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

Måns Rullgård
2014-11-06T20:49:08Z

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.

--
Måns Rullgård
mans@mansr.com