[PATCH 1/3 v2] namei: add LOOKUP_DFD_ROOT to use dfd as root

From: Andrey Vagin
Date: Wed Jul 20 2016 - 16:43:31 EST


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.

Changes since v1:
* initialize nd->root_seq (thanks to Omar Sandoval for reporting and
fixing this issue)

Cc: Omar Sandoval <osandov@xxxxxxxxxxx>
Signed-off-by: Andrey Vagin <avagin@xxxxxxxxxx>
---
fs/namei.c | 16 +++++++++++++++-
include/linux/namei.h | 2 ++
2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/fs/namei.c b/fs/namei.c
index 28cb1cd..17548b1 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2141,7 +2141,7 @@ 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);
@@ -2167,6 +2167,13 @@ 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)
+ nd->root_seq = nd->seq;
+ else
+ path_get(&nd->root);
+ }
return s;
} else {
/* Caller must check execute permissions on the starting path component */
@@ -2195,6 +2202,13 @@ 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)
+ nd->root_seq = nd->seq;
+ else
+ path_get(&nd->root);
+ }
return s;
}
}
diff --git a/include/linux/namei.h b/include/linux/namei.h
index ec5ec28..59ca78c 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 user_path_at_empty(int, const char __user *, unsigned, struct path *, int *empty);

static inline int user_path_at(int dfd, const char __user *name, unsigned flags,
--
2.5.5