Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 25 Jan 2022 14:00:48 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Christian Brauner <brauner@kernel.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        paulmck <paulmck@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api <linux-api@vger.kernel.org>, shuah <shuah@kernel.org>,
        linux-kselftest <linux-kselftest@vger.kernel.org>,
        Florian Weimer <fw@deneb.enyo.de>,
        Andy Lutomirski <luto@amacapital.net>,
        Dave Watson <davejwatson@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Andi Kleen <andi@firstfloor.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Ben Maurer <bmaurer@fb.com>, rostedt <rostedt@goodmis.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Joel Fernandes <joelaf@google.com>
Message-ID: <1445357149.71067.1643137248305.JavaMail.zimbra@efficios.com>
In-Reply-To: <1234069751.70438.1643121673355.JavaMail.zimbra@efficios.com>
References: <20220124171253.22072-1-mathieu.desnoyers@efficios.com> <20220124171253.22072-3-mathieu.desnoyers@efficios.com> <20220125122156.v2f5anzcs35i3rii@wittgenstein> <1234069751.70438.1643121673355.JavaMail.zimbra@efficios.com>
Subject: Re: [RFC PATCH 02/15] rseq: Remove broken uapi field layout on
 32-bit little endian
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 7bit
Thread-Topic: rseq: Remove broken uapi field layout on 32-bit little endian
Thread-Index: jBpzgk+GT1oWkc+mziLmJCxydp6hDf1GHpYP
Precedence: bulk

----- On Jan 25, 2022, at 9:41 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:

> ----- On Jan 25, 2022, at 7:21 AM, Christian Brauner brauner@kernel.org wrote:
[...]
>>>  include/uapi/linux/rseq.h | 17 ++++-------------
[...]
>>>  	union {
>> 
>> A bit unfortunate we seem to have to keep the union around even though
>> it's just one field now.
> 
> Well, as far as the user-space projects that I know of which use rseq
> are concerned (glibc, librseq, tcmalloc), those end up with their own
> copy of the uapi header anyway to deal with the big/little endian field
> on 32-bit. So I'm very much open to remove the union if we accept that
> this uapi header is really just meant to express the ABI and is not
> expected to be used as an API by user-space.
> 
> That would mean we also bring a uapi header copy into the kernel
> rseq selftests as well to minimize the gap between librseq and
> the kernel sefltests (the kernel sefltests pretty much include a
> copy of librseq for convenience. librseq is maintained out of tree).
> 
> Thoughts ?

Actually, if we go ahead and remove the union, and replace:

struct rseq {
  union {
    __u64 ptr64;
  } rseq_cs;
[...]
} v;

by:

struct rseq {
  __u64 rseq_cs;
} v;

expressions such as these are unchanged:

- sizeof(v.rseq_cs),
- &v.rseq_cs,
- __alignof__(v.rseq_cs),
- offsetof(struct rseq, rseq_cs).

So users of the uapi rseq.h (as an API) can still use rseq_abi->rseq_cs before
and after the change.

Based on this, I am inclined to remove the union, and just make the rseq_cs field
a __u64.

Any objections ?

Thanks,

Mathieu


> 
> Thanks,
> 
> Mathieu
> 
> 
> --
> Mathieu Desnoyers
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com