linux-kernel - Re: [PATCH v1 02/14] futex: Extend the FUTEX2 flags

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Date:   Mon, 31 Jul 2023 23:14:11 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     axboe@...nel.dk, linux-kernel@...r.kernel.org, mingo@...hat.com,
        dvhart@...radead.org, dave@...olabs.net, andrealmeid@...lia.com,
        Andrew Morton <akpm@...ux-foundation.org>, urezki@...il.com,
        hch@...radead.org, lstoakes@...il.com,
        Arnd Bergmann <arnd@...db.de>, linux-api@...r.kernel.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org,
        malteskarupke@....de
Subject: Re: [PATCH v1 02/14] futex: Extend the FUTEX2 flags

On Mon, Jul 31 2023 at 21:20, Peter Zijlstra wrote:
> -#define FUTEX2_MASK (FUTEX2_64 | FUTEX2_PRIVATE)
> +#define FUTEX2_MASK (FUTEX2_SIZE_MASK | FUTEX2_PRIVATE)

Along with some comment which documents that the size "flags" constitute
a number field and not flags in the sense of binary flags.

And please name these size constants so it really becomes obvious:

#define FUTEX2_SIZE_U32		2

>  /**
>   * futex_parse_waitv - Parse a waitv array from userspace
> @@ -208,11 +208,11 @@ static int futex_parse_waitv(struct fute
>  			return -EINVAL;
>  
>  		if (!IS_ENABLED(CONFIG_64BIT) || in_compat_syscall()) {
> -			if ((aux.flags & FUTEX2_64) == FUTEX2_64)
> +			if ((aux.flags & FUTEX2_SIZE_MASK) == FUTEX2_64)
>  				return -EINVAL;
>  		}

That should be part of the actual 64bit futex enablement, no?
  
> -		if ((aux.flags & FUTEX2_64) != FUTEX2_32)
> +		if ((aux.flags & FUTEX2_SIZE_MASK) != FUTEX2_32)
>  			return -EINVAL;

In hindsight I think it was as mistake just to have this __u32 flags
field in the new interface. Soemthing like the incomplete below might be
retrofittable, no?

--- a/include/uapi/linux/futex.h
+++ b/include/uapi/linux/futex.h
@@ -74,7 +74,12 @@
 struct futex_waitv {
 	__u64 val;
 	__u64 uaddr;
-	__u32 flags;
+	union {
+		__u32	flags;
+		__u32	size	: 2,
+				: 5,
+			private	: 1;
+	};
 	__u32 __reserved;
 };
 
--- a/kernel/futex/syscalls.c
+++ b/kernel/futex/syscalls.c
@@ -204,10 +204,10 @@ static int futex_parse_waitv(struct fute
 		if (copy_from_user(&aux, &uwaitv[i], sizeof(aux)))
 			return -EFAULT;
 
-		if ((aux.flags & ~FUTEX2_MASK) || aux.__reserved)
+		if ((aux.flags & ~FUTEX2_VALID_FLAGBITS) || aux.__reserved)
 			return -EINVAL;
 
-		if (!(aux.flags & FUTEX2_32))
+		if (aux.size != FUTEX2_SIZE_U32)
 			return -EINVAL;
 
 		futexv[i].w.flags = aux.flags;


If this muck already confused me right now, then I don't want to
experience the confusion factor 6 month down the road :)

Thanks,

        tglx