Re: [PATCH] n_tty: Add memory barrier to fix race condition in receive path

Greg Kroah-Hartman
2014-11-06T21:17:57Z

On Thu, Nov 06, 2014 at 09:01:36PM +0000, Måns Rullgård wrote:
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>
> > On Thu, Nov 06, 2014 at 08:49:01PM +0000, Måns Rullgård wrote:
> >> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
> >>
> >> > On Thu, Nov 06, 2014 at 12:39:59PM +0100, Christian Riesch wrote:
> >> >> The current implementation of put_tty_queue() causes a race condition
> >> >> when re-arranged by the compiler.
> >> >>
> >> >> On my build with gcc 4.8.3, cross-compiling for ARM, the line
> >> >>
> >> >> *read_buf_addr(ldata, ldata->read_head++) = c;
> >> >>
> >> >> was re-arranged by the compiler to something like
> >> >>
> >> >> x = ldata->read_head
> >> >> ldata->read_head++
> >> >> *read_buf_addr(ldata, x) = c;
> >> >>
> >> >> which causes a race condition. Invalid data is read if data is read
> >> >> before it is actually written to the read buffer.
> >> >
> >> > Really? A compiler can rearange things like that and expect things to
> >> > actually work? How is that valid?
> >>
> >> This is actually required by the C spec. There is a sequence point
> >> before a function call, after the arguments have been evaluated. Thus
> >> all side-effects, such as the post-increment, must be complete before
> >> the function is called, just like in the example.
> >>
> >> There is no "re-arranging" here. The code is simply wrong.
> >
> > Ah, ok, time to dig out the C spec...
> >
> > Anyway, because of this, no need for the wmb() calls, just rearrange the
> > logic and all should be good, right? Christian, can you test that
> > instead?
>
> Weakly ordered SMP systems probably need some kind of barrier. I didn't
> look at it carefully.

It shouldn't need a barier, as it is a sequence point with the function
call. Well, it's an inline function, but that "shouldn't" matter here,
right?

thanks,

greg k-h