From: Arnd Bergmann <arnd@arndb.de>
To: Lee Jones <lee.jones@linaro.org>
Cc: linux-arm-kernel@lists.infradead.org, Bjorn Andersson <bjorn@kryo.se>,
        Ohad Ben-Cohen <ohad@wizery.com>,
        "devicetree@vger.kernel.org" <devicetree@vger.kernel.org>,
        Florian Fainelli <f.fainelli@gmail.com>, kernel@stlinux.com,
        Nathan_Lynch@mentor.com,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        ludovic.barre@st.com, Maxime Coquelin <maxime.coquelin@st.com>
Subject: Re: [RESEND v4 2/6] remoteproc: debugfs: Add ability to boot remote processor using debugfs
Date: Thu, 03 Dec 2015 14:19:47 +0100
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On Thursday 03 December 2015 13:03:41 Lee Jones wrote:
> On Thu, 03 Dec 2015, Arnd Bergmann wrote:
> 
> > On Thursday 03 December 2015 12:26:34 Lee Jones wrote:
> > > > >
> > > > > +static ssize_t rproc_state_write(struct file *filp, const char __user *userbuf,
> > > > > +                                size_t count, loff_t *ppos)
> > > > > +{
> > > > > +       struct rproc *rproc = filp->private_data;
> > > > > +       char buf[10];
> > > > > +       int ret;
> > > > > +
> > > > > +       if (count > sizeof(buf))
> > > > > +               return count;
> > > > > +       ret = copy_from_user(buf, userbuf, count);
> > > > > +       if (ret)
> > > > > +               return -EFAULT;
> > > > > +
> > > > > +       if (buf[count - 1] == '\n')
> > > > > +               buf[count - 1] = '\0';
> > > > 
> > > > I believe you can get here with count = 0.
> > > 
> > > I'm pretty sure you can't.
> > > 
> > > If you are sure that you can, if you can provide me with a way of
> > > testing, I'd be happy to put in provisions.
> > > 
> > 
> > I think that a zero-length write() from user space ends up in the write
> > file operation.
> 
> I tested this and didn't see it enter write().  My conclusion was that
> if the file doesn't change, then nothing is triggered.
> 

Ah, interesting. I haven't tried myself, and just tried to read the
code. Maybe glibc already catches zero-length writes before it gets
into the kernel, or I just missed the part of the syscall that checks
for this.

	arnd
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