linux-kernel - Re: [PATCH 1/3] siginfo: Move si_trapno inside the union inside _si

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Date:   Sun, 02 May 2021 13:27:21 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Marco Elver <elver@...gle.com>
Cc:     Arnd Bergmann <arnd@...db.de>, Florian Weimer <fweimer@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Collingbourne <pcc@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Alexander Potapenko <glider@...gle.com>,
        sparclinux <sparclinux@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>
Subject: Re: [PATCH 1/3] siginfo: Move si_trapno inside the union inside _si_fault

Marco Elver <elver@...gle.com> writes:

> On Sat, 1 May 2021 at 00:50, Eric W. Biederman <ebiederm@...ssion.com> wrote:
>>
>> It turns out that linux uses si_trapno very sparingly, and as such it
>> can be considered extra information for a very narrow selection of
>> signals, rather than information that is present with every fault
>> reported in siginfo.
>>
>> As such move si_trapno inside the union inside of _si_fault.  This
>> results in no change in placement, and makes it eaiser to extend
>> _si_fault in the future as this reduces the number of special cases.
>> In particular with si_trapno included in the union it is no longer a
>> concern that the union must be pointer alligned on most architectures
>> because the union followes immediately after si_addr which is a
>> pointer.
>>
>
> Maybe add "Link:
> https://lkml.kernel.org/r/CAK8P3a0+uKYwL1NhY6Hvtieghba2hKYGD6hcKx5n8=4Gtt+pHA@mail.gmail.com"
>
>> Signed-off-by: "Eric W. Biederman" <ebiederm@...ssion.com>
>
> Acked-by: Marco Elver <elver@...gle.com>
>
> By no longer guarding it with __ARCH_SI_TRAPNO we run the risk that it
> will be used by something else at some point. Is that intentional?

The motivation was letting the code be tested on other architectures.

But once si_trapno falls inside the union instead of being present for
every signal reporting a fault it doesn't really matter.

I think it would be poor taste but harmless to use si_trapno, mostly
because defining a new entry in the union could be more specific and
well defined.

Eric