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

From: Greg Kroah-Hartman <[email-masked]>
Date: Tue, 3 Mar 2020 10:58:49 +0100

On Tue, Mar 03, 2020 at 08:45:23PM +1100, Michael Ellerman wrote:
> Greg Kroah-Hartman <[email-masked]> writes:
> > On Tue, Mar 03, 2020 at 06:46:23PM +1100, Michael Ellerman wrote:
> >> Greg Kroah-Hartman <[email-masked]> 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;
> }

Fair enough, but I doubt it ever matters :)

Thanks for the fixups, sorry for sending a broken patch, my fault.

greg k-h