linux-kernel - Re: [PATCH v2] KVM: SVM: hyper-v: placate modpost section mismatch error

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Message-ID: <Y/ZG8u6/aUtpsVDa@google.com>
Date:   Wed, 22 Feb 2023 08:46:42 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     linux-kernel@...r.kernel.org,
        Vineeth Pillai <viremana@...ux.microsoft.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v2] KVM: SVM: hyper-v: placate modpost section mismatch error

On Wed, Feb 22, 2023, Randy Dunlap wrote:
> modpost reports section mismatch errors/warnings:
> WARNING: modpost: vmlinux.o: section mismatch in reference: svm_hv_hardware_setup (section: .text) -> (unknown) (section: .init.data)
> WARNING: modpost: vmlinux.o: section mismatch in reference: svm_hv_hardware_setup (section: .text) -> (unknown) (section: .init.data)
> WARNING: modpost: vmlinux.o: section mismatch in reference: svm_hv_hardware_setup (section: .text) -> (unknown) (section: .init.data)
> 
> Marking svm_hv_hardware_setup() as __init fixes the warnings.
> 
> I don't know why this should be needed -- it seems like a compiler
> problem to me since the calling function is marked as __init.

It's not a compiler issue.  __initdata is freed after init and so must not be
accessed by __init-less functions.

This as a changelog?

  Tag svm_hv_hardware_setup() with __init to fix a modpost warning as the
  non-stub implementation accesses __initdata (svm_x86_ops), i.e. would
  generate a use-after-free if svm_hv_hardware_setup() were actually invoked
  post-init.  The helper is only called from svm_hardware_setup(), which is
  also __init, i.e. other than the modpost warning, lack of __init is benign.

With that (in case Paolo grabs this directly):

Reviewed-by: Sean Christopherson <seanjc@...gle.com>
  
> This "(unknown) (section: .init.data)" all refer to svm_x86_ops.
> 
> Fixes: 1e0c7d40758b ("KVM: SVM: hyper-v: Remote TLB flush for SVM")
> Signed-off-by: Randy Dunlap <rdunlap@...radead.org>
> Cc: Vineeth Pillai <viremana@...ux.microsoft.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Vitaly Kuznetsov <vkuznets@...hat.com>
> Cc: Sean Christopherson <seanjc@...gle.com>
> Cc: kvm@...r.kernel.org
> ---
> v2: also make the empty stub function be __init (Vitaly)
> 
>  arch/x86/kvm/svm/svm_onhyperv.h |    4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff -- a/arch/x86/kvm/svm/svm_onhyperv.h b/arch/x86/kvm/svm/svm_onhyperv.h
> --- a/arch/x86/kvm/svm/svm_onhyperv.h
> +++ b/arch/x86/kvm/svm/svm_onhyperv.h
> @@ -30,7 +30,7 @@ static inline void svm_hv_init_vmcb(stru
>  		hve->hv_enlightenments_control.msr_bitmap = 1;
>  }
>  
> -static inline void svm_hv_hardware_setup(void)
> +static inline __init void svm_hv_hardware_setup(void)
>  {
>  	if (npt_enabled &&
>  	    ms_hyperv.nested_features & HV_X64_NESTED_ENLIGHTENED_TLB) {
> @@ -84,7 +84,7 @@ static inline void svm_hv_init_vmcb(stru
>  {
>  }
>  
> -static inline void svm_hv_hardware_setup(void)
> +static inline __init void svm_hv_hardware_setup(void)
>  {
>  }
>  
>