mm: allow handling of stacked mmap_prepare hooks in more drivers

While the conversion of mmap hooks to mmap_prepare is underway, we will
encounter situations where mmap hooks need to invoke nested mmap_prepare
hooks.

The nesting of mmap hooks is termed 'stacking'.  In order to flexibly
facilitate the conversion of custom mmap hooks in drivers which stack, we
must split up the existing __compat_vma_mmap() function into two separate
functions:

* compat_set_desc_from_vma() - This allows the setting of a vm_area_desc
  object's fields to the relevant fields of a VMA.

* __compat_vma_mmap() - Once an mmap_prepare hook has been executed upon a
  vm_area_desc object, this function performs any mmap actions specified by
  the mmap_prepare hook and then invokes its vm_ops->mapped() hook if any
  were specified.

In ordinary cases, where a file's f_op->mmap_prepare() hook simply needs
to be invoked in a stacked mmap() hook, compat_vma_mmap() can be used.

However some drivers define their own nested hooks, which are invoked in
turn by another hook.

A concrete example is vmbus_channel->mmap_ring_buffer(), which is invoked
in turn by bin_attribute->mmap():

vmbus_channel->mmap_ring_buffer() has a signature of:

int (*mmap_ring_buffer)(struct vmbus_channel *channel,
			struct vm_area_struct *vma);

And bin_attribute->mmap() has a signature of:

	int (*mmap)(struct file *, struct kobject *,
		    const struct bin_attribute *attr,
		    struct vm_area_struct *vma);

And so compat_vma_mmap() cannot be used here for incremental conversion of
hooks from mmap() to mmap_prepare().

There are many such instances like this, where conversion to mmap_prepare
would otherwise cascade to a huge change set due to nesting of this kind.

The changes in this patch mean we could now instead convert
vmbus_channel->mmap_ring_buffer() to
vmbus_channel->mmap_prepare_ring_buffer(), and implement something like:

	struct vm_area_desc desc;
	int err;

	compat_set_desc_from_vma(&desc, file, vma);
	err = channel->mmap_prepare_ring_buffer(channel, &desc);
	if (err)
		return err;

	return __compat_vma_mmap(&desc, vma);

Allowing us to incrementally update this logic, and other logic like it.

Unfortunately, as part of this change, we need to be able to flexibly
assign to the VMA descriptor, so have to remove some of the const
declarations within the structure.

Also update the VMA tests to reflect the changes.

Link: https://lkml.kernel.org/r/24aac3019dd34740e788d169fccbe3c62781e648.1774045440.git.ljs@kernel.org
Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Acked-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexandre Torgue <alexandre.torgue@foss.st.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Bodo Stroesser <bostroesser@gmail.com>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Clemens Ladisch <clemens@ladisch.de>
Cc: David Hildenbrand <david@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Long Li <longli@microsoft.com>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Pedro Falcato <pfalcato@suse.de>
Cc: Richard Weinberger <richard@nod.at>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Wei Liu <wei.liu@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

2 files changed, 5 insertions, 2 deletions

diff --git a/include/linux/fs.h b/include/linux/fs.h
index c390f5c667e3..0bdccfa70b44 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2058,6 +2058,9 @@ static inline bool can_mmap_file(struct file *file)
 	return true;
 }
 
+void compat_set_desc_from_vma(struct vm_area_desc *desc, const struct file *file,
+			      const struct vm_area_struct *vma);
+int __compat_vma_mmap(struct vm_area_desc *desc, struct vm_area_struct *vma);
 int compat_vma_mmap(struct file *file, struct vm_area_struct *vma);
 int __vma_check_mmap_hook(struct vm_area_struct *vma);
 
diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 91a3db174d78..b702c63bf0e0 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -891,8 +891,8 @@ static __always_inline bool vma_flags_empty(const vma_flags_t *flags)
  */
 struct vm_area_desc {
 	/* Immutable state. */
-	const struct mm_struct *const mm;
-	struct file *const file; /* May vary from vm_file in stacked callers. */
+	struct mm_struct *mm;
+	struct file *file; /* May vary from vm_file in stacked callers. */
 	unsigned long start;
 	unsigned long end;