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

[Greg Kroah-Hartman]

On Thu, Nov 06, 2014 at 09:38:59PM +0000, Måns Rullgård wrote:
> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
> 
> > On Thu, Nov 06, 2014 at 09:01:36PM +0000, Måns Rullgård wrote:
> >> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> writes:
> >> 
> >> > On Thu, Nov 06, 2014 at 08:49:01PM +0000, Måns Rullgård wrote:
> >> >> Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> 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
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