mm: add mmap_prepare() compatibility layer for nested file systems

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.

[akpm@linux-foundation.org: export compat_vma_mmap_prepare tp fix build]
[lorenzo.stoakes@oracle.com: remove unused declarations]
  Link: https://lkml.kernel.org/r/ac3ae324-4c65-432a-8c6d-2af988b18ac8@lucifer.local
Link: https://lkml.kernel.org/r/20250609165749.344976-1-lorenzo.stoakes@oracle.com
Fixes: c84bf6dd2b83 ("mm: introduce new .mmap_prepare() file callback").
Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Reported-by: Jann Horn <jannh@google.com>
Closes: https://lore.kernel.org/linux-mm/CAG48ez04yOEVx1ekzOChARDDBZzAKwet8PEoPM4Ln3_rk91AzQ@mail.gmail.com/
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 47 insertions, 0 deletions

diff --git a/mm/vma.h b/mm/vma.h
index 0db066e7a45d..f47112a352db 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,