linux-kernel - Re: [PATCH] KVM: replace DEFINE_SIMPLE_ATTRIBUTE with DEFINE_DEBUGFS

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Message-ID: <c0b98151-16b6-6d8f-1765-0f7d46682d60@redhat.com>
Date:   Wed, 2 Nov 2022 18:48:19 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Bo Liu <liubo03@...pur.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: replace DEFINE_SIMPLE_ATTRIBUTE with
 DEFINE_DEBUGFS_ATTRIBUTE

On 11/1/22 08:25, Bo Liu wrote:
> Fix the following coccicheck warning:
>    virt/kvm/kvm_main.c:3847:0-23: WARNING
>      vcpu_get_pid_fops should be defined with DEFINE_DEBUGFS_ATTRIBUTE
> 
> Signed-off-by: Bo Liu <liubo03@...pur.com>
> ---
>   virt/kvm/kvm_main.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index f1df24c2bc84..3f383f27d3d7 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3844,7 +3844,7 @@ static int vcpu_get_pid(void *data, u64 *val)
>   	return 0;
>   }
>   
> -DEFINE_SIMPLE_ATTRIBUTE(vcpu_get_pid_fops, vcpu_get_pid, NULL, "%llu\n");
> +DEFINE_DEBUGFS_ATTRIBUTE(vcpu_get_pid_fops, vcpu_get_pid, NULL, "%llu\n");
>   
>   static void kvm_create_vcpu_debugfs(struct kvm_vcpu *vcpu)
>   {

If you really wanted to do this, you would also have to replace 
debugfs_create_file with debugfs_create_file_unsafe.

However, this is not a good idea.  The rationale in the .cocci file is 
that "DEFINE_SIMPLE_ATTRIBUTE + debugfs_create_file() imposes some 
significant overhead", but this should not really be relevant for a 
debugfs file.

Such a patch would only make sense if there was a version of 
debugfs_create_file_unsafe() with a less-terrible name (e.g. 
debugfs_create_simple_attr?), which could _only_ be used with fops 
created by DEFINE_DEBUGFS_ATTRIBUTE.  Without such a type-safe trick, 
the .cocci file is only adding confusion to perfectly fine code.

Paolo