linux-kernel - Re: [PATCH] seccomp: rework define for SECCOMP_USER_NOTIF_FLAG

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Date:   Mon, 28 Oct 2019 12:27:40 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-kernel@...r.kernel.org, luto@...capital.net
Subject: Re: [PATCH] seccomp: rework define for
 SECCOMP_USER_NOTIF_FLAG_CONTINUE

On Thu, Oct 24, 2019 at 11:25:39PM +0200, Christian Brauner wrote:
> Switch from BIT(0) to (1UL << 0).
> First, there are already two different forms used in the header, so there's
> no need to add a third. Second, the BIT() macros is kernel internal and
> afaict not actually exposed to userspace. Maybe there's some magic there
> I'm missing but it definitely causes issues when compiling a program that
> tries to use SECCOMP_USER_NOTIF_FLAG_CONTINUE. It currently fails in the
> following way:
> 
> 	# github.com/lxc/lxd/lxd
> 	/usr/bin/ld: $WORK/b001/_x003.o: in function
> 	`__do_user_notification_continue':
> 	lxd/main_checkfeature.go:240: undefined reference to `BIT'
> 	collect2: error: ld returned 1 exit status
> 
> Switching to (1UL << 0) should prevent that and is more in line what is
> already done in the rest of the header.

Hmpf. I thought those were already exported into the UAPI. Ah well.
Thanks! Applied.

-Kees

> 
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Andy Lutomirski <luto@...capital.net>
> Signed-off-by: Christian Brauner <christian.brauner@...ntu.com>
> ---
>  include/uapi/linux/seccomp.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
> index 61fbbb7c1ee9..9099972200cd 100644
> --- a/include/uapi/linux/seccomp.h
> +++ b/include/uapi/linux/seccomp.h
> @@ -102,7 +102,7 @@ struct seccomp_notif {
>   * SECCOMP_USER_NOTIF_FLAG_CONTINUE. Note that SECCOMP_RET_USER_NOTIF can
>   * equally be overriden by SECCOMP_USER_NOTIF_FLAG_CONTINUE.
>   */
> -#define SECCOMP_USER_NOTIF_FLAG_CONTINUE BIT(0)
> +#define SECCOMP_USER_NOTIF_FLAG_CONTINUE (1UL << 0)
>  
>  struct seccomp_notif_resp {
>  	__u64 id;
> -- 
> 2.23.0
> 

-- 
Kees Cook