linux-kernel - Re: [PATCH] KVM: x86: Fix BUILD_BUG() in __cpuid_entry_get_reg() w/ CONFIG

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Date:   Tue, 31 Mar 2020 16:51:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] KVM: x86: Fix BUILD_BUG() in __cpuid_entry_get_reg() w/
 CONFIG_UBSAN=y

On 25/03/20 20:12, Sean Christopherson wrote:
> Take the target reg in __cpuid_entry_get_reg() instead of a pointer to a
> struct cpuid_reg.  When building with -fsanitize=alignment (enabled by
> CONFIG_UBSAN=y), some versions of gcc get tripped up on the pointer and
> trigger the BUILD_BUG().
> 
> Reported-by: Randy Dunlap <rdunlap@...radead.org>
> Fixes: d8577a4c238f8 ("KVM: x86: Do host CPUID at load time to mask KVM cpu caps")
> Fixes: 4c61534aaae2a ("KVM: x86: Introduce cpuid_entry_{get,has}() accessors")
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/cpuid.c | 2 +-
>  arch/x86/kvm/cpuid.h | 8 ++++----
>  2 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 08280d8a2ac9..16d3ae432420 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -269,7 +269,7 @@ static __always_inline void kvm_cpu_cap_mask(enum cpuid_leafs leaf, u32 mask)
>  	cpuid_count(cpuid.function, cpuid.index,
>  		    &entry.eax, &entry.ebx, &entry.ecx, &entry.edx);
>  
> -	kvm_cpu_caps[leaf] &= *__cpuid_entry_get_reg(&entry, &cpuid);
> +	kvm_cpu_caps[leaf] &= *__cpuid_entry_get_reg(&entry, cpuid.reg);
>  }
>  
>  void kvm_set_cpu_caps(void)
> diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
> index 23b4cd1ad986..63a70f6a3df3 100644
> --- a/arch/x86/kvm/cpuid.h
> +++ b/arch/x86/kvm/cpuid.h
> @@ -99,9 +99,9 @@ static __always_inline struct cpuid_reg x86_feature_cpuid(unsigned int x86_featu
>  }
>  
>  static __always_inline u32 *__cpuid_entry_get_reg(struct kvm_cpuid_entry2 *entry,
> -						  const struct cpuid_reg *cpuid)
> +						  u32 reg)
>  {
> -	switch (cpuid->reg) {
> +	switch (reg) {
>  	case CPUID_EAX:
>  		return &entry->eax;
>  	case CPUID_EBX:
> @@ -121,7 +121,7 @@ static __always_inline u32 *cpuid_entry_get_reg(struct kvm_cpuid_entry2 *entry,
>  {
>  	const struct cpuid_reg cpuid = x86_feature_cpuid(x86_feature);
>  
> -	return __cpuid_entry_get_reg(entry, &cpuid);
> +	return __cpuid_entry_get_reg(entry, cpuid.reg);
>  }
>  
>  static __always_inline u32 cpuid_entry_get(struct kvm_cpuid_entry2 *entry,
> @@ -189,7 +189,7 @@ static __always_inline u32 *guest_cpuid_get_register(struct kvm_vcpu *vcpu,
>  	if (!entry)
>  		return NULL;
>  
> -	return __cpuid_entry_get_reg(entry, &cpuid);
> +	return __cpuid_entry_get_reg(entry, cpuid.reg);
>  }
>  
>  static __always_inline bool guest_cpuid_has(struct kvm_vcpu *vcpu,
> 

Queued, thanks.

Paolo