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

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Message-ID: <YhZ16cMMcHQIvS9d@google.com>
Date:   Wed, 23 Feb 2022 17:59:05 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Nathan Chancellor <nathan@...nel.org>
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, 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;

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.