linux-kernel - Re: [PATCH v2] x86/hyper-v: Mark TLFS structures packed

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Date:   Fri, 30 Nov 2018 22:21:35 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>
Cc:     kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Roman Kagan <rkagan@...tuozzo.com>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>, x86@...nel.org,
        "Michael Kelley (EOSG)" <Michael.H.Kelley@...rosoft.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] x86/hyper-v: Mark TLFS structures packed

> On Nov 30, 2018, at 4:54 AM, Vitaly Kuznetsov <vkuznets@...hat.com> wrote:
> 
> The TLFS structures are used for hypervisor-guest communication and must
> exactly meet the specification.
> 
> Compilers can add alignment padding to structures or reorder struct members
> for randomization and optimization, which would break the hypervisor ABI.
> 
> Mark the structures as packed to prevent this.

Seems good to me (I made sure you remembered to set __packed for the nested
structs ;-) )

If needed:

Acked-by: Nadav Amit <nadav.amit@...il.com>


Having said that, something else is sort of strange in the TLFS definitions,
I think (I really know little about this whole protocol). Look at the
following definitions from hyperv-tlfs.h:

> struct hv_vpset {
>         u64 format;
>         u64 valid_bank_mask;
>         u64 bank_contents[];
> };
> 
> struct hv_tlb_flush_ex {
>         u64 address_space;
>         u64 flags;
>         struct hv_vpset hv_vp_set;
>         u64 gva_list[];
> };

It seems you have two flexible array members at the end of hv_tlb_flush_ex.
This causes bank_contents[x] and gva_list[x] to overlap. So unless they have
the same meaning, this asks for trouble IMHO.