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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [patch V4 part 1 29/36] x86/mce: Send #MC singal from task work
Date:   Thu, 14 May 2020 09:19:32 -0700
Message-Id: <19476304-8EA8-4B68-9894-3056308035EE@amacapital.net>
References: <1278452779.21179.1589472210183.JavaMail.zimbra@efficios.com>
Cc:     Borislav Petkov <bp@alien8.de>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        x86 <x86@kernel.org>, paulmck <paulmck@kernel.org>,
        Andy Lutomirski <luto@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>, rostedt <rostedt@goodmis.org>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <JGross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Tony Luck <tony.luck@intel.com>
In-Reply-To: <1278452779.21179.1589472210183.JavaMail.zimbra@efficios.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On May 14, 2020, at 9:03 AM, Mathieu Desnoyers <mathieu.desnoyers@efficios=
.com> wrote:
>=20
> =EF=BB=BF----- On May 14, 2020, at 10:17 AM, Borislav Petkov bp@alien8.de w=
rote:
>=20
>> + Tony.
>>=20
>>> On Tue, May 05, 2020 at 03:16:31PM +0200, Thomas Gleixner wrote:
>>> From: Peter Zijlstra <peterz@infradead.org>
>>>=20
>>> Convert #MC over to using task_work_add(); it will run the same code
>>> slightly later, on the return to user path of the same exception.
>>>=20
>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>>> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
>>> Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
>>> ---
>>> arch/x86/kernel/cpu/mce/core.c |   56 ++++++++++++++++++++++------------=
-------
>>> include/linux/sched.h          |    6 ++++
>>> 2 files changed, 37 insertions(+), 25 deletions(-)
>>=20
>> I like this:
>>=20
>> Reviewed-by: Borislav Petkov <bp@suse.de>
>=20
> What I am not fully grasping here is whether this patch preserves the inst=
ruction
> pointer (and possibly other relevant information for siginfo_t) triggering=
 the
> exception in a scenario where we have:
>=20
> - #MC triggered, queuing task work,
> - unrelated signal happens to be delivered to task,
> - exit to usermode loop handles do_signal first,
> - then it runs task work.

If anyone wants to ponder this, I suspect that we have lots of delightful bu=
gs in our handling of cr2, trapnr, and error_code in signals. We should move=
 them to the sigcontext, at least in kernel, and fix up ucontext when we del=
iver the signal.  The current code can=E2=80=99t possibly be correct.=