[PATCH] KVM: x86: Fix pointer mistmatch warning when patching RET0 static calls

[PATCH] KVM: x86: Fix pointer mistmatch warning when patching RET0 static calls

[Sean Christopherson]

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@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/kvm_host.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 713e08f62385..f285ddb8b66b 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1547,8 +1547,8 @@ static inline void kvm_ops_static_call_update(void)
 	WARN_ON(!kvm_x86_ops.func); __KVM_X86_OP(func)
 #define KVM_X86_OP_OPTIONAL __KVM_X86_OP
 #define KVM_X86_OP_OPTIONAL_RET0(func) \
-	static_call_update(kvm_x86_##func, kvm_x86_ops.func ? : \
-			   (void *) __static_call_return0);
+	static_call_update(kvm_x86_##func, (void *)kvm_x86_ops.func ? : \
+					   (void *)__static_call_return0);
 #include <asm/kvm-x86-ops.h>
 #undef __KVM_X86_OP
 }

base-commit: f4bc051fc91ab9f1d5225d94e52d369ef58bec58
-- 
2.35.1.473.g83b2b277ed-goog

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

[Nathan Chancellor]

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@gmail.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Thank you for the patch! Is this a bug in clang? I ended up creating a
small reproducer in case we need to file a bug with clang upstream:

https://godbolt.org/z/P7nEdzejE

This does shut up the warning and I can still spawn guests on my AMD and
Intel systems so:

Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>

> ---
>  arch/x86/include/asm/kvm_host.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 713e08f62385..f285ddb8b66b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1547,8 +1547,8 @@ static inline void kvm_ops_static_call_update(void)
>  	WARN_ON(!kvm_x86_ops.func); __KVM_X86_OP(func)
>  #define KVM_X86_OP_OPTIONAL __KVM_X86_OP
>  #define KVM_X86_OP_OPTIONAL_RET0(func) \
> -	static_call_update(kvm_x86_##func, kvm_x86_ops.func ? : \
> -			   (void *) __static_call_return0);
> +	static_call_update(kvm_x86_##func, (void *)kvm_x86_ops.func ? : \
> +					   (void *)__static_call_return0);
>  #include <asm/kvm-x86-ops.h>
>  #undef __KVM_X86_OP
>  }
> 
> base-commit: f4bc051fc91ab9f1d5225d94e52d369ef58bec58
> -- 
> 2.35.1.473.g83b2b277ed-goog
> 

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

[Sean Christopherson]

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@gmail.com>
> > Reported-by: kernel test robot <lkp@intel.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.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.

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

[Nathan Chancellor]

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@gmail.com>
> > > Reported-by: kernel test robot <lkp@intel.com>
> > > Signed-off-by: Sean Christopherson <seanjc@google.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

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

[Sean Christopherson]

On Wed, Feb 23, 2022, Nathan Chancellor wrote:
> 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@gmail.com>
> > > > Reported-by: kernel test robot <lkp@intel.com>
> > > > Signed-off-by: Sean Christopherson <seanjc@google.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).

Aha!  Looks like clang's behavior is correct, assuming a function is not considered
an "object".  From C99 "6.5.15 Conditional operator":

  One of the following shall hold for the second and third operands:
    — both operands have arithmetic type;
    — both operands have the same structure or union type;
    — both operands have void type;
    — both operands are pointers to qualified or unqualified versions of compatible types;
    — one operand is a pointer and the other is a null pointer constant; or
    — one operand is a pointer to an object or incomplete type and the other is a pointer to a
      qualified or unqualified version of void.

That last case would explain why clang warns about a function pointer but not a
object pointer when the third operand is a 'void *'.

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

[David Dunn]

Reviewed-by: David Dunn <daviddunn@google.com>

On Wed, Feb 23, 2022 at 8:24 AM Sean Christopherson <seanjc@google.com> wrote:

> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 713e08f62385..f285ddb8b66b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1547,8 +1547,8 @@ static inline void kvm_ops_static_call_update(void)
>         WARN_ON(!kvm_x86_ops.func); __KVM_X86_OP(func)
>  #define KVM_X86_OP_OPTIONAL __KVM_X86_OP
>  #define KVM_X86_OP_OPTIONAL_RET0(func) \
> -       static_call_update(kvm_x86_##func, kvm_x86_ops.func ? : \
> -                          (void *) __static_call_return0);
> +       static_call_update(kvm_x86_##func, (void *)kvm_x86_ops.func ? : \
> +                                          (void *)__static_call_return0);
>  #include <asm/kvm-x86-ops.h>
>  #undef __KVM_X86_OP
>  }

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

[Paolo Bonzini]

On 2/23/22 17:23, 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@gmail.com>
> Reported-by: kernel test robot <lkp@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/include/asm/kvm_host.h | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 713e08f62385..f285ddb8b66b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1547,8 +1547,8 @@ static inline void kvm_ops_static_call_update(void)
>   	WARN_ON(!kvm_x86_ops.func); __KVM_X86_OP(func)
>   #define KVM_X86_OP_OPTIONAL __KVM_X86_OP
>   #define KVM_X86_OP_OPTIONAL_RET0(func) \
> -	static_call_update(kvm_x86_##func, kvm_x86_ops.func ? : \
> -			   (void *) __static_call_return0);
> +	static_call_update(kvm_x86_##func, (void *)kvm_x86_ops.func ? : \
> +					   (void *)__static_call_return0);
>   #include <asm/kvm-x86-ops.h>
>   #undef __KVM_X86_OP
>   }
> 
> base-commit: f4bc051fc91ab9f1d5225d94e52d369ef58bec58

Queued, thanks.

Paolo