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    SubjectRe: [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@efficios.com>
    > Cc: Florian Weimer <fw@deneb.enyo.de>
    > Cc: Thomas Gleixner <tglx@linutronix.de>
    > Cc: linux-api@vger.kernel.org
    > Cc: Peter Zijlstra <peterz@infradead.org>
    > Cc: Boqun Feng <boqun.feng@gmail.com>
    > Cc: Andy Lutomirski <luto@amacapital.net>
    > Cc: Dave Watson <davejwatson@fb.com>
    > Cc: Paul Turner <pjt@google.com>
    > Cc: Andrew Morton <akpm@linux-foundation.org>
    > Cc: Russell King <linux@arm.linux.org.uk>
    > Cc: "H . Peter Anvin" <hpa@zytor.com>
    > Cc: Andi Kleen <andi@firstfloor.org>
    > Cc: Christian Brauner <christian.brauner@ubuntu.com>
    > Cc: Ben Maurer <bmaurer@fb.com>
    > Cc: Steven Rostedt <rostedt@goodmis.org>
    > Cc: Josh Triplett <josh@joshtriplett.org>
    > Cc: Linus Torvalds <torvalds@linux-foundation.org>
    > Cc: Catalin Marinas <catalin.marinas@arm.com>
    > Cc: Will Deacon <will.deacon@arm.com>
    > Cc: Michael Kerrisk <mtk.manpages@gmail.com>
    > Cc: Joel Fernandes <joelaf@google.com>
    > Cc: Paul E. McKenney <paulmck@kernel.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.

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