Re: [PATCH v16 16/16] selftests, arm64: add a selftest for passing tagged pointers to kernel

[Andrey Konovalov]

On Tue, Jun 11, 2019 at 7:50 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
>
> On Tue, Jun 11, 2019 at 07:18:04PM +0200, Andrey Konovalov wrote:
> > On Tue, Jun 11, 2019 at 5:01 PM Catalin Marinas <catalin.marinas@xxxxxxx> wrote:
> > > static void *tag_ptr(void *ptr)
> > > {
> > >         static int tagged_addr_err = 1;
> > >         unsigned long tag = 0;
> > >
> > >         if (tagged_addr_err == 1)
> > >                 tagged_addr_err = prctl(PR_SET_TAGGED_ADDR_CTRL,
> > >                                         PR_TAGGED_ADDR_ENABLE, 0, 0, 0);
> >
> > I think this requires atomics. malloc() can be called from multiple threads.
>
> It's slightly racy but I assume in a real libc it can be initialised
> earlier than the hook calls while still in single-threaded mode (I had
> a quick attempt with __attribute__((constructor)) but didn't get far).
>
> Even with the race, under normal circumstances calling the prctl() twice
> is not a problem. I think the risk here is that someone disables the ABI
> via sysctl and the ABI is enabled for some of the threads only.

OK, I'll keep the code racy, but add a comment pointing it out. Thanks!

>
> --
> Catalin