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

Måns Rullgård
2014-11-06T21:01:41Z

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.

--
Måns Rullgård
mans@mansr.com