linux-kernel - Re: [PATCH V8 30/44] mm/pkeys: Test setting a PKS key in a custom fault callback

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Date:   Tue, 1 Mar 2022 07:39:02 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "hpa@...or.com" <hpa@...or.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V8 30/44] mm/pkeys: Test setting a PKS key in a custom
 fault callback

On Mon, Jan 31, 2022 at 04:55:47PM -0800, Edgecombe, Rick P wrote:
> On Thu, 2022-01-27 at 09:54 -0800, ira.weiny@...el.com wrote:
> > Add a test which does this.
> >
> >         $ echo 5 > /sys/kernel/debug/x86/run_pks
> >         $ cat /sys/kernel/debug/x86/run_pks
> >         PASS
> 
> Hmm, when I run this on qemu TCG, I get:
> 
> root@(none):/# echo 5 > /sys/kernel/debug/x86/run_pks
> [   29.438159] pks_test: Failed to see the callback
> root@(none):/# cat /sys/kernel/debug/x86/run_pks
> FAIL
> 
> I think it's a problem with the test though. The generated code is not
> expecting fault_callback_ctx.callback_seen to get changed in the
> exception. The following fixed it for me:
> 
> diff --git a/lib/pks/pks_test.c b/lib/pks/pks_test.c
> index 1528df0bb283..d979d2afe921 100644
> --- a/lib/pks/pks_test.c
> +++ b/lib/pks/pks_test.c
> @@ -570,6 +570,7 @@ static bool run_fault_clear_test(void)
>         /* fault */
>         memcpy(test_page, ctx->data, 8);
> 
> +       barrier();
>         if (!fault_callback_ctx.callback_seen) {
>                 pr_err("Failed to see the callback\n");
>                 rc = false;
> 
> But, I wonder if volatile is also needed on the read to be fully
> correct. I usually have to consult the docs when I deal with that
> stuff...

I was not able to reproduce this.  However, I've done a lot of reading and I
think you are correct that the barrier is needed.  I thought WRITE_ONCE was
sufficient and I had used it in other calls but I missed it here.

As part of the test rework I've added a call to barrier() for all the tests.
In addition I've simplified, and hopefully clarified, which variables are
being shared with the fault handler.

Thanks for the testing and review!
Ira