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From:   Arnd Bergmann <arnd@arndb.de>
Date:   Fri, 30 Apr 2021 22:43:09 +0200
Message-ID: <CAK8P3a3mb6X4+4Q1WBQp22O2Bvc3w-Q=L25jh+WPvp2kJFwHiQ@mail.gmail.com>
Subject: Re: siginfo_t ABI break on sparc64 from si_addr_lsb move 3y ago
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Marco Elver <elver@google.com>,
        Florian Weimer <fweimer@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Collingbourne <pcc@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Alexander Potapenko <glider@google.com>,
        sparclinux <sparclinux@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        kasan-dev <kasan-dev@googlegroups.com>
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On Fri, Apr 30, 2021 at 7:08 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
>
> The code is only safe if the analysis that says we can move si_trapno
> and perhaps the ia64 fields into the union is correct.  It looks like
> ia64 much more actively uses it's signal extension fields including for
> SIGTRAP, so I am not at all certain the generic definition of
> perf_sigtrap is safe on ia64.
>
> > I suppose in theory sparc64 or alpha might start using the other
> > fields in the future, and an application might be compiled against
> > mismatched headers, but that is unlikely and is already broken
> > with the current headers.
>
> If we localize the use of si_trapno to just a few special cases on alpha
> and sparc I think we don't even need to worry about breaking userspace
> on any architecture.  It will complicate siginfo_layout, but it is a
> complication that reflects reality.

Ok.

> I don't have a clue how any of this affects ia64.  Does perf work on
> ia64?  Does perf work on sparc, and alpha?
>
> If perf works on ia64 we need to take a hard look at what is going on
> there as well.

ia64 never had perf support. It had oprofile until very recently, and it
had a custom thing before that. My feeling is that it's increasingly
unlikely to ever gain perf support in the future, given that oprofile
(in user space) has required kernel perf support (in kernel) for a
long time and nobody cared about that being broken either.

sparc64 has perf support for Sun UltraSPARC 3/3+/3i/4+/T1/T2/T3
and Oracle SPARC T4/T5/M7 but lacks support for most CPUs from
Oracle, Fujitsu and the rest, in particular anything from the last
ten years.
Alpha has perf support for EV67, EV68, EV7, EV79, and EV69, i.e.
anything from 1996 to the end in 2004.

      Arnd