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

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Date:   Mon, 03 May 2021 14:38:39 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Peter Zijlstra <peterz@...radead.org>
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

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?

Eric