linux-kernel - Re: [PATCH] KVM: Add const to file

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Date:   Tue, 31 May 2022 19:47:06 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Xiang wangx <wangxiang@...rlc.com>
Cc:     pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: Add const to file_operations

On Mon, May 30, 2022, Xiang wangx wrote:
> Struct file_operations should normally be const.

Operative word being "should".  kvm_chardev_ops isn't constant because kvm_init()
modifies the "owner".  The same was true for kvm_vcpu_fops and kvm_vm_fops until
commit 70375c2d8fa3 ("Revert "KVM: set owner of cpu and vm file operations"").

So, this patch is both stale, and completely broken when applied against current
KVM as well as whatever old version of KVM it was generated with.

arch/x86/kvm/../../../virt/kvm/kvm_main.c:4873:37: error: conflicting type qualifiers for ‘kvm_chardev_ops’
 4873 | static const struct file_operations kvm_chardev_ops = {
      |                                     ^~~~~~~~~~~~~~~
arch/x86/kvm/../../../virt/kvm/kvm_main.c:120:31: note: previous declaration of ‘kvm_chardev_ops’ with type ‘struct file_operations’
  120 | static struct file_operations kvm_chardev_ops;
      |                               ^~~~~~~~~~~~~~~
arch/x86/kvm/../../../virt/kvm/kvm_main.c: In function ‘kvm_init’:
arch/x86/kvm/../../../virt/kvm/kvm_main.c:5777:31: error: assignment of member ‘owner’ in read-only object
 5777 |         kvm_chardev_ops.owner = module;
      |                               ^
  CC      arch/x86/events/intel/p6.o
make[3]: *** [scripts/Makefile.build:288: arch/x86/kvm/../../../virt/kvm/kvm_main.o] Error 1
make[2]: *** [scripts/Makefile.build:550: arch/x86/kvm] Error 2


> Signed-off-by: Xiang wangx <wangxiang@...rlc.com>
> ---
>  virt/kvm/kvm_main.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 3f6d450355f0..7dc2433f1b01 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3550,7 +3550,7 @@ static int kvm_vcpu_release(struct inode *inode, struct file *filp)
>  	return 0;
>  }
>  
> -static struct file_operations kvm_vcpu_fops = {
> +static const struct file_operations kvm_vcpu_fops = {
>  	.release        = kvm_vcpu_release,
>  	.unlocked_ioctl = kvm_vcpu_ioctl,
>  	.mmap           = kvm_vcpu_mmap,
> @@ -4599,7 +4599,7 @@ static long kvm_vm_compat_ioctl(struct file *filp,
>  }
>  #endif
>  
> -static struct file_operations kvm_vm_fops = {
> +static const struct file_operations kvm_vm_fops = {
>  	.release        = kvm_vm_release,
>  	.unlocked_ioctl = kvm_vm_ioctl,
>  	.llseek		= noop_llseek,
> @@ -4701,7 +4701,7 @@ static long kvm_dev_ioctl(struct file *filp,
>  	return r;
>  }
>  
> -static struct file_operations kvm_chardev_ops = {
> +static const struct file_operations kvm_chardev_ops = {
>  	.unlocked_ioctl = kvm_dev_ioctl,
>  	.llseek		= noop_llseek,
>  	KVM_COMPAT(kvm_dev_ioctl),
> -- 
> 2.36.1
>