linux-kernel - Re: [PATCH gmem FIXUP] kvm: guestmem: do not use a file system

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Date:   Mon, 9 Oct 2023 07:32:48 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Al Viro <viro@...iv.linux.org.uk>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH gmem FIXUP] kvm: guestmem: do not use a file system

On Mon, Oct 09, 2023, Al Viro wrote:
> On Thu, Sep 28, 2023 at 07:22:16PM -0700, Sean Christopherson wrote:
> > On Thu, 28 Sep 2023 14:06:51 -0400, Paolo Bonzini wrote:
> > > Use a run-of-the-mill anonymous inode, there is nothing useful
> > > being provided by kvm_gmem_fs.
> > > 
> > > 
> > 
> > Applied to kvm-x86 guest_memfd, thanks!
> > 
> > [1/1] kvm: guestmem: do not use a file system
> >       https://github.com/kvm-x86/linux/commit/0f7e60a5f42a
> 
> Please, revert; this is completely broken.  anon_inode_getfile()
> yields a file with the same ->f_inode every time it is called.
> 
> Again, ->f_inode of those things is shared to hell and back,
> very much by design.  You can't modify its ->i_op or anything
> other field, for that matter.  No information can be stored
> in that thing - you are only allowed to use the object you've
> passed via 'priv' argument.

Yeah, we found that out the hard way.  Is using the "secure" variant to get a
per-file inode a sane approach, or is that abuse that's going to bite us too?

	/*
	 * Use the so called "secure" variant, which creates a unique inode
	 * instead of reusing a single inode.  Each guest_memfd instance needs
	 * its own inode to track the size, flags, etc.
	 */
	file = anon_inode_getfile_secure(anon_name, &kvm_gmem_fops, gmem,
					 O_RDWR, NULL);