[PATCH 08/10] mm: replace __access_remote_vm() write parameter with gup_flags

[Lorenzo Stoakes]

This patch removes the write parameter from __access_remote_vm() and replaces it
with a gup_flags parameter as use of this function previously _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 <lstoa...@gmail.com>
---
 mm/memory.c | 23 +++++++++++++++--------
 mm/nommu.c  |  9 ++++++---
 2 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/mm/memory.c b/mm/memory.c
index 20a9adb..79ebed3 100644
--- a/mm/memory.c
+++ b/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;
diff --git a/mm/nommu.c b/mm/nommu.c
index 70cb844..bde7df3 100644
--- a/mm/nommu.c
+++ b/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;
-- 
2.10.0