linux-kernel - Re: [PATCH 2/2] x86: kvm: avoid constant-conversion warning

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Date:   Fri, 12 Jul 2019 19:47:05 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Arnd Bergmann <arnd@...db.de>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Junaid Shahid <junaids@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Lan Tianyu <Tianyu.Lan@...rosoft.com>,
        Wei Yang <richard.weiyang@...il.com>,
        Kai Huang <kai.huang@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com
Subject: Re: [PATCH 2/2] x86: kvm: avoid constant-conversion warning

On 12/07/19 11:12, Arnd Bergmann wrote:
> clang finds a contruct suspicious that converts an unsigned
> character to a signed integer and back, causing an overflow:

I like how the commit message conveys the braindead-ness of the warning.

Queued, thanks.

Paolo

> arch/x86/kvm/mmu.c:4605:39: error: implicit conversion from 'int' to 'u8' (aka 'unsigned char') changes value from -205 to 51 [-Werror,-Wconstant-conversion]
>                 u8 wf = (pfec & PFERR_WRITE_MASK) ? ~w : 0;
>                    ~~                               ^~
> arch/x86/kvm/mmu.c:4607:38: error: implicit conversion from 'int' to 'u8' (aka 'unsigned char') changes value from -241 to 15 [-Werror,-Wconstant-conversion]
>                 u8 uf = (pfec & PFERR_USER_MASK) ? ~u : 0;
>                    ~~                              ^~
> arch/x86/kvm/mmu.c:4609:39: error: implicit conversion from 'int' to 'u8' (aka 'unsigned char') changes value from -171 to 85 [-Werror,-Wconstant-conversion]
>                 u8 ff = (pfec & PFERR_FETCH_MASK) ? ~x : 0;
>                    ~~                               ^~
> 
> Add an explicit cast to tell clang that everything works as
> intended here.
> 
> Signed-off-by: Arnd Bergmann <arnd@...db.de>
> ---
>  arch/x86/kvm/mmu.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 17ece7b994b1..aea7f969ecb8 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4602,11 +4602,11 @@ static void update_permission_bitmask(struct kvm_vcpu *vcpu,
>  		 */
>  
>  		/* Faults from writes to non-writable pages */
> -		u8 wf = (pfec & PFERR_WRITE_MASK) ? ~w : 0;
> +		u8 wf = (pfec & PFERR_WRITE_MASK) ? (u8)~w : 0;
>  		/* Faults from user mode accesses to supervisor pages */
> -		u8 uf = (pfec & PFERR_USER_MASK) ? ~u : 0;
> +		u8 uf = (pfec & PFERR_USER_MASK) ? (u8)~u : 0;
>  		/* Faults from fetches of non-executable pages*/
> -		u8 ff = (pfec & PFERR_FETCH_MASK) ? ~x : 0;
> +		u8 ff = (pfec & PFERR_FETCH_MASK) ? (u8)~x : 0;
>  		/* Faults from kernel mode fetches of user pages */
>  		u8 smepf = 0;
>  		/* Faults from kernel mode accesses of user pages */
>