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Date:   Mon, 3 May 2021 14:44:21 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Marco Elver <elver@google.com>, Arnd Bergmann <arnd@arndb.de>,
        Florian Weimer <fweimer@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        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>
Subject: Re: [PATCH 7/3] signal: Deliver all of the perf_data in si_perf
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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.