Re: [PATCH v3] getrandom system call wrapper [BZ #17252]

[Florian Weimer <fweimer at redhat dot com>]

On 09/09/2016 04:21 PM, Torvald Riegel wrote:
> On Thu, 2016-09-08 at 13:44 +0200, Florian Weimer wrote:
> > I have made the system call wrapper a cancellation point.  (If we
> > implement the simpler getentropy interface, it would not be a
> > cancellation point.)
>
> Why did you do that?

I have to, because it can block indefinitely.

> Even though the system call is new, and thus can't
> have been used in existing code directly, making it a cancellation point
> will make all callers cancellation points too.  Therefore, for example,
> we couldn't use it in the implementation of any POSIX functions (that
> are not cancellation points) in glibc without having to disable and
> restore the cancellation state around it every time.

The system call definition facility also provides a 
__getrandom_notcancel entry point, which can be called if we don't 
expect blocking.  Obviously, this is for internal use only.

> It might be even more convenient to have one wrapper that is a
> cancellation point and one that is not.
>
> Can't we just let cancellation rot in its corner?

No, we have many customers who use it (and this despite the fact that 
the current implementation has a critical race condition).

Synchronous cancellation is often easier to use than the alternative 
because our API does not allow waiting for arbitrary combination of events.

Asynchronous cancellation is not very useful, though.

Thanks,
Florian