openwrt overlayfs 2.6.36内核补丁

openwrt的rootfs默认都是squashfs的，只读属性，压缩性高。为了实现可写的需求，openwrt利用了overlay来实现。本文在broadcom的sdk 2.6.36上实现overlayfs支持。
    从openwrt的官方站上找到早期的2.6kernel的overlayfs补丁文件，实际对比下，比较合适的就这个：
https://dev.openwrt.org/browser/trunk/target/linux/generic/patches-2.6.38/209-overlayfs.patch?rev=26213，
在此基础上，因为涉及到了struct inode_operations，故proc,sysfs,fuse模块都得必须手动修改，对照编译错误，修改下 *_inode_operations.permission 的回调实现即可。
   

diff -uNr linux-2.6.36-ori/fs/sysfs/inode.c linux-2.6.36/fs/sysfs/inode.c
--- linux-2.6.36-ori/fs/sysfs/inode.c    2014-10-29 17:25:24.203842130 +0800
+++ linux-2.6.36/fs/sysfs/inode.c    2014-10-29 17:46:47.328092412 +0800
@@ -348,13 +348,16 @@
         return -ENOENT;
 }

-int sysfs_permission(struct inode *inode, int mask)
+int sysfs_permission(struct inode *inode, int mask, unsigned int flags)
 {
     struct sysfs_dirent *sd = inode->i_private;

-    mutex_lock(&sysfs_mutex);
+    if (flags & IPERM_FLAG_RCU)
+        return -ECHILD;
+
+ mutex_lock(&sysfs_mutex);
     sysfs_refresh_inode(sd, inode);
     mutex_unlock(&sysfs_mutex);

-    return generic_permission(inode, mask, NULL);
+    return generic_permission(inode, mask, flags, NULL);
 }
diff -uNr linux-2.6.36-ori/fs/sysfs/sysfs.h linux-2.6.36/fs/sysfs/sysfs.h
--- linux-2.6.36-ori/fs/sysfs/sysfs.h    2014-10-29 17:25:24.203842130 +0800
+++ linux-2.6.36/fs/sysfs/sysfs.h    2014-10-29 17:46:47.328092412 +0800
@@ -200,7 +200,7 @@
 struct inode *sysfs_get_inode(struct super_block *sb, struct sysfs_dirent *sd);
 void sysfs_evict_inode(struct inode *inode);
 int sysfs_sd_setattr(struct sysfs_dirent *sd, struct iattr *iattr);
-int sysfs_permission(struct inode *inode, int mask);
+int sysfs_permission(struct inode *inode, int mask, unsigned int flags);
 int sysfs_setattr(struct dentry *dentry, struct iattr *iattr);
 int sysfs_getattr(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat);
 int sysfs_setxattr(struct dentry *dentry, const char *name, const void *value,

diff -uNr linux-2.6.36-ori/fs/proc/base.c linux-2.6.36/fs/proc/base.c
--- linux-2.6.36-ori/fs/proc/base.c    2014-10-29 17:25:24.208842063 +0800
+++ linux-2.6.36/fs/proc/base.c    2014-10-29 17:46:47.326092612 +0800
@@ -2050,11 +2050,14 @@
  * /proc/pid/fd needs a special permission handler so that a process can still
  * access /proc/self/fd after it has executed a setuid().
  */
-static int proc_fd_permission(struct inode *inode, int mask)
+static int proc_fd_permission(struct inode *inode, int mask, unsigned int flags)
 {
     int rv;

-    rv = generic_permission(inode, mask, NULL);
+    if (flags & IPERM_FLAG_RCU)
+        return -ECHILD;
+
+    rv = generic_permission(inode, mask, flags, NULL);
     if (rv == 0)
         return 0;
     if (task_pid(current) == proc_pid(inode))

diff -uNr linux-2.6.36-ori/fs/fuse/dir.c linux-2.6.36/fs/fuse/dir.c
--- linux-2.6.36-ori/fs/fuse/dir.c    2014-10-29 17:25:24.287842062 +0800
+++ linux-2.6.36/fs/fuse/dir.c    2014-10-29 17:46:47.318092493 +0800
@@ -981,12 +981,15 @@
  * access request is sent. Execute permission is still checked
  * locally based on file mode.
  */
-static int fuse_permission(struct inode *inode, int mask)
+static int fuse_permission(struct inode *inode, int mask, unsigned int flags)
 {
     struct fuse_conn *fc = get_fuse_conn(inode);
     bool refreshed = false;
     int err = 0;

+    if (flags & IPERM_FLAG_RCU)
+        return -ECHILD;
+
     if (!fuse_allow_task(fc, current))
         return -EACCES;

@@ -1001,7 +1004,7 @@
     }

     if (fc->flags & FUSE_DEFAULT_PERMISSIONS) {
-        err = generic_permission(inode, mask, NULL);
+        err = generic_permission(inode, mask, flags, NULL);

         /* If permission is denied, try to refresh file
            attributes. This is also needed, because the root
@@ -1009,7 +1012,7 @@
         if (err == -EACCES && !refreshed) {
             err = fuse_do_getattr(inode, NULL, NULL);
             if (!err)
-                err = generic_permission(inode, mask, NULL);
+                err = generic_permission(inode, mask, flags, NULL);
         }

         /* Note: the opposite of the above test does not

还有一个overlayfs的patch非常重要，
https://dev.openwrt.org/browser/trunk/target/linux/generic/patches-2.6.38/210-overlayfs_fix_readdir_deadlock.patch?rev=26213
解决overlayfs上死锁问题。问题现象是进入overlay的目录后ls，内核就会挂住。