Re: [PATCH 2/6] powerpc: kvm: no need to check return value of debugfs_create functions

[Michael Ellerman <mpe@ellerman.id.au>]

Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
> On Tue, Mar 03, 2020 at 06:46:23PM +1100, Michael Ellerman wrote:
>> Greg Kroah-Hartman <gregkh@linuxfoundation.org> writes:
>> > When calling debugfs functions, there is no need to ever check the
>> > return value.  The function can work or not, but the code logic should
>> > never do something different based on this.
>> 
>> Except it does need to do something different, if the file was created
>> it needs to be removed in the remove path.
>> 
>> > diff --git a/arch/powerpc/kvm/timing.c b/arch/powerpc/kvm/timing.c
>> > index bfe4f106cffc..8e4791c6f2af 100644
>> > --- a/arch/powerpc/kvm/timing.c
>> > +++ b/arch/powerpc/kvm/timing.c
>> > @@ -207,19 +207,12 @@ static const struct file_operations kvmppc_exit_timing_fops = {
>> >  void kvmppc_create_vcpu_debugfs(struct kvm_vcpu *vcpu, unsigned int id)
>> >  {
>> >  	static char dbg_fname[50];
>> > -	struct dentry *debugfs_file;
>> >  
>> >  	snprintf(dbg_fname, sizeof(dbg_fname), "vm%u_vcpu%u_timing",
>> >  		 current->pid, id);
>> > -	debugfs_file = debugfs_create_file(dbg_fname, 0666,
>> > -					kvm_debugfs_dir, vcpu,
>> > -					&kvmppc_exit_timing_fops);
>> > -
>> > -	if (!debugfs_file) {
>> > -		printk(KERN_ERR"%s: error creating debugfs file %s\n",
>> > -			__func__, dbg_fname);
>> > -		return;
>> > -	}
>> > +	debugfs_create_file(dbg_fname, 0666, kvm_debugfs_dir, vcpu,
>> > +			    &kvmppc_exit_timing_fops);
>> > +
>> >  
>> >  	vcpu->arch.debugfs_exit_timing = debugfs_file;
>
> Ugh, you are right, how did I miss that?  How is 0-day missing this?
> It's been in my tree for a long time, odd.

This code isn't enabled by default, or in any defconfig. So it's only
allmodconfig that would trip it, I guess 0-day isn't doing powerpc
allmodconfig builds.

>> I squashed this in, which seems to work:
...
>>  
>>  void kvmppc_remove_vcpu_debugfs(struct kvm_vcpu *vcpu)
>>  {
>> -       if (vcpu->arch.debugfs_exit_timing) {
>> +       if (!IS_ERR_OR_NULL(vcpu->arch.debugfs_exit_timing)) {
>>                 debugfs_remove(vcpu->arch.debugfs_exit_timing);
>>                 vcpu->arch.debugfs_exit_timing = NULL;
>>         }
>
> No, this can just be:
> 	debugfs_remove(vcpu->arch.debugfs_exit_timing);
>
> No need to check anything, just call it and the debugfs code can handle
> it just fine.

Oh duh, of course, I should have checked.

I'd still like to NULL out the debugfs_exit_timing member, so I'll do:

void kvmppc_remove_vcpu_debugfs(struct kvm_vcpu *vcpu)
{
	debugfs_remove(vcpu->arch.debugfs_exit_timing);
	vcpu->arch.debugfs_exit_timing = NULL;
}


cheers