Re: [PATCH v2] vfio-ccw: Do not read region ret_code after write

[Cornelia Huck <cohuck@redhat.com>]

On Wed,  3 Mar 2021 17:07:39 +0100
Eric Farman <farman@linux.ibm.com> wrote:

> A pwrite() call returns the number of bytes written (or -1 on error),
> and vfio-ccw compares this number with the size of the region to
> determine if an error had occurred or not.
> 
> If they are not equal, this is a failure and the errno is used to
> determine exactly how things failed. An errno of zero is possible
> (though unlikely) in this situation and would be translated to a
> successful operation.
> 
> If they ARE equal, the ret_code field is read from the region to
> determine how to proceed. While the kernel sets the ret_code field
> as necessary, the region and thus this field is not "written back"
> to the user. So the value can only be what it was initialized to,
> which is zero.
> 
> So, let's convert an unexpected length with errno of zero to a
> return code of -EFAULT, and explicitly set an expected length to
> a return code of zero. This will be a little safer and clearer.
> 
> Suggested-by: Matthew Rosato <mjrosato@linux.ibm.com>
> Signed-off-by: Eric Farman <farman@linux.ibm.com>
> ---
> 
> Notes:
>     Changes v1->v2:
>      - Add code to handle an unexpected length being returned,
>        but errno is not set. [CH]
>     
>     v1: https://lore.kernel.org/qemu-devel/cc440ee8-28c5-3208-19db-ebb48ea60e9d@linux.ibm.com/
> 
>  hw/vfio/ccw.c | 12 ++++++------
>  1 file changed, 6 insertions(+), 6 deletions(-)

Thanks, applied.