Messages in this thread | | | From | Andrey Vagin <> | Subject | [PATCH 0/3 v2] fs: allow to use dirfd as root for openat and other *at syscalls | Date | Tue, 28 Jun 2016 10:38:27 -0700 |
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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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