linux-kernel - Re: [PATCH 2/2] mm: memfd

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Message-ID: <20260126183115.GU1134360@nvidia.com>
Date: Mon, 26 Jan 2026 14:31:15 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Mike Rapoport <rppt@...nel.org>
Cc: Pratyush Yadav <pratyush@...nel.org>, Alexander Graf <graf@...zon.com>,
	Pasha Tatashin <pasha.tatashin@...een.com>,
	Hugh Dickins <hughd@...gle.com>,
	Baolin Wang <baolin.wang@...ux.alibaba.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Samiullah Khawaja <skhawaja@...gle.com>, kexec@...ts.infradead.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] mm: memfd_luo: preserve file seals

On Sun, Jan 25, 2026 at 02:03:29PM +0200, Mike Rapoport wrote:
> > @@ -67,11 +72,13 @@ struct memfd_luo_folio_ser {
> >  struct memfd_luo_ser {
> >  	u64 pos;
> >  	u64 size;
> > +	u64 seals:8;
> 
> Kernel uABI defines seals as unsigned int, I think we can spare u32 for
> them and reserve a u32 flags for other memfd flags (MFD_CLOEXEC,
> MFD_HUGETLB etc).

It is a bit worse than that, the "v2" version is only going to support
some set of seals (probably the set defined in v6.19) and if there are
new seals down the road then this needs a version bump.

So I'd check that only supported seals are set here:

> > +     seals = memfd_get_seals(args->file);
> > +     if (seals < 0) {
> > +             err = seals;
> > +             goto err_free_ser;
> > +     }
> > +
> >       ser->pos = args->file->f_pos;
> >       ser->size = i_size_read(inode);
> > +     ser->seals = seals;

..

> > @@ -444,13 +453,23 @@ static int memfd_luo_retrieve(struct liveupdate_file_op_args *args)
> >  	if (!ser)
> >  		return -EINVAL;
> >  
> > -	file = memfd_alloc_file("", 0);
> > +	/*
> > +	 * The seals are preserved. Allow sealing here so they can be added
> > +	 * later.
> > +	 */
> > +	file = memfd_alloc_file("", MFD_ALLOW_SEALING);
> >  	if (IS_ERR(file)) {
> >  		pr_err("failed to setup file: %pe\n", file);
> >  		err = PTR_ERR(file);
> >  		goto free_ser;
> >  	}
> >  
> > +	err = memfd_add_seals(file, ser->seals);

Because we really don't want this to fail :\

Jason