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

[Catalin Marinas]

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.

-- 
Catalin