linux-kernel - Re: [RFC PATCH 02/15] rseq: Remove broken uapi field layout on 32-bit little endian

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Date:   Tue, 25 Jan 2022 14:00:48 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Christian Brauner <brauner@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api <linux-api@...r.kernel.org>, shuah <shuah@...nel.org>,
        linux-kselftest <linux-kselftest@...r.kernel.org>,
        Florian Weimer <fw@...eb.enyo.de>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Andi Kleen <andi@...stfloor.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Ben Maurer <bmaurer@...com>, rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH 02/15] rseq: Remove broken uapi field layout on
 32-bit little endian

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

> ----- On Jan 25, 2022, at 7:21 AM, Christian Brauner brauner@...nel.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