Message-ID: <24f504b5-afa2-43db-9655-3a42fd46108f@lucifer.local>
Date: Mon, 9 Jun 2025 12:45:18 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Pedro Falcato <pfalcato@...e.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <brauner@...nel.org>, Jan Kara <jack@...e.cz>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] mm: add mmap_prepare() compatibility layer for nested
 file systems

On Mon, Jun 09, 2025 at 12:35:40PM +0100, Pedro Falcato wrote:
> On Mon, Jun 09, 2025 at 10:24:13AM +0100, Lorenzo Stoakes wrote:
> > Nested file systems, that is those which invoke call_mmap() within their
> > own f_op->mmap() handlers, may encounter underlying file systems which
> > provide the f_op->mmap_prepare() hook introduced by commit
> > c84bf6dd2b83 ("mm: introduce new .mmap_prepare() file callback").
> >
> > We have a chicken-and-egg scenario here - until all file systems are
> > converted to using .mmap_prepare(), we cannot convert these nested
> > handlers, as we can't call f_op->mmap from an .mmap_prepare() hook.
> >
> > So we have to do it the other way round - invoke the .mmap_prepare() hook
> > from an .mmap() one.
> >
> > in order to do so, we need to convert VMA state into a struct vm_area_desc
> > descriptor, invoking the underlying file system's f_op->mmap_prepare()
> > callback passing a pointer to this, and then setting VMA state accordingly
> > and safely.
> >
> > This patch achieves this via the compat_vma_mmap_prepare() function, which
> > we invoke from call_mmap() if f_op->mmap_prepare() is specified in the
> > passed in file pointer.
> >
> > We place the fundamental logic into mm/vma.c where VMA manipulation
> > belongs. We also update the VMA userland tests to accommodate the changes.
> >
> > The compat_vma_mmap_prepare() function and its associated machinery is
> > temporary, and will be removed once the conversion of file systems is
> > complete.
> >
>
> Thanks, this is annoying but looks mostly cromulent!

You do love that word :P I have to look it up every time... Maybe time to
up my vocabulary?? ;)

>
> > Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> > Reported-by: Jann Horn <jannh@...gle.com>
> > Closes: https://lore.kernel.org/linux-mm/CAG48ez04yOEVx1ekzOChARDDBZzAKwet8PEoPM4Ln3_rk91AzQ@mail.gmail.com/
> > Fixes: c84bf6dd2b83 ("mm: introduce new .mmap_prepare() file callback").
> > ---
> >  include/linux/fs.h               |  6 +++--
> >  mm/mmap.c                        | 39 +++++++++++++++++++++++++++
> >  mm/vma.c                         | 46 +++++++++++++++++++++++++++++++-
> >  mm/vma.h                         |  4 +++
> >  tools/testing/vma/vma_internal.h | 16 +++++++++++
> >  5 files changed, 108 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/fs.h b/include/linux/fs.h
> > index 05abdabe9db7..8fe41a2b7527 100644
> > --- a/include/linux/fs.h
> > +++ b/include/linux/fs.h
> > @@ -2274,10 +2274,12 @@ static inline bool file_has_valid_mmap_hooks(struct file *file)
> >  	return true;
> >  }
> >
> > +int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma);
> > +
> >  static inline int call_mmap(struct file *file, struct vm_area_struct *vma)
> >  {
> > -	if (WARN_ON_ONCE(file->f_op->mmap_prepare))
> > -		return -EINVAL;
> > +	if (file->f_op->mmap_prepare)
> > +		return compat_vma_mmap_prepare(file, vma);
> >
> >  	return file->f_op->mmap(file, vma);
> >  }
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 09c563c95112..0755cb5d89d1 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -1891,3 +1891,42 @@ __latent_entropy int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
> >  	vm_unacct_memory(charge);
> >  	goto loop_out;
> >  }
> > +
> > +/**
> > + * compat_vma_mmap_prepare() - Apply the file's .mmap_prepare() hook to an
> > + * existing VMA
> > + * @file: The file which possesss an f_op->mmap_prepare() hook
> > + * @vma; The VMA to apply the .mmap_prepare() hook to.
> > + *
> > + * Ordinarily, .mmap_prepare() is invoked directly upon mmap(). However, certain
> > + * 'wrapper' file systems invoke a nested mmap hook of an underlying file.
> > + *
> > + * Until all filesystems are converted to use .mmap_prepare(), we must be
> > + * conservative and continue to invoke these 'wrapper' filesystems using the
> > + * deprecated .mmap() hook.
> > + *
> > + * However we have a problem if the underlying file system possesses an
> > + * .mmap_prepare() hook, as we are in a different context when we invoke the
> > + * .mmap() hook, already having a VMA to deal with.
> > + *
> > + * compat_vma_mmap_prepare() is a compatibility function that takes VMA state,
> > + * establishes a struct vm_area_desc descriptor, passes to the underlying
> > + * .mmap_prepare() hook and applies any changes performed by it.
> > + *
> > + * Once the conversion of filesystems is complete this function will no longer
> > + * be required and will be removed.
> > + *
> > + * Returns: 0 on success or error.
> > + */
> > +int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
> > +{
> > +	struct vm_area_desc desc;
> > +	int err;
> > +
> > +	err = file->f_op->mmap_prepare(vma_to_desc(vma, &desc));
> > +	if (err)
> > +		return err;
> > +	set_vma_from_desc(vma, &desc);
> > +
> > +	return 0;
> > +}
> > diff --git a/mm/vma.c b/mm/vma.c
> > index 01b1d26d87b4..d771750f8f76 100644
> > --- a/mm/vma.c
> > +++ b/mm/vma.c
> > @@ -3153,7 +3153,6 @@ int __vm_munmap(unsigned long start, size_t len, bool unlock)
> >  	return ret;
> >  }
> >
> > -
> >  /* Insert vm structure into process list sorted by address
> >   * and into the inode's i_mmap tree.  If vm_file is non-NULL
> >   * then i_mmap_rwsem is taken here.
> > @@ -3195,3 +3194,48 @@ int insert_vm_struct(struct mm_struct *mm, struct vm_area_struct *vma)
> >
> >  	return 0;
> >  }
> > +
> > +/*
> > + * Temporary helper functions for file systems which wrap an invocation of
> > + * f_op->mmap() but which might have an underlying file system which implements
> > + * f_op->mmap_prepare().
> > + */
> > +
> > +struct vm_area_desc *vma_to_desc(struct vm_area_struct *vma,
> > +		struct vm_area_desc *desc)
> > +{
> > +	desc->mm = vma->vm_mm;
> > +	desc->start = vma->vm_start;
> > +	desc->end = vma->vm_end;
> > +
> > +	desc->pgoff = vma->vm_pgoff;
> > +	desc->file = vma->vm_file;
> > +	desc->vm_flags = vma->vm_flags;
> > +	desc->page_prot = vma->vm_page_prot;
> > +
> > +	desc->vm_ops = NULL;
> > +	desc->private_data = NULL;
> > +
> > +	return desc;
> > +}
> > +
> > +void set_vma_from_desc(struct vm_area_struct *vma, struct vm_area_desc *desc)
> > +{
> > +	/*
> > +	 * Since we're invoking .mmap_prepare() despite having a partially
> > +	 * established VMA, we must take care to handle setting fields
> > +	 * correctly.
> > +	 */
> > +
> > +	/* Mutable fields. Populated with initial state. */
> > +	vma->vm_pgoff = desc->pgoff;
> > +	if (vma->vm_file != desc->file)
> > +		vma_set_file(vma, desc->file);
> > +	if (vma->vm_flags != desc->vm_flags)
> > +		vm_flags_set(vma, desc->vm_flags);
>
> I think we don't need vm_flags_set in this case, since the VMA isn't exposed yet.
> __vm_flags_mod should work just fine. Of course this isn't a big deal, but I would
> like it if we reduced vm_flags_set to core mm and conceptually attached things.

Yeah I considered doing something like this, but I really want to keep this as
close to how .mmap() handlers do this ordinarily, even though it's a bit
unnecessary here.

This will (genuinely!) be temporary either way, and we can address the
unnecesary use of vm_flags_set() as a whole elsewhere.

>
> In any case, with or without that addressed:
>
> Reviewed-by: Pedro Falcato <pfalcato@...e.de>

Thanks!

>
> --
> Pedro

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