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

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@arm.com 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.