[PATCH] staging: comedi: support vm_access_process for mmap'd buffer

[Ian Abbott]

On 20/04/17 18:39, Ian Abbott wrote:
> If a process that has mmap'd a COMEDI buffer is being run under a
> debugger such as GDB, the buffer contents are inaccessible from the
> debugger.  Support the `access()` VM operation to allow the buffer
> contents to be accessed by another process.
>
> Signed-off-by: Ian Abbott <abbotti at mev.co.uk>
> ---
>  drivers/staging/comedi/comedi_buf.c      | 24 ++++++++++++++++++++++++
>  drivers/staging/comedi/comedi_fops.c     | 15 +++++++++++++++
>  drivers/staging/comedi/comedi_internal.h |  2 ++
>  3 files changed, 41 insertions(+)
>
> diff --git a/drivers/staging/comedi/comedi_buf.c b/drivers/staging/comedi/comedi_buf.c
> index 1e1df89b5018..8e9b30b26810 100644
> --- a/drivers/staging/comedi/comedi_buf.c
> +++ b/drivers/staging/comedi/comedi_buf.c
> @@ -161,6 +161,30 @@ int comedi_buf_map_put(struct comedi_buf_map *bm)
>  	return 1;
>  }
>
> +/* helper for "access" vm operation */
> +int comedi_buf_map_access(struct comedi_buf_map *bm, unsigned long offset,
> +			  void *buf, int len, int write)
> +{
> +	unsigned int pgoff = offset & ~PAGE_MASK;
> +	unsigned long pg = offset >> PAGE_SHIFT;
> +	int done = 0;
> +
> +	while (done < len && pg < bm->n_pages) {
> +		int l = min_t(int, len - done, PAGE_SIZE - pgoff);
> +		void *b = bm->page_list[pg].virt_addr + pgoff;
> +
> +		if (write)
> +			memcpy(b, buf, l);
> +		else
> +			memcpy(buf, b, l);
> +		buf += l;
> +		done += l;
> +		pg++;
> +		pgoff = 0;
> +	}
> +	return done;
> +}
> +
>  /* returns s->async->buf_map and increments its kref refcount */
>  struct comedi_buf_map *
>  comedi_buf_map_from_subdev_get(struct comedi_subdevice *s)
> diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
> index 92d864fc08ac..2a8a39653c94 100644
> --- a/drivers/staging/comedi/comedi_fops.c
> +++ b/drivers/staging/comedi/comedi_fops.c
> @@ -2165,9 +2165,24 @@ static void comedi_vm_close(struct vm_area_struct *area)
>  	comedi_buf_map_put(bm);
>  }
>
> +static int comedi_vm_access(struct vm_area_struct *vma, unsigned long addr,
> +			    void *buf, int len, int write)
> +{
> +	struct comedi_buf_map *bm = vma->vm_private_data;
> +	unsigned long offset =
> +	    addr - vma->vm_start + (vma->vm_pgoff << PAGE_SHIFT);
> +
> +	if (len < 0)
> +		return -EINVAL;
> +	if (len > vma->vm_end - vma->vm_start)
> +		len = vma->vm_end - vma->vm_start;

Actually, this limit isn't correct as it doesn't take the offset from 
`vma->vm_start` to `addr` into account.  I'll send another version of 
the patch to sort that out.

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti at mev.co.uk> )=-
-=(                          Web: http://www.mev.co.uk/  )=-