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

Greg Kroah-Hartman
2014-11-06T22:02:55Z

On Thu, Nov 06, 2014 at 09:38:59PM +0000, Måns Rullgård wrote:
> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>
> > 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?
>
> Sequence points say nothing about the order in which stores become
> visible to other CPUs. That's why there are barrier instructions.

Yes, but "order" matters.

If I write code that does:

100 x = ldata->read_head;
101 &ldata->read_head[x & SOME_VALUE] = y;
102 ldata->read_head++;

the compiler can not reorder lines 102 and 101 just because it feels
like it, right? Or is it time to go spend some reading of the C spec
again...

thanks,

greg k-h