mm: replace __access_remote_vm() write parameter with gup_flags

This removes the 'write' argument from __access_remote_vm() and replaces
it with 'gup_flags' as use of this function previously silently implied
FOLL_FORCE, whereas after this patch callers explicitly pass this flag.

We make this explicit as use of FOLL_FORCE can result in surprising
behaviour (and hence bugs) within the mm subsystem.

Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -3869,14 +3869,11 @@ EXPORT_SYMBOL_GPL(generic_access_phys);
  * given task for page fault accounting.
  */
 static int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
-		unsigned long addr, void *buf, int len, int write)
+		unsigned long addr, void *buf, int len, unsigned int gup_flags)
 {
 	struct vm_area_struct *vma;
 	void *old_buf = buf;
-	unsigned int flags = FOLL_FORCE;
-
-	if (write)
-		flags |= FOLL_WRITE;
+	int write = gup_flags & FOLL_WRITE;
 
 	down_read(&mm->mmap_sem);
 	/* ignore errors, just check how much was successfully transferred */
@@ -3886,7 +3883,7 @@ static int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
 		struct page *page = NULL;
 
 		ret = get_user_pages_remote(tsk, mm, addr, 1,
-				flags, &page, &vma);
+				gup_flags, &page, &vma);
 		if (ret <= 0) {
 #ifndef CONFIG_HAVE_IOREMAP_PROT
 			break;
@@ -3945,7 +3942,12 @@ static int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
 int access_remote_vm(struct mm_struct *mm, unsigned long addr,
 		void *buf, int len, int write)
 {
-	return __access_remote_vm(NULL, mm, addr, buf, len, write);
+	unsigned int flags = FOLL_FORCE;
+
+	if (write)
+		flags |= FOLL_WRITE;
+
+	return __access_remote_vm(NULL, mm, addr, buf, len, flags);
 }
 
 /*
@@ -3958,12 +3960,17 @@ int access_process_vm(struct task_struct *tsk, unsigned long addr,
 {
 	struct mm_struct *mm;
 	int ret;
+	unsigned int flags = FOLL_FORCE;
 
 	mm = get_task_mm(tsk);
 	if (!mm)
 		return 0;
 
-	ret = __access_remote_vm(tsk, mm, addr, buf, len, write);
+	if (write)
+		flags |= FOLL_WRITE;
+
+	ret = __access_remote_vm(tsk, mm, addr, buf, len, flags);
+
 	mmput(mm);
 
 	return ret;

mm/nommu.c

@@ -1809,9 +1809,10 @@ void filemap_map_pages(struct fault_env *fe,
 EXPORT_SYMBOL(filemap_map_pages);
 
 static int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
-		unsigned long addr, void *buf, int len, int write)
+		unsigned long addr, void *buf, int len, unsigned int gup_flags)
 {
 	struct vm_area_struct *vma;
+	int write = gup_flags & FOLL_WRITE;
 
 	down_read(&mm->mmap_sem);
 
@@ -1853,7 +1854,8 @@ static int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
 int access_remote_vm(struct mm_struct *mm, unsigned long addr,
 		void *buf, int len, int write)
 {
-	return __access_remote_vm(NULL, mm, addr, buf, len, write);
+	return __access_remote_vm(NULL, mm, addr, buf, len,
+			write ? FOLL_WRITE : 0);
 }
 
 /*
@@ -1871,7 +1873,8 @@ int access_process_vm(struct task_struct *tsk, unsigned long addr, void *buf, in
 	if (!mm)
 		return 0;
 
-	len = __access_remote_vm(tsk, mm, addr, buf, len, write);
+	len = __access_remote_vm(tsk, mm, addr, buf, len,
+			write ? FOLL_WRITE : 0);
 
 	mmput(mm);
 	return len;