linux-kernel - Re: [PATCH] KVM: s390: add explicit padding to struct kvm_s390

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Message-ID: <20260206113319.49a600f2@p-imbrenda>
Date: Fri, 6 Feb 2026 11:33:19 +0100
From: Claudio Imbrenda <imbrenda@...ux.ibm.com>
To: Arnd Bergmann <arnd@...nel.org>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Heiko Carstens <hca@...ux.ibm.com>,
        Arnd Bergmann <arnd@...db.de>, Oliver Upton <oupton@...nel.org>,
        Sean
 Christopherson <seanjc@...gle.com>,
        Anup Patel <anup@...infault.org>,
        Binbin Wu <binbin.wu@...ux.intel.com>, Marc Zyngier <maz@...nel.org>,
        Jiaqi
 Yan <jiaqiyan@...gle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: s390: add explicit padding to struct
 kvm_s390_keyop

On Fri,  6 Feb 2026 10:17:30 +0100
Arnd Bergmann <arnd@...nel.org> wrote:

> From: Arnd Bergmann <arnd@...db.de>
> 
> The newly added structure causes a warning about implied padding:
> 
> ./usr/include/linux/kvm.h:1247:1: error: padding struct size to alignment boundary with 6 bytes [-Werror=padded]
> 
> The padding can lead to leaking kernel data and ABI incompatibilies
> when used on x86. Neither of these is a problem in this specific
> patch, but it's best to avoid it and use explicit padding fields
> in general.
> 
> Fixes: 0ee4ddc1647b ("KVM: s390: Storage key manipulation IOCTL")
> Signed-off-by: Arnd Bergmann <arnd@...db.de>

Reviewed-by: Claudio Imbrenda <imbrenda@...ux.ibm.com>

> ---
> I have a long series to add annotations to all existing structures,
> but that will take a while. For now, I'm sending fixes when new
> instances show up in linux-next.
> ---
>  include/uapi/linux/kvm.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index a68b1741045c..1225fbd017e5 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -1255,6 +1255,7 @@ struct kvm_s390_keyop {
>  	__u64 guest_addr;
>  	__u8  key;
>  	__u8  operation;
> +	__u8  pad[6];
>  };
>  
>  /*