The problem is that a pathname can contain absolute symlinks and now
they are resolved relative to the current root.
If we want to open a file in another mount namespaces and we have a file
descriptor to its root directory, we probably want to resolve pathname
in the target mount namespace. For this we add this new flag.

If LOOKUP_DFD_ROOT is set, path_init() initializes nd->root and nd->path
to the same value.

Signed-off-by: Andrey Vagin <ava...@openvz.org>
---
 fs/namei.c            | 12 +++++++++++-
 include/linux/namei.h |  2 ++
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 70580ab..5f08b69 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2148,7 +2148,7 @@ static const char *path_init(struct nameidata *nd, 
unsigned flags)
        nd->path.dentry = NULL;
 
        nd->m_seq = read_seqbegin(&mount_lock);
-       if (*s == '/') {
+       if (*s == '/' && !(flags & LOOKUP_DFD_ROOT)) {
                if (flags & LOOKUP_RCU)
                        rcu_read_lock();
                set_root(nd);
@@ -2174,6 +2174,11 @@ static const char *path_init(struct nameidata *nd, 
unsigned flags)
                        get_fs_pwd(current->fs, &nd->path);
                        nd->inode = nd->path.dentry->d_inode;
                }
+               if (flags & LOOKUP_DFD_ROOT) {
+                       nd->root = nd->path;
+                       if (!(flags & LOOKUP_RCU))
+                               path_get(&nd->root);
+               }
                return s;
        } else {
                /* Caller must check execute permissions on the starting path 
component */
@@ -2202,6 +2207,11 @@ static const char *path_init(struct nameidata *nd, 
unsigned flags)
                        nd->inode = nd->path.dentry->d_inode;
                }
                fdput(f);
+               if (flags & LOOKUP_DFD_ROOT) {
+                       nd->root = nd->path;
+                       if (!(flags & LOOKUP_RCU))
+                               path_get(&nd->root);
+               }
                return s;
        }
 }
diff --git a/include/linux/namei.h b/include/linux/namei.h
index d3d0398..ca2ea13 100644
--- a/include/linux/namei.h
+++ b/include/linux/namei.h
@@ -45,6 +45,8 @@ enum {LAST_NORM, LAST_ROOT, LAST_DOT, LAST_DOTDOT, LAST_BIND};
 #define LOOKUP_ROOT            0x2000
 #define LOOKUP_EMPTY           0x4000
 
+#define LOOKUP_DFD_ROOT                0x8000
+
 extern int path_pts(struct path *path);
 
 extern int user_path_at_empty(int, const char __user *, unsigned, struct path 
*, int *empty);
-- 
2.5.5

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