Re: [PATCH v2 6/6] misc: fastrpc: Add support for compat ioctls

[Greg KH <gregkh@linuxfoundation.org>]

On Wed, Dec 12, 2018 at 12:26:26PM +0000, Srinivas Kandagatla wrote:
> 
> 
> > > > 
> > > > What prevents you from doing that and requiring compat support?
> > > > 
> > > I removed most of the compat IOCTLS except this one.
> > > The reason is that this ioctl takes arguments which can vary in number for
> > > each call.
> > 
> > Then do not do that :)
> > 
> > Remember, you get to design the api, fix the structure size to work
> > properly everywhere.
> > 
> > > So args are passed as pointer to structure, rather than fixed
> > > size. I could not find better way to rearrange this to give a fixed size
> > > data structure. In theory number of arguments can vary from 0-255 for both
> > > in & out.
> > > 
> > > current data structure looks like this:
> > > 
> > > struct fastrpc_invoke_args {
> > > 	__s32 fd;
> > > 	size_t length;
> > > 	void *ptr;
> > > };
> > 
> > Make length and ptr both __u64 and you should be fine, right?  If you do
> > that, might as well make fd __u64 as well to align things better.
> > 
> 
> That is fine for the args structure, but below "args" pointer in "struct
> fastrpc_invoke" is still not fixed size, unless we change that to __u64
> pointing to array of struct fastrpc_invoke_args. I have seen such usages in
> i915_drm.h.
> Is that the preferred?

Yes, force all pointers to be __u64 and you should be fine.

thanks,

greg k-h