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

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Message-ID: <YI/wJSwQitisM8Xf@hirez.programming.kicks-ass.net>
Date:   Mon, 3 May 2021 14:44:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Marco Elver <elver@...gle.com>, 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 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.