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Subject[PATCH 0/3 v2] fs: allow to use dirfd as root for openat and other *at syscalls
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The problem is that a pathname can contain absolute symlinks and now
they are resolved relative to the current root.

It may be a problem if you want to open a file in another namespace.
For example, you open /proc/PID/root for a process from the target
namespace and then you use openat() to open a file from this namespace.

If a path to the file contains an absolute symlink, you will open a file
from the current namespace, because a symlink will be resolved relative
to the current root.

A proposed solution adds a new flag which means that dirfd should be
set as a root for a current system call (openat(), statat(), etc).

Here are examples how we can open a file in a contex of another process.

How we can do this without these changes:

old_root = open("/", O_PATH);
old_cwd = open(".", O_PATH);
chroot("/proc/PID/root");

fd = open(pathname, O_RDONLY);

fchdir(old_root); /* emulate fchroot() */
chroot(".");
fchdir(old_cwd);

close(old_cwd);
close(old_root);

How this code is simplified with new flags:
dirfd = open("/proc/PID/root", O_PATH);
fd = open(dirfd, pathname, O_RDONLY | O_ATROOT);
close(dirfd);

One more thing is that chroot isn't available for unprivileged users.

We met this problem, when we tryed to dump an ubuntu container and
failed to resolve /proc/PID/root/var/run/mysqld/mysqld.sock, because
/var/run was a symlink to /run.

Changes since the first version:
- change a value of O_ATROOT to not intersect with other constants.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: "J. Bruce Fields" <bfields@redhat.com>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: NeilBrown <neilb@suse.de>
Cc: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Andrey Vagin <avagin@openvz.org>

Andrey Vagin (3):
namei: add LOOKUP_DFD_ROOT to use dfd as root
fs: allow to use dirfd as root for openat and other *at syscalls
selftests: check O_ATROOT and AT_FDROOT flags

fs/exec.c | 4 +-
fs/namei.c | 22 ++++++--
fs/open.c | 6 ++-
fs/stat.c | 4 +-
fs/utimes.c | 4 +-
include/linux/namei.h | 2 +
include/uapi/asm-generic/fcntl.h | 4 ++
include/uapi/linux/fcntl.h | 1 +
tools/testing/selftests/Makefile | 1 +
tools/testing/selftests/lookup/.gitignore | 1 +
tools/testing/selftests/lookup/Makefile | 8 +++
tools/testing/selftests/lookup/lookup_at_root.c | 69 +++++++++++++++++++++++++
tools/testing/selftests/lookup/run.sh | 14 +++++
13 files changed, 131 insertions(+), 9 deletions(-)
create mode 100644 tools/testing/selftests/lookup/.gitignore
create mode 100644 tools/testing/selftests/lookup/Makefile
create mode 100644 tools/testing/selftests/lookup/lookup_at_root.c
create mode 100755 tools/testing/selftests/lookup/run.sh

--
2.5.5

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