linux-kernel - Re: [PATCH] KVM: x86: Fix pointer mistmatch warning when patching RET0 static calls

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Date:   Wed, 23 Feb 2022 11:13:23 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        llvm@...ts.linux.dev, linux-kernel@...r.kernel.org,
        Like Xu <like.xu.linux@...il.com>
Subject: Re: [PATCH] KVM: x86: Fix pointer mistmatch warning when patching
 RET0 static calls

On Wed, Feb 23, 2022 at 05:59:05PM +0000, Sean Christopherson wrote:
> On Wed, Feb 23, 2022, Nathan Chancellor wrote:
> > Hi Sean,
> > 
> > On Wed, Feb 23, 2022 at 04:23:55PM +0000, Sean Christopherson wrote:
> > > Cast kvm_x86_ops.func to 'void *' when updating KVM static calls that are
> > > conditionally patched to __static_call_return0().  clang complains about
> > > using mismatching pointers in the ternary operator, which breaks the
> > > build when compiling with CONFIG_KVM_WERROR=y.
> > > 
> > >   >> arch/x86/include/asm/kvm-x86-ops.h:82:1: warning: pointer type mismatch
> > >   ('bool (*)(struct kvm_vcpu *)' and 'void *') [-Wpointer-type-mismatch]
> > > 
> > > Fixes: 5be2226f417d ("KVM: x86: allow defining return-0 static calls")
> > > Reported-by: Like Xu <like.xu.linux@...il.com>
> > > Reported-by: kernel test robot <lkp@...el.com>
> > > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > 
> > Thank you for the patch! Is this a bug in clang?
> 
> IMO, no.  I think it's completely reasonable for the compiler to complain that KVM
> is generating two different pointer types out of a ternary operator.
> 
> clang is somewhat inconsistent, though it may be deliberate.  clang doesn't complain
> about implicitly casting a 'void *' to another data type, e.g. this complies clean,
> where "data" is a 'void *'
> 
> 	struct kvm_vcpu *x = vcpu ? : data;

Right, I would assume this is deliberate. I think warning in this case
might be quite noisy, as the kernel implicitly converts 'void *' to
typed pointers for certain function pointer callbacks (although this
particular case is probably pretty rare).

> But changing it to a function on the lhs triggers the warn:
> 
> 	typeof(kvm_x86_ops.vcpu_run) x = kvm_x86_ops.vcpu_run ? : data;
> 
> Again, complaining in the function pointer case seems reasonable.

Ack, thank you for the clarification and explanation!

Cheers,
Nathan