linux-kernel - [PATCH 0/2] [RFC] fs: allow to use dirfd as root for openat and other *at syscalls

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Date:	Fri, 17 Jun 2016 13:20:46 -0700
From:	Andrey Vagin <avagin@...nvz.org>
To:	linux-fsdevel@...r.kernel.org
Cc:	linux-kernel@...r.kernel.org, Andrey Vagin <avagin@...nvz.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: [PATCH 0/2] [RFC] fs: allow to use dirfd as root for openat and other *at syscalls

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.

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 avaliable 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.

Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: "Eric W. Biederman" <ebiederm@...ssion.com>
Signed-off-by: Andrey Vagin <avagin@...nvz.org>

Andrey Vagin (2):
  namei: add LOOKUP_DFD_ROOT to use dfd as root
  fs: allow to use dirfd as root for openat and other *at syscalls

 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 |  3 +++
 include/uapi/linux/fcntl.h       |  1 +
 8 files changed, 37 insertions(+), 9 deletions(-)

-- 
2.5.5