +
> +               return 0;
> +
> +       case MSHAREFS_SET_SIZE:
> +               if (copy_from_user(&minfo, (struct mshare_info __user *)arg,
> +                       sizeof(minfo)))
> +                       return -EFAULT;
> +
> +               /*
> +                * If this mshare region has been set up once already, bail out
> +                */
> +               spin_lock(&m_data->m_lock);
> +               if (m_data->minfo.start != 0) {

Is there actually anything that prevents msharefs_set_size() from
setting up m_data with ->minfo.start==0, so that a second
MSHAREFS_SET_SIZE invocation will succeed? It would probably be more
reliable to have a separate flag for "has this thing been set up yet".


> +                       spin_unlock(&m_data->m_lock);
> +                       return -EINVAL;
> +               }
> +
> +               return msharefs_set_size(mm, m_data, &minfo);
> +
> +       default:
> +               return -ENOTTY;
> +       }
> +}