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

[Greg KH <gregkh@linuxfoundation.org>]

On Sun, Jan 23, 2022 at 02:31:54PM -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.
> 
> While working on fixing the ppc32 support in librseq [1], I made sure
> all 32-bit little endian architectures stopped depending on little
> endian byte ordering by using the ptr32 field. It led me to discover
> this wrong ptr32 field ordering on little endian.
> 
> Because it is already exposed as a UAPI, all we can do for the existing
> fields is document the wrong behavior and encourage users to use
> alternative mechanisms.
> 
> Introduce a new rseq_cs.arch field with correct field ordering. Use this
> opportunity to improve the layout so accesses to architecture fields on
> both 32-bit and 64-bit architectures are done through the same field
> hierarchy, which is much nicer than the previous scheme.
> 
> The intended use is now:
> 
> * rseq_thread_area->rseq_cs.ptr64: Access the 64-bit value of the rseq_cs
> 				   pointer. Available on all
>                                    architectures (unchanged).
> 
> * rseq_thread_area->rseq_cs.arch.ptr: Access the architecture specific
> 				      layout of the rseq_cs pointer. This
> 				      is a 32-bit field on 32-bit
> 				      architectures, and a 64-bit field on
>                                       64-bit architectures.
> 
> Link: https://git.kernel.org/pub/scm/libs/librseq/librseq.git/ [1]
> 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 | 23 +++++++++++++++++++++++
>  1 file changed, 23 insertions(+)

<formletter>

This is not the correct way to submit patches for inclusion in the
stable kernel tree.  Please read:
    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
for how to do this properly.

</formletter>