linux-kernel - Re: [PATCH] KVM: guest_memfd: Drop a superfluous local var in kvm_gmem_fault_user

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Message-ID: <aOmEpZw7DXnuu--l@google.com>
Date: Fri, 10 Oct 2025 15:11:49 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Ackerley Tng <ackerleytng@...gle.com>
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: guest_memfd: Drop a superfluous local var in kvm_gmem_fault_user_mapping()

On Thu, Oct 09, 2025, Ackerley Tng wrote:
> Sean Christopherson <seanjc@...gle.com> writes:
> 
> > Drop the local "int err" that's buried in the middle guest_memfd's user
> > fault handler to avoid the potential for variable shadowing, e.g. if an
> > "err" variable were also declared at function scope.
> >
> 
> Is the takeaway here that the variable name "err", if used, should be
> defined at function scope?

Generally speaking, for generic variables like "err", "r", and "ret", yes, because
the danger of shadowing is high, while the odds of _wanting_ to contain a return
code are low.

But as a broad rule, there's simply no "right" answer other than "it depends".
As Paolo put it, damned if you do, damned if you don't.  See this thread for more:

https://lore.kernel.org/all/YZL1ZiKQVRQd8rZi@google.com

> IOW, would this code have been okay if any other variable name were
> used, like if err_folio were used instead of err?

Probably not?  Because that has it's own problems.  E.g. then you can end up
introducing bugs like this:

	int err;

	err = blah();
	if (err)
		goto fault_err;

        folio = kvm_gmem_get_folio(inode, vmf->pgoff);
        if (IS_ERR(folio)) {
		int folio_err = PTR_ERR(folio);

		if (folio_err == -EAGAIN)
                        return VM_FAULT_RETRY;

		goto fault_err;
        }

	...

fault_err:
	return vmf_error(err);