linux-kernel - Re: [PATCH v13 04/26] KVM: guest_memfd: Fix PTR_ERR() handling in __kvm_gmem_get

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Message-ID: <20240419151109.bo6kz4s24jgrmaj4@amd.com>
Date: Fri, 19 Apr 2024 10:11:09 -0500
From: Michael Roth <michael.roth@....com>
To: David Hildenbrand <david@...hat.com>
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Subject: Re: [PATCH v13 04/26] KVM: guest_memfd: Fix PTR_ERR() handling in
 __kvm_gmem_get_pfn()

On Fri, Apr 19, 2024 at 02:58:43PM +0200, David Hildenbrand wrote:
> On 18.04.24 21:41, Michael Roth wrote:
> > kvm_gmem_get_folio() may return a PTR_ERR() rather than just NULL. In
> > particular, for cases where EEXISTS is returned when FGP_CREAT_ONLY
> > flag is used. Handle this properly in __kvm_gmem_get_pfn().
> > 
> > Signed-off-by: Michael Roth <michael.roth@....com>
> > ---
> >   virt/kvm/guest_memfd.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> > index ccf22e44f387..9d7c6a70c547 100644
> > --- a/virt/kvm/guest_memfd.c
> > +++ b/virt/kvm/guest_memfd.c
> > @@ -580,8 +580,8 @@ static int __kvm_gmem_get_pfn(struct file *file, struct kvm_memory_slot *slot,
> >   	}
> >   	folio = kvm_gmem_get_folio(file_inode(file), index, prepare);
> > -	if (!folio)
> > -		return -ENOMEM;
> > +	if (IS_ERR_OR_NULL(folio))
> > +		return folio ? PTR_ERR(folio) : -ENOMEM;
> 
> Will it even return NULL?  Staring at other filemap_grab_folio() users, they
> all check for IS_ERR().

Looks like the NULL case is handled with PTR_ERR(-ENOENT), so IS_ERR()
would be sufficient. I think in the past kvm_gmem_get_folio() itself
would return NULL in some cases, but as of commit 2b01b7e994e95 that's
no longer the case.

I'll fix this up to expect only PTR_ERR() when I re-spin v14, and also
address the other kvm_gmem_get_folio() / __filemap_get_folio() call
sites.

> 
> >   	if (folio_test_hwpoison(folio)) {
> >   		r = -EHWPOISON;
> 
> Do we have a Fixes: tag?

Fixes: 2b01b7e994e95 ("KVM: guest_memfd: pass error up from filemap_grab_folio")

Will add that in the re-spin as well.

Thanks!

-Mike

> 
> -- 
> Cheers,
> 
> David / dhildenb
>