linux-kernel - Re: [RFC PATCH] rseq: Fix broken uapi field layout on 32-bit little endian

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Message-ID: <1704063895.68277.1643024990042.JavaMail.zimbra@efficios.com>
Date:   Mon, 24 Jan 2022 06:49:50 -0500 (EST)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Linus Torvalds <torvalds@...ux-foundation.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>,
        stable <stable@...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>,
        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] rseq: Fix broken uapi field layout on 32-bit little
 endian

----- On Jan 24, 2022, at 2:42 AM, Linus Torvalds torvalds@...ux-foundation.org wrote:

> On Sun, Jan 23, 2022 at 9:32 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> The rseq rseq_cs.ptr.{ptr32,padding} uapi endianness handling is
>> entirely wrong on 32-bit little endian: a preprocessor logic mistake
>> wrongly uses the big endian field layout on 32-bit little endian
>> architectures.
>>
>> Fortunately, those ptr32 accessors were never used within the kernel,
>> and only meant as a convenience for user-space.
> 
> Please don't double down on something that was already broken once.
> 
> Just remove the broken 32-bit one entirely that the kernel doesn't
> even use, and make everybody use
> 
>   __u64 ptr64;
> 
> and be done with it.

OK, should I just leave:

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

and remove all the other content from the union, so users of
rseq_abi->rseq_cs.ptr64 will continue to work as-is with either
old and new headers ? This keeps a union in place with a single
element, so I just want to confirm with you that is what you
have in mind.

It does make tons of sense to just remove the broken convenience
code and let user-space handle this based on the ptr64 field, so
it will work fine with old and new headers.

Thanks for your feedback, and travel safe!

Mathieu

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