[PATCH v2 4/7] mm/gup: introduce the FOLL_SAME_FILE GUP flag

[Lorenzo Stoakes <lstoakes@gmail.com>]

This flag causes GUP to assert that all VMAs within the input range possess
the same vma->vm_file. If not, the operation fails.

This is part of a patch series which eliminates the vmas parameter from the
GUP API, implementing the one remaining assertion within the entire kernel
that requires access to the VMAs associated with a GUP range.

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
---
 include/linux/mm_types.h |  2 ++
 mm/gup.c                 | 16 ++++++++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 3fc9e680f174..84d1aec9dbab 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -1185,6 +1185,8 @@ enum {
 	FOLL_PCI_P2PDMA = 1 << 10,
 	/* allow interrupts from generic signals */
 	FOLL_INTERRUPTIBLE = 1 << 11,
+	/* assert that the range spans VMAs with the same vma->vm_file */
+	FOLL_SAME_FILE = 1 << 12,
 
 	/* See also internal only FOLL flags in mm/internal.h */
 };
diff --git a/mm/gup.c b/mm/gup.c
index 9440aa54c741..3954ce499a4a 100644
--- a/mm/gup.c
+++ b/mm/gup.c
@@ -959,7 +959,8 @@ static int faultin_page(struct vm_area_struct *vma,
 	return 0;
 }
 
-static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
+static int check_vma_flags(struct vm_area_struct *vma, struct file *file,
+			   unsigned long gup_flags)
 {
 	vm_flags_t vm_flags = vma->vm_flags;
 	int write = (gup_flags & FOLL_WRITE);
@@ -968,7 +969,7 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
 	if (vm_flags & (VM_IO | VM_PFNMAP))
 		return -EFAULT;
 
-	if (gup_flags & FOLL_ANON && !vma_is_anonymous(vma))
+	if ((gup_flags & FOLL_ANON) && !vma_is_anonymous(vma))
 		return -EFAULT;
 
 	if ((gup_flags & FOLL_LONGTERM) && vma_is_fsdax(vma))
@@ -977,6 +978,9 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
 	if (vma_is_secretmem(vma))
 		return -EFAULT;
 
+	if ((gup_flags & FOLL_SAME_FILE) && vma->vm_file != file)
+		return -EFAULT;
+
 	if (write) {
 		if (!(vm_flags & VM_WRITE)) {
 			if (!(gup_flags & FOLL_FORCE))
@@ -1081,6 +1085,7 @@ static long __get_user_pages(struct mm_struct *mm,
 	long ret = 0, i = 0;
 	struct vm_area_struct *vma = NULL;
 	struct follow_page_context ctx = { NULL };
+	struct file *file = NULL;
 
 	if (!nr_pages)
 		return 0;
@@ -1111,10 +1116,13 @@ static long __get_user_pages(struct mm_struct *mm,
 				ret = -EFAULT;
 				goto out;
 			}
-			ret = check_vma_flags(vma, gup_flags);
+			ret = check_vma_flags(vma, i == 0 ? vma->vm_file : file,
+					      gup_flags);
 			if (ret)
 				goto out;
 
+			file = vma->vm_file;
+
 			if (is_vm_hugetlb_page(vma)) {
 				i = follow_hugetlb_page(mm, vma, pages, vmas,
 						&start, &nr_pages, i,
@@ -1595,7 +1603,7 @@ long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
 	 * We want to report -EINVAL instead of -EFAULT for any permission
 	 * problems or incompatible mappings.
 	 */
-	if (check_vma_flags(vma, gup_flags))
+	if (check_vma_flags(vma, vma->vm_file, gup_flags))
 		return -EINVAL;
 
 	ret = __get_user_pages(mm, start, nr_pages, gup_flags,
-- 
2.40.0