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 13:21:56 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        "H . Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        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>,
        Steven 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 Mon, Jan 24, 2022 at 12:12:40PM -0500, Mathieu Desnoyers 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.
> 
> Remove those and only leave the "ptr64" union field, as this is the only
> thing really needed to express the ABI. Document how 32-bit
> architectures are meant to interact with this "ptr64" union field.
> 
> Fixes: ec9c82e03a74 ("rseq: uapi: Declare rseq_cs field as union, update includes")
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> Cc: Florian Weimer <fw@...eb.enyo.de>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: linux-api@...r.kernel.org
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Boqun Feng <boqun.feng@...il.com>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Dave Watson <davejwatson@...com>
> Cc: Paul Turner <pjt@...gle.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Russell King <linux@....linux.org.uk>
> Cc: "H . Peter Anvin" <hpa@...or.com>
> Cc: Andi Kleen <andi@...stfloor.org>
> Cc: Christian Brauner <christian.brauner@...ntu.com>
> Cc: Ben Maurer <bmaurer@...com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Cc: Will Deacon <will.deacon@....com>
> Cc: Michael Kerrisk <mtk.manpages@...il.com>
> Cc: Joel Fernandes <joelaf@...gle.com>
> Cc: Paul E. McKenney <paulmck@...nel.org>
> ---
>  include/uapi/linux/rseq.h | 17 ++++-------------
>  1 file changed, 4 insertions(+), 13 deletions(-)
> 
> diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
> index 9a402fdb60e9..31290f2424a7 100644
> --- a/include/uapi/linux/rseq.h
> +++ b/include/uapi/linux/rseq.h
> @@ -105,22 +105,13 @@ struct rseq {
>  	 * Read and set by the kernel. Set by user-space with single-copy
>  	 * atomicity semantics. This field should only be updated by the
>  	 * thread which registered this data structure. Aligned on 64-bit.
> +	 *
> +	 * 32-bit architectures should update the low order bits of the
> +	 * rseq_cs.ptr64 field, leaving the high order bits initialized
> +	 * to 0.
>  	 */
>  	union {

A bit unfortunate we seem to have to keep the union around even though
it's just one field now.