[PATCH v2 1/2] mm: specify separate file and vm_file params in vm_area_desc

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

Stacked filesystems and drivers may invoke mmap hooks with a struct file
pointer that differs from the overlying file. We will make this
functionality possible in a subsequent patch.

In order to prepare for this, let's update vm_area_struct to separately
provide desc->file and desc->vm_file parameters.

The desc->file parameter is the file that the hook is expected to operate
upon, and is not assignable (though the hok may wish to e.g. update the
file's accessed time for instance).

The desc->vm_file defaults to what will become vma->vm_file and is what the
hook must reassign should it wish to change the VMA"s vma->vm_file.

For now we keep desc->file, vm_file the same to remain consistent.

No f_op->mmap_prepare() callback sets a new vma->vm_file currently, so this
is safe to change.

While we're here, make the mm_struct desc->mm pointers at immutable as well
as the desc->mm field itself.

As part of this change, also update the single hook which this would
otherwise break - mlock_future_ok(), invoked by secretmem_mmap_prepare()).

We additionally update set_vma_from_desc() to compare fields in a more
logical fashion, checking the (possibly) user-modified fields as the
first operand against the existing value as the second one.

Additionally, update VMA tests to accommodate changes.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 include/linux/mm_types.h         |  5 +++--
 mm/internal.h                    |  4 ++--
 mm/mmap.c                        |  2 +-
 mm/util.c                        | 14 ++++++++++++--
 mm/vma.c                         |  5 +++--
 mm/vma.h                         | 28 ++++------------------------
 tools/testing/vma/vma_internal.h | 28 ++++++++++++++++++----------
 7 files changed, 43 insertions(+), 43 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index d934a3a5b443..73c6c0340064 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -785,13 +785,14 @@ struct pfnmap_track_ctx {
  */
 struct vm_area_desc {
 	/* Immutable state. */
-	struct mm_struct *mm;
+	const struct mm_struct *const mm;
+	struct file *const file; /* May vary from vm_file in stacked callers. */
 	unsigned long start;
 	unsigned long end;
 
 	/* Mutable fields. Populated with initial state. */
 	pgoff_t pgoff;
-	struct file *file;
+	struct file *vm_file;
 	vm_flags_t vm_flags;
 	pgprot_t page_prot;
 
diff --git a/mm/internal.h b/mm/internal.h
index 9b0129531d00..456a41e8ed28 100644
--- a/mm/internal.h
+++ b/mm/internal.h
@@ -962,8 +962,8 @@ extern long populate_vma_page_range(struct vm_area_struct *vma,
 		unsigned long start, unsigned long end, int *locked);
 extern long faultin_page_range(struct mm_struct *mm, unsigned long start,
 		unsigned long end, bool write, int *locked);
-extern bool mlock_future_ok(struct mm_struct *mm, vm_flags_t vm_flags,
-			       unsigned long bytes);
+bool mlock_future_ok(const struct mm_struct *mm, vm_flags_t vm_flags,
+		unsigned long bytes);
 
 /*
  * NOTE: This function can't tell whether the folio is "fully mapped" in the
diff --git a/mm/mmap.c b/mm/mmap.c
index 7a057e0e8da9..5fd3b80fda1d 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -225,7 +225,7 @@ static inline unsigned long round_hint_to_min(unsigned long hint)
 	return hint;
 }
 
-bool mlock_future_ok(struct mm_struct *mm, vm_flags_t vm_flags,
+bool mlock_future_ok(const struct mm_struct *mm, vm_flags_t vm_flags,
 			unsigned long bytes)
 {
 	unsigned long locked_pages, limit_pages;
diff --git a/mm/util.c b/mm/util.c
index bb4b47cd6709..ee2544566ac3 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -1161,10 +1161,20 @@ EXPORT_SYMBOL(flush_dcache_folio);
  */
 int compat_vma_mmap_prepare(struct file *file, struct vm_area_struct *vma)
 {
-	struct vm_area_desc desc;
+	struct vm_area_desc desc = {
+		.mm = vma->vm_mm,
+		.file = vma->vm_file,
+		.start = vma->vm_start,
+		.end = vma->vm_end,
+
+		.pgoff = vma->vm_pgoff,
+		.vm_file = vma->vm_file,
+		.vm_flags = vma->vm_flags,
+		.page_prot = vma->vm_page_prot,
+	};
 	int err;
 
-	err = file->f_op->mmap_prepare(vma_to_desc(vma, &desc));
+	err = file->f_op->mmap_prepare(&desc);
 	if (err)
 		return err;
 	set_vma_from_desc(vma, &desc);
diff --git a/mm/vma.c b/mm/vma.c
index 3b12c7579831..abe0da33c844 100644
--- a/mm/vma.c
+++ b/mm/vma.c
@@ -2572,11 +2572,12 @@ static int call_mmap_prepare(struct mmap_state *map)
 	int err;
 	struct vm_area_desc desc = {
 		.mm = map->mm,
+		.file = map->file,
 		.start = map->addr,
 		.end = map->end,
 
 		.pgoff = map->pgoff,
-		.file = map->file,
+		.vm_file = map->file,
 		.vm_flags = map->vm_flags,
 		.page_prot = map->page_prot,
 	};
@@ -2588,7 +2589,7 @@ static int call_mmap_prepare(struct mmap_state *map)
 
 	/* Update fields permitted to be changed. */
 	map->pgoff = desc.pgoff;
-	map->file = desc.file;
+	map->file = desc.vm_file;
 	map->vm_flags = desc.vm_flags;
 	map->page_prot = desc.page_prot;
 	/* User-defined fields. */
diff --git a/mm/vma.h b/mm/vma.h
index bcdc261c5b15..9183fe549009 100644
--- a/mm/vma.h
+++ b/mm/vma.h
@@ -222,31 +222,11 @@ static inline int vma_iter_store_gfp(struct vma_iterator *vmi,
 	return 0;
 }
 
-
 /*
- * Temporary helper functions for file systems which wrap an invocation of
+ * Temporary helper function for stacked mmap handlers which specify
  * 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)
 {
@@ -258,9 +238,9 @@ static inline void set_vma_from_desc(struct vm_area_struct *vma,
 
 	/* 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)
+	if (desc->vm_file != vma->vm_file)
+		vma_set_file(vma, desc->vm_file);
+	if (desc->vm_flags != vma->vm_flags)
 		vm_flags_set(vma, desc->vm_flags);
 	vma->vm_page_prot = desc->page_prot;
 
diff --git a/tools/testing/vma/vma_internal.h b/tools/testing/vma/vma_internal.h
index 6f95ec14974f..a519cf4c45d3 100644
--- a/tools/testing/vma/vma_internal.h
+++ b/tools/testing/vma/vma_internal.h
@@ -283,13 +283,14 @@ struct vm_area_struct;
  */
 struct vm_area_desc {
 	/* Immutable state. */
-	struct mm_struct *mm;
+	const struct mm_struct *const mm;
+	struct file *const file; /* May vary from vm_file in stacked callers. */
 	unsigned long start;
 	unsigned long end;
 
 	/* Mutable fields. Populated with initial state. */
 	pgoff_t pgoff;
-	struct file *file;
+	struct file *vm_file;
 	vm_flags_t vm_flags;
 	pgprot_t page_prot;
 
@@ -1264,8 +1265,8 @@ static inline bool capable(int cap)
 	return true;
 }
 
-static inline bool mlock_future_ok(struct mm_struct *mm, vm_flags_t vm_flags,
-			unsigned long bytes)
+static inline bool mlock_future_ok(const struct mm_struct *mm,
+		vm_flags_t vm_flags, unsigned long bytes)
 {
 	unsigned long locked_pages, limit_pages;
 
@@ -1413,16 +1414,23 @@ static inline void free_anon_vma_name(struct vm_area_struct *vma)
 static inline void set_vma_from_desc(struct vm_area_struct *vma,
 		struct vm_area_desc *desc);
 
-static inline struct vm_area_desc *vma_to_desc(struct vm_area_struct *vma,
-		struct vm_area_desc *desc);
-
-static int compat_vma_mmap_prepare(struct file *file,
+static inline int compat_vma_mmap_prepare(struct file *file,
 		struct vm_area_struct *vma)
 {
-	struct vm_area_desc desc;
+	struct vm_area_desc desc = {
+		.mm = vma->vm_mm,
+		.file = vma->vm_file,
+		.start = vma->vm_start,
+		.end = vma->vm_end,
+
+		.pgoff = vma->vm_pgoff,
+		.vm_file = vma->vm_file,
+		.vm_flags = vma->vm_flags,
+		.page_prot = vma->vm_page_prot,
+	};
 	int err;
 
-	err = file->f_op->mmap_prepare(vma_to_desc(vma, &desc));
+	err = file->f_op->mmap_prepare(&desc);
 	if (err)
 		return err;
 	set_vma_from_desc(vma, &desc);
-- 
2.50.1