On Thu, 03 Dec 2015, Arnd Bergmann wrote:
> 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.
Glibc is responsible indeed:
http://osxr.org/glibc/source/io/write.c
--
Lee Jones
Linaro STMicroelectronics Landing Team Lead
Linaro.org │ Open source software for ARM SoCs
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