linux-kernel - Re: [PATCH v2 4/6] iommufd: Deliver fault messages to user space

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Message-ID: <20231207171742.GU1489931@ziepe.ca>
Date:   Thu, 7 Dec 2023 13:17:42 -0400
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Joel Granados <j.granados@...sung.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Kevin Tian <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        Nicolin Chen <nicolinc@...dia.com>,
        Yi Liu <yi.l.liu@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        iommu@...ts.linux.dev, linux-kselftest@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/6] iommufd: Deliver fault messages to user space

On Thu, Dec 07, 2023 at 05:34:10PM +0100, Joel Granados wrote:
> > @@ -58,6 +255,8 @@ static void hw_pagetable_fault_free(struct hw_pgtable_fault *fault)
> >  	WARN_ON(!list_empty(&fault->deliver));
> >  	WARN_ON(!list_empty(&fault->response));
> >  
> > +	fput(fault->fault_file);
> > +	put_unused_fd(fault->fault_fd);

> I have resolved this in a naive way by just not calling the
> put_unused_fd function.

That is correct.

put_unused_fd() should only be called on error paths prior to the
syscall return.

The design of a FD must follow this pattern

 syscall():
   fdno = get_unused_fd_flags(O_CLOEXEC);
   filep = [..]
 
   // syscall MUST succeed after this statement:
   fd_install(fdno, filep);
   return 0;

  err:
    put_unused_fd(fdno)
    return -ERRNO

Also the refcounting looks a little strange, the filep reference is
consumed by fd_install, so what is that fput pairing with in fault_free?

Jason