Re: [PATCH 1/3] mm: introduce new .mmap_prepare() file callback

[David Hildenbrand <david@redhat.com>]

On 07.05.25 13:03, Lorenzo Stoakes wrote:
> Provide a means by which drivers can specify which fields of those
> permitted to be changed should be altered to prior to mmap()'ing a
> range (which may either result from a merge or from mapping an entirely new
> VMA).
> 
> Doing so is substantially safer than the existing .mmap() calback which
> provides unrestricted access to the part-constructed VMA and permits
> drivers and file systems to do 'creative' things which makes it hard to
> reason about the state of the VMA after the function returns.
> 
> The existing .mmap() callback's freedom has caused a great deal of issues,
> especially in error handling, as unwinding the mmap() state has proven to
> be non-trivial and caused significant issues in the past, for instance
> those addressed in commit 5de195060b2e ("mm: resolve faulty mmap_region()
> error path behaviour").
> 
> It also necessitates a second attempt at merge once the .mmap() callback
> has completed, which has caused issues in the past, is awkward, adds
> overhead and is difficult to reason about.
> 
> The .mmap_prepare() callback eliminates this requirement, as we can update
> fields prior to even attempting the first merge. It is safer, as we heavily
> restrict what can actually be modified, and being invoked very early in the
> mmap() process, error handling can be performed safely with very little
> unwinding of state required.
> 
> The .mmap_prepare() and deprecated .mmap() callbacks are mutually
> exclusive, so we permit only one to be invoked at a time.
> 
> Update vma userland test stubs to account for changes.
> 

In general, looks very good to me.

Some comments, especially regarding suboptimal code duplciation with the 
stubs. (unless I am missing fine details :) )

> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> ---
>   include/linux/fs.h               | 38 +++++++++++++++
>   include/linux/mm_types.h         | 24 ++++++++++
>   mm/memory.c                      |  3 +-
>   mm/mmap.c                        |  2 +-
>   mm/vma.c                         | 70 +++++++++++++++++++++++++++-
>   tools/testing/vma/vma_internal.h | 79 ++++++++++++++++++++++++++++++--
>   6 files changed, 208 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 016b0fe1536e..d6c5a703a215 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2169,6 +2169,7 @@ struct file_operations {
>   	int (*uring_cmd)(struct io_uring_cmd *ioucmd, unsigned int issue_flags);
>   	int (*uring_cmd_iopoll)(struct io_uring_cmd *, struct io_comp_batch *,
>   				unsigned int poll_flags);
> +	int (*mmap_prepare)(struct vm_area_desc *);
>   } __randomize_layout;
>   
>   /* Supports async buffered reads */
> @@ -2238,11 +2239,48 @@ struct inode_operations {
>   	struct offset_ctx *(*get_offset_ctx)(struct inode *inode);
>   } ____cacheline_aligned;
>   
> +static inline bool file_has_deprecated_mmap_hook(struct file *file)
> +{
> +	return file->f_op->mmap;
> +}
> +
> +static inline bool file_has_mmap_prepare_hook(struct file *file)
> +{
> +	return file->f_op->mmap_prepare;
> +}

I am usually not a fan of such dummy helper functions .. I mean, how far 
do we go?

file_has_f_op()

file_is_non_null()

...

Or is this required for some stubbing regarding vma tests? But even the 
stubs below confuse me a bit, because they do exactly the same thing :(

:)

> +
> +/* Did the driver provide valid mmap hook configuration? */
> +static inline bool file_has_valid_mmap_hooks(struct file *file)
> +{
> +	bool has_mmap = file_has_deprecated_mmap_hook(file);
> +	bool has_mmap_prepare = file_has_mmap_prepare_hook(file);
> +
> +	/* Hooks are mutually exclusive. */
> +	if (has_mmap && has_mmap_prepare)

Should this be WARN_ON_ONCE() ?

> +		return false;
> +
> +	/* But at least one must be specified. */
> +	if (!has_mmap && !has_mmap_prepare)
> +		return false;
> +
 > +	return true;

return has_mmap || has_mmap_prepare;

And I think you can drop the comment about "at least one" with that, 
should be quite clear from that simplified version.

> +}
> +
>   static inline int call_mmap(struct file *file, struct vm_area_struct *vma)
>   {
> +	/* If the driver specifies .mmap_prepare() this call is invalid. */
> +	if (file_has_mmap_prepare_hook(file))

Should this be WARN_ON_ONCE() ?

> +		return -EINVAL;
> +
>   	return file->f_op->mmap(file, vma);
>   }
>   
> +static inline int __call_mmap_prepare(struct file *file,
> +		struct vm_area_desc *desc)
> +{
> +	return file->f_op->mmap_prepare(desc);
> +}
> +

[...]

>   struct file {
>   	struct address_space	*f_mapping;
> +	const struct file_operations	*f_op;
>   };
>   
>   #define VMA_LOCK_OFFSET	0x40000000
> @@ -1125,11 +1157,6 @@ static inline void vm_flags_clear(struct vm_area_struct *vma,
>   	vma->__vm_flags &= ~flags;
>   }
>   
> -static inline int call_mmap(struct file *, struct vm_area_struct *)
> -{
> -	return 0;
> -}
> -
>   static inline int shmem_zero_setup(struct vm_area_struct *)
>   {
>   	return 0;
> @@ -1405,4 +1432,46 @@ static inline void free_anon_vma_name(struct vm_area_struct *vma)
>   	(void)vma;
>   }
>   
> +static inline bool file_has_deprecated_mmap_hook(struct file *file)
> +{
> +	return file->f_op->mmap;
> +}
> +
> +static inline bool file_has_mmap_prepare_hook(struct file *file)
> +{
> +	return file->f_op->mmap_prepare;
> +}
 > +> +/* Did the driver provide valid mmap hook configuration? */
> +static inline bool file_has_valid_mmap_hooks(struct file *file)
> +{
> +	bool has_mmap = file_has_deprecated_mmap_hook(file);
> +	bool has_mmap_prepare = file_has_mmap_prepare_hook(file);
> +
> +	/* Hooks are mutually exclusive. */
> +	if (has_mmap && has_mmap_prepare)
> +		return false;
 > +> +	/* But at least one must be specified. */
> +	if (!has_mmap && !has_mmap_prepare)
> +		return false;
> +
 > +	return true;> +}
> +
> +static inline int call_mmap(struct file *file, struct vm_area_struct *vma)
> +{
> +	/* If the driver specifies .mmap_prepare() this call is invalid. */
> +	if (file_has_mmap_prepare_hook(file))
 > +		return -EINVAL;> +
> +	return file->f_op->mmap(file, vma);
> +}
> +
> +static inline int __call_mmap_prepare(struct file *file,
> +		struct vm_area_desc *desc)
> +{
> +	return file->f_op->mmap_prepare(desc);
> +}

Hm, is there a way avoid a copy of the exact same code from fs.h, and 
essentially test the implementation in fs.h (-> more coverage by using 
less duplciated stubs?).

-- 
Cheers,

David / dhildenb