[PATCH 2/3] fs: allow to use dirfd as root for openat and other *at syscalls

From: Andrey Vagin
Date: Tue Jun 28 2016 - 13:39:22 EST

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The problem is that a pathname can contain absolute symlinks and now
they are resolved relative to the current root.

But if we want to open a file in another mount namespace and we have
a file descriptor to its root directory, we want that the pathname is
resolved in the target mount namespace and in this case we need these
new flags O_ATROOT or AT_FDROOT.

If O_ATROOT is set for openat() or AT_FDROOT is set for fstatat, linkat,
unlinkat, path_init is executed with the LOOKUP_DFD_ROOT flag.

v2: fix a value of O_ATROOT to not intersect with other constans
Signed-off-by: Andrey Vagin <avagin@xxxxxxxxxx>
---
fs/exec.c | 4 +++-
fs/namei.c | 26 +++++++++++++++++---------
fs/open.c | 6 +++++-
fs/stat.c | 4 +++-
fs/utimes.c | 4 +++-
include/uapi/asm-generic/fcntl.h | 4 ++++
include/uapi/linux/fcntl.h | 1 +
7 files changed, 36 insertions(+), 13 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 887c1c9..473b709 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -775,12 +775,14 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
.lookup_flags = LOOKUP_FOLLOW,
};

- if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
+ if ((flags & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH | AT_FDROOT)) != 0)
return ERR_PTR(-EINVAL);
if (flags & AT_SYMLINK_NOFOLLOW)
open_exec_flags.lookup_flags &= ~LOOKUP_FOLLOW;
if (flags & AT_EMPTY_PATH)
open_exec_flags.lookup_flags |= LOOKUP_EMPTY;
+ if (flags & AT_FDROOT)
+ open_exec_flags.lookup_flags |= LOOKUP_DFD_ROOT;

file = do_filp_open(fd, name, &open_exec_flags);
if (IS_ERR(file))
diff --git a/fs/namei.c b/fs/namei.c
index 5f08b69..9958b60 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2581,7 +2581,8 @@ user_path_parent(int dfd, const char __user *path,
unsigned int flags)
{
/* only LOOKUP_REVAL is allowed in extra flags */
- return filename_parentat(dfd, getname(path), flags & LOOKUP_REVAL,
+ return filename_parentat(dfd, getname(path),
+ flags & (LOOKUP_REVAL | LOOKUP_DFD_ROOT),
parent, last, type);
}

@@ -3577,7 +3578,7 @@ static struct dentry *filename_create(int dfd, struct filename *name,
* Note that only LOOKUP_REVAL and LOOKUP_DIRECTORY matter here. Any
* other flags passed in are ignored!
*/
- lookup_flags &= LOOKUP_REVAL;
+ lookup_flags &= LOOKUP_REVAL | LOOKUP_DFD_ROOT;

name = filename_parentat(dfd, name, lookup_flags, path, &last, &type);
if (IS_ERR(name))
@@ -3975,7 +3976,8 @@ EXPORT_SYMBOL(vfs_unlink);
* writeout happening, and we don't want to prevent access to the directory
* while waiting on the I/O.
*/
-static long do_unlinkat(int dfd, const char __user *pathname)
+static long do_unlinkat(int dfd, const char __user *pathname,
+ unsigned int lookup_flags)
{
int error;
struct filename *name;
@@ -3985,7 +3987,6 @@ static long do_unlinkat(int dfd, const char __user *pathname)
int type;
struct inode *inode = NULL;
struct inode *delegated_inode = NULL;
- unsigned int lookup_flags = 0;
retry:
name = user_path_parent(dfd, pathname,
&path, &last, &type, lookup_flags);
@@ -4050,18 +4051,23 @@ slashes:

SYSCALL_DEFINE3(unlinkat, int, dfd, const char __user *, pathname, int, flag)
{
- if ((flag & ~AT_REMOVEDIR) != 0)
+ unsigned int lookup_flags = 0;
+
+ if ((flag & ~(AT_REMOVEDIR | AT_FDROOT)) != 0)
return -EINVAL;

if (flag & AT_REMOVEDIR)
return do_rmdir(dfd, pathname);

- return do_unlinkat(dfd, pathname);
+ if (flag & AT_FDROOT)
+ lookup_flags |= LOOKUP_DFD_ROOT;
+
+ return do_unlinkat(dfd, pathname, lookup_flags);
}

SYSCALL_DEFINE1(unlink, const char __user *, pathname)
{
- return do_unlinkat(AT_FDCWD, pathname);
+ return do_unlinkat(AT_FDCWD, pathname, 0);
}

int vfs_symlink(struct inode *dir, struct dentry *dentry, const char *oldname)
@@ -4212,7 +4218,7 @@ SYSCALL_DEFINE5(linkat, int, olddfd, const char __user *, oldname,
int how = 0;
int error;

- if ((flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH)) != 0)
+ if ((flags & ~(AT_SYMLINK_FOLLOW | AT_EMPTY_PATH | AT_FDROOT)) != 0)
return -EINVAL;
/*
* To use null names we require CAP_DAC_READ_SEARCH
@@ -4227,13 +4233,15 @@ SYSCALL_DEFINE5(linkat, int, olddfd, const char __user *, oldname,

if (flags & AT_SYMLINK_FOLLOW)
how |= LOOKUP_FOLLOW;
+ if (flags & AT_FDROOT)
+ how |= LOOKUP_DFD_ROOT;
retry:
error = user_path_at(olddfd, oldname, how, &old_path);
if (error)
return error;

new_dentry = user_path_create(newdfd, newname, &new_path,
- (how & LOOKUP_REVAL));
+ (how & (LOOKUP_REVAL | LOOKUP_DFD_ROOT)));
error = PTR_ERR(new_dentry);
if (IS_ERR(new_dentry))
goto out;
diff --git a/fs/open.c b/fs/open.c
index 93ae3cd..e0bc8d0 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -613,12 +613,14 @@ SYSCALL_DEFINE5(fchownat, int, dfd, const char __user *, filename, uid_t, user,
int error = -EINVAL;
int lookup_flags;

- if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH)) != 0)
+ if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_EMPTY_PATH | AT_FDROOT)) != 0)
goto out;

lookup_flags = (flag & AT_SYMLINK_NOFOLLOW) ? 0 : LOOKUP_FOLLOW;
if (flag & AT_EMPTY_PATH)
lookup_flags |= LOOKUP_EMPTY;
+ if (flag & AT_FDROOT)
+ lookup_flags |= LOOKUP_DFD_ROOT;
retry:
error = user_path_at(dfd, filename, lookup_flags, &path);
if (error)
@@ -941,6 +943,8 @@ static inline int build_open_flags(int flags, umode_t mode, struct open_flags *o
lookup_flags |= LOOKUP_DIRECTORY;
if (!(flags & O_NOFOLLOW))
lookup_flags |= LOOKUP_FOLLOW;
+ if (flags & O_ATROOT)
+ lookup_flags |= LOOKUP_DFD_ROOT;
op->lookup_flags = lookup_flags;
return 0;
}
diff --git a/fs/stat.c b/fs/stat.c
index bc045c7..d71e7f2 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -95,13 +95,15 @@ int vfs_fstatat(int dfd, const char __user *filename, struct kstat *stat,
unsigned int lookup_flags = 0;

if ((flag & ~(AT_SYMLINK_NOFOLLOW | AT_NO_AUTOMOUNT |
- AT_EMPTY_PATH)) != 0)
+ AT_EMPTY_PATH | AT_FDROOT)) != 0)
goto out;

if (!(flag & AT_SYMLINK_NOFOLLOW))
lookup_flags |= LOOKUP_FOLLOW;
if (flag & AT_EMPTY_PATH)
lookup_flags |= LOOKUP_EMPTY;
+ if (flag & AT_FDROOT)
+ lookup_flags |= LOOKUP_DFD_ROOT;
retry:
error = user_path_at(dfd, filename, lookup_flags, &path);
if (error)
diff --git a/fs/utimes.c b/fs/utimes.c
index 85c40f4..78a9eb9 100644
--- a/fs/utimes.c
+++ b/fs/utimes.c
@@ -143,7 +143,7 @@ long do_utimes(int dfd, const char __user *filename, struct timespec *times,
goto out;
}

- if (flags & ~AT_SYMLINK_NOFOLLOW)
+ if (flags & ~(AT_SYMLINK_NOFOLLOW | AT_FDROOT))
goto out;

if (filename == NULL && dfd != AT_FDCWD) {
@@ -165,6 +165,8 @@ long do_utimes(int dfd, const char __user *filename, struct timespec *times,

if (!(flags & AT_SYMLINK_NOFOLLOW))
lookup_flags |= LOOKUP_FOLLOW;
+ if (flags & AT_FDROOT)
+ lookup_flags |= LOOKUP_DFD_ROOT;
retry:
error = user_path_at(dfd, filename, lookup_flags, &path);
if (error)
diff --git a/include/uapi/asm-generic/fcntl.h b/include/uapi/asm-generic/fcntl.h
index e063eff..0436b1d 100644
--- a/include/uapi/asm-generic/fcntl.h
+++ b/include/uapi/asm-generic/fcntl.h
@@ -88,6 +88,10 @@
#define __O_TMPFILE 020000000
#endif

+#ifndef O_ATROOT
+#define O_ATROOT 040000000 /* dfd is a root */
+#endif
+
/* a horrid kludge trying to make sure that this will fail on old kernels */
#define O_TMPFILE (__O_TMPFILE | O_DIRECTORY)
#define O_TMPFILE_MASK (__O_TMPFILE | O_DIRECTORY | O_CREAT)
diff --git a/include/uapi/linux/fcntl.h b/include/uapi/linux/fcntl.h
index beed138..4f3b631 100644
--- a/include/uapi/linux/fcntl.h
+++ b/include/uapi/linux/fcntl.h
@@ -62,6 +62,7 @@
#define AT_SYMLINK_FOLLOW 0x400 /* Follow symbolic links. */
#define AT_NO_AUTOMOUNT 0x800 /* Suppress terminal automount traversal */
#define AT_EMPTY_PATH 0x1000 /* Allow empty relative pathname */
+#define AT_FDROOT 0x2000 /* Resolve a path as if dirfd is root */

#endif /* _UAPI_LINUX_FCNTL_H */
--
2.5.5

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