Message-ID: <2d4750fd-10d8-42f6-a396-ceaf386e65f5@lucifer.local>
Date: Mon, 9 Jun 2025 19:37:33 +0100
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: 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>,
        Pedro Falcato <pfalcato@...e.de>, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2] mm: add mmap_prepare() compatibility layer for nested
 file systems

Andrew - apologies, please attach the tags as below which I mistakenly didn't
propagate...

Thanks!

(note to self, figure out b4 :P)

On Mon, Jun 09, 2025 at 05:57:49PM +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.h 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.
> 
> We carefully place this code so it can be used with CONFIG_MMU and also
> with cutting edge nommu silicon.
> 
> 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").

Reviewed-by: Pedro Falcato <pfalcato@...e.de>
Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

> ---
> 
> Apologies for the quick turn-around here, but I'm keen to address the silly
> kernel-doc and nommu issues here.
> 
> v2:
> * Propagated tags (thanks everyone!)
> * Corrected nommu issue by carefully positioning code in mm/util.c and mm/vma.h.
> * Fixed ';' typo in kernel-doc comment.
> 
> v1:
> https://lore.kernel.org/all/20250609092413.45435-1-lorenzo.stoakes@oracle.com/
> 
>  include/linux/fs.h               |  6 ++--
>  mm/util.c                        | 39 ++++++++++++++++++++++++
>  mm/vma.c                         |  1 -
>  mm/vma.h                         | 51 ++++++++++++++++++++++++++++++++
>  tools/testing/vma/vma_internal.h | 16 ++++++++++
>  5 files changed, 110 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/util.c b/mm/util.c
> index 448117da071f..23a9bc26ef68 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -1131,3 +1131,42 @@ void flush_dcache_folio(struct folio *folio)
>  }
>  EXPORT_SYMBOL(flush_dcache_folio);
>  #endif
> +
> +/**
> + * 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..3cdd0aaa10aa 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.
> diff --git a/mm/vma.h b/mm/vma.h
> index 0db066e7a45d..d92e6c906c96 100644
> --- a/mm/vma.h
> +++ b/mm/vma.h
> @@ -222,6 +222,53 @@ static inline int vma_iter_store_gfp(struct vma_iterator *vmi,
>  	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().
> + */
> +
> +static inline 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;
> +}
> +
> +static inline 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);
> +	vma->vm_page_prot = desc->page_prot;
> +
> +	/* User-defined fields. */
> +	vma->vm_ops = desc->vm_ops;
> +	vma->vm_private_data = desc->private_data;
> +}
> +
>  int
>  do_vmi_align_munmap(struct vma_iterator *vmi, struct vm_area_struct *vma,
>  		    struct mm_struct *mm, unsigned long start,
> @@ -570,4 +617,8 @@ int create_init_stack_vma(struct mm_struct *mm, struct vm_area_struct **vmap,
>  int relocate_vma_down(struct vm_area_struct *vma, unsigned long shift);
>  #endif
>  
> +struct vm_area_desc *vma_to_desc(struct vm_area_struct *vma,
> +		struct vm_area_desc *desc);
> +void set_vma_from_desc(struct vm_area_struct *vma, struct vm_area_desc *desc);
> +
>  #endif	/* __MM_VMA_H */
> diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
> index 77b2949d874a..675a55216607 100644
> --- a/tools/testing/vma/vma_internal.h
> +++ b/tools/testing/vma/vma_internal.h
> @@ -159,6 +159,14 @@ typedef __bitwise unsigned int vm_fault_t;
>  
>  #define ASSERT_EXCLUSIVE_WRITER(x)
>  
> +/**
> + * swap - swap values of @a and @b
> + * @a: first value
> + * @b: second value
> + */
> +#define swap(a, b) \
> +	do { typeof(a) __tmp = (a); (a) = (b); (b) = __tmp; } while (0)
> +
>  struct kref {
>  	refcount_t refcount;
>  };
> @@ -1479,4 +1487,12 @@ static inline vm_flags_t ksm_vma_flags(const struct mm_struct *, const struct fi
>  	return vm_flags;
>  }
>  
> +static inline void vma_set_file(struct vm_area_struct *vma, struct file *file)
> +{
> +	/* Changing an anonymous vma with this is illegal */
> +	get_file(file);
> +	swap(vma->vm_file, file);
> +	fput(file);
> +}
> +
>  #endif	/* __MM_VMA_INTERNAL_H */
> -- 
> 2.49.0
> 

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