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Subject[PATCH 06/39] vfs: add f_op->pre_mmap()
Date
This is needed by overlayfs to be able to copy up a file from a read-only
lower layer to a writable layer when being mapped shared. When copying up,
overlayfs takes VFS locks that would violate locking order when nested
inside mmap_sem.

Add a new f_op->pre_mmap method, which is called before taking mmap_sem.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
---
Documentation/filesystems/Locking | 1 +
Documentation/filesystems/vfs.txt | 3 +++
include/linux/fs.h | 1 +
mm/util.c | 5 +++++
4 files changed, 10 insertions(+)

diff --git a/Documentation/filesystems/Locking b/Documentation/filesystems/Locking
index 75d2d57e2c44..60e76060baff 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -442,6 +442,7 @@ prototypes:
unsigned int (*poll) (struct file *, struct poll_table_struct *);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
+ int (*pre_mmap) (struct file *, unsigned long, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
int (*open) (struct inode *, struct file *);
int (*flush) (struct file *);
diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index 5fd325df59e2..2bc77ea8aef4 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -859,6 +859,7 @@ struct file_operations {
unsigned int (*poll) (struct file *, struct poll_table_struct *);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
+ int (*pre_mmap) (struct file *, unsigned long, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
int (*mremap)(struct file *, struct vm_area_struct *);
int (*open) (struct inode *, struct file *);
@@ -906,6 +907,8 @@ otherwise noted.
compat_ioctl: called by the ioctl(2) system call when 32 bit system calls
are used on 64 bit kernels.

+ pre_mmap: called before mmap, without mmap_sem being held yet.
+
mmap: called by the mmap(2) system call

open: called by the VFS when an inode should be opened. When the VFS
diff --git a/include/linux/fs.h b/include/linux/fs.h
index ecc854c75611..1ea3f153b7f8 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1716,6 +1716,7 @@ struct file_operations {
__poll_t (*poll) (struct file *, struct poll_table_struct *);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
+ int (*pre_mmap) (struct file *, unsigned long, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
unsigned long mmap_supported_flags;
int (*open) (struct inode *, struct file *);
diff --git a/mm/util.c b/mm/util.c
index 45fc3169e7b0..11cd375e1a19 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -352,6 +352,11 @@ unsigned long vm_mmap_pgoff(struct file *file, unsigned long addr,

ret = security_mmap_file(file, prot, flag);
if (!ret) {
+ if (file && file->f_op->pre_mmap) {
+ ret = file->f_op->pre_mmap(file, prot, flag);
+ if (ret)
+ return ret;
+ }
if (down_write_killable(&mm->mmap_sem))
return -EINTR;
ret = do_mmap_pgoff(file, addr, len, prot, flag, pgoff,
--
2.14.3
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