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

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Date:   Wed, 12 Dec 2018 12:50:37 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
Cc:     robh+dt@...nel.org, arnd@...db.de, mark.rutland@....com,
        devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
        bjorn.andersson@...aro.org, linux-arm-msm@...r.kernel.org,
        bkumar@....qualcomm.com, thierry.escande@...aro.org
Subject: Re: [PATCH v2 6/6] misc: fastrpc: Add support for compat ioctls

On Wed, Dec 12, 2018 at 11:44:05AM +0000, Srinivas Kandagatla wrote:
> Thanks for the comments,
> 
> On 12/12/18 10:59, Greg KH wrote:
> > > This patch adds support for compat ioctl from 32 bits userland to
> > > Qualcomm fastrpc driver.
> > Ick, why?
> > 
> > Why not just fix up your ioctl structures to not need that at all?  For
> > new code being added to the kernel, there's no excuse to have to have
> > compat structures anymore, just make your api sane and all should be
> > fine.
> 
> Yes, I did try that after comments from Arnd,
> 
> 
> > 
> > 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.

> struct fastrpc_invoke {
> 	__u32 handle;
> 	__u32 sc;
> 	struct fastrpc_invoke_args *args;
> };
> 
> Other option is to split this IOCTL into 3 parts
> SET_INVOKE_METHOD, SET_INVOKE_ARGS and INVOKE
> with some kinda handle to bind these three.
> 
> with below structures:
> struct fastrpc_invoke {
> 	__u32 handle;
> 	__u32 sc;
> };
> 
> struct fastrpc_invoke_arg {
> 	__s32 fd;
> 	__u64 length;
> 	__u64 ptr;
> };
> I can try this and see how this works before I send next version of patch!

No need to have 3 calls, just change your one structure and you should
be fine.

thanks,

greg k-h