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

Greg Kroah-Hartman
2014-11-06T20:56:48Z

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?

thanks,

greg k-h