linux-kernel - Re: [PATCH 7/3] signal: Deliver all of the perf_data in si

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Date:   Mon, 3 May 2021 21:53:39 +0200
From:   Marco Elver <elver@...gle.com>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Arnd Bergmann <arnd@...db.de>,
        Florian Weimer <fweimer@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        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 7/3] signal: Deliver all of the perf_data in si_perf

On Mon, 3 May 2021 at 21:38, Eric W. Biederman <ebiederm@...ssion.com> wrote:
> Peter Zijlstra <peterz@...radead.org> writes:
>
> > On Sun, May 02, 2021 at 01:39:16PM -0500, Eric W. Biederman wrote:
> >
> >> The one thing that this doesn't do is give you a 64bit field
> >> on 32bit architectures.
> >>
> >> On 32bit builds the layout is:
> >>
> >>      int si_signo;
> >>      int si_errno;
> >>      int si_code;
> >>      void __user *_addr;
> >>
> >> So I believe if the first 3 fields were moved into the _sifields union
> >> si_perf could define a 64bit field as it's first member and it would not
> >> break anything else.
> >>
> >> Given that the data field is 64bit that seems desirable.
> >
> > The data field is fundamentally an address, it is internally a u64
> > because the perf ring buffer has u64 alignment and it saves on compat
> > crap etc.
> >
> > So for the 32bit/compat case the high bits will always be 0 and
> > truncating into an unsigned long is fine.
>
> I see why it is fine to truncate the data field into an unsigned long.
>
> Other than technical difficulties in extending siginfo_t is there any
> reason not to define data as a __u64?

No -- like I pointed at earlier, si_perf used to be __u64, but we
can't because of the siginfo_t limitation. What we have now is fine,
and not worth dwelling over given siginfo limitations.

Thanks,
-- Marco