linux-kernel - Re: [PATCH v2 4/8] cxl/mem: Add basic IOCTL interface

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Date:   Sun, 14 Feb 2021 15:14:56 -0800
From:   Ben Widawsky <ben.widawsky@...el.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-cxl@...r.kernel.org, linux-acpi@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-nvdimm@...ts.01.org,
        linux-pci@...r.kernel.org, Bjorn Helgaas <helgaas@...nel.org>,
        Chris Browy <cbrowy@...ry-design.com>,
        Christoph Hellwig <hch@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>,
        David Hildenbrand <david@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Ira Weiny <ira.weiny@...el.com>,
        Jon Masters <jcm@...masters.org>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "John Groves (jgroves)" <jgroves@...ron.com>,
        "Kelley, Sean V" <sean.v.kelley@...el.com>,
        kernel test robot <lkp@...el.com>,
        Dan Williams <dan.j.willams@...el.com>
Subject: Re: [PATCH v2 4/8] cxl/mem: Add basic IOCTL interface

On 21-02-14 16:30:09, Al Viro wrote:
> On Tue, Feb 09, 2021 at 04:02:55PM -0800, Ben Widawsky wrote:
> 
> > +static int handle_mailbox_cmd_from_user(struct cxl_memdev *cxlmd,
> > +					const struct cxl_mem_command *cmd,
> > +					u64 in_payload, u64 out_payload,
> > +					struct cxl_send_command __user *s)
> > +{
> > +	struct cxl_mem *cxlm = cxlmd->cxlm;
> > +	struct device *dev = &cxlmd->dev;
> > +	struct mbox_cmd mbox_cmd = {
> > +		.opcode = cmd->opcode,
> > +		.size_in = cmd->info.size_in,
> > +	};
> > +	s32 user_size_out;
> > +	int rc;
> > +
> > +	if (get_user(user_size_out, &s->out.size))
> > +		return -EFAULT;
> 
> You have already copied it in.  Never reread stuff from userland - it *can*
> change under you.

As it turns out, this is some leftover logic which doesn't need to exist at all,
and I'm happy to change it. Thanks for reviewing.

I wasn't familiar with this restriction though. For my edification could you
explain how that could happen? Also, is this something that should go in the
kdocs, because I don't see anything about this restriction there.

Thanks.
Ben