Re: [RFC PATCH v2] rseq: Remove broken uapi field layout on 32-bit little endian

[Christian Brauner <brauner@kernel.org>]

On Thu, Jan 27, 2022 at 10:27:20AM -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 replace the whole rseq_cs union by a __u64 type, as
> this is the only thing really needed to express the ABI. Document how
> 32-bit architectures are meant to interact with this 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>
> ---

Looks way cleaner now! Fwiw,
Acked-by: Christian Brauner <brauner@kernel.org>

>  include/uapi/linux/rseq.h | 20 ++++----------------
>  kernel/rseq.c             |  8 ++++----
>  2 files changed, 8 insertions(+), 20 deletions(-)
> 
> diff --git a/include/uapi/linux/rseq.h b/include/uapi/linux/rseq.h
> index 9a402fdb60e9..77ee207623a9 100644
> --- a/include/uapi/linux/rseq.h
> +++ b/include/uapi/linux/rseq.h
> @@ -105,23 +105,11 @@ 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 field, leaving the high order bits initialized to 0.
>  	 */
> -	union {
> -		__u64 ptr64;
> -#ifdef __LP64__
> -		__u64 ptr;
> -#else
> -		struct {
> -#if (defined(__BYTE_ORDER) && (__BYTE_ORDER == __BIG_ENDIAN)) || defined(__BIG_ENDIAN)
> -			__u32 padding;		/* Initialized to zero. */
> -			__u32 ptr32;
> -#else /* LITTLE */
> -			__u32 ptr32;
> -			__u32 padding;		/* Initialized to zero. */
> -#endif /* ENDIAN */
> -		} ptr;
> -#endif
> -	} rseq_cs;
> +	__u64 rseq_cs;
>  
>  	/*
>  	 * Restartable sequences flags field.
> diff --git a/kernel/rseq.c b/kernel/rseq.c
> index 6d45ac3dae7f..97ac20b4f738 100644
> --- a/kernel/rseq.c
> +++ b/kernel/rseq.c
> @@ -128,10 +128,10 @@ static int rseq_get_rseq_cs(struct task_struct *t, struct rseq_cs *rseq_cs)
>  	int ret;
>  
>  #ifdef CONFIG_64BIT
> -	if (get_user(ptr, &t->rseq->rseq_cs.ptr64))
> +	if (get_user(ptr, &t->rseq->rseq_cs))
>  		return -EFAULT;
>  #else
> -	if (copy_from_user(&ptr, &t->rseq->rseq_cs.ptr64, sizeof(ptr)))
> +	if (copy_from_user(&ptr, &t->rseq->rseq_cs, sizeof(ptr)))
>  		return -EFAULT;
>  #endif
>  	if (!ptr) {
> @@ -217,9 +217,9 @@ static int clear_rseq_cs(struct task_struct *t)
>  	 * Set rseq_cs to NULL.
>  	 */
>  #ifdef CONFIG_64BIT
> -	return put_user(0UL, &t->rseq->rseq_cs.ptr64);
> +	return put_user(0UL, &t->rseq->rseq_cs);
>  #else
> -	if (clear_user(&t->rseq->rseq_cs.ptr64, sizeof(t->rseq->rseq_cs.ptr64)))
> +	if (clear_user(&t->rseq->rseq_cs, sizeof(t->rseq->rseq_cs)))
>  		return -EFAULT;
>  	return 0;
>  #endif
> -- 
> 2.17.1
>