[RFC PATCH 06/27] containers, vfs: Allow syscall dirfd arguments to take a container fd

David Howells
2019-02-15T16:08:24Z

Some filesystem system calls, such as mkdirat(), take a 'directory fd' to
specify the pathwalk origin. This takes either AT_FDCWD or a file
descriptor that refers to an open directory.

Make it possible to supply a container fd, as obtained from
container_create(), instead thereby specifying the container's root as the
origin. This performs the filesystem operation into the container's mount
namespace. For example:

int cfd = container_create("fred", CONTAINER_NEW_MNT_NS, 0);
mkdirat(cfd, "/fred", 0755);

A better way to do this might be to temporarily override current->fs and
current->nsproxy, but this requires splitting those fields so that procfs
doesn't see the override.

A sequence number and lock are available to protect the root pointer in
case container_chroot() and/or container_pivot_root() are implemented.

Signed-off-by: David Howells <dhowells@redhat.com>
---

fs/namei.c | 45 ++++++++++++++++++++++++++++++++++-----------
1 file changed, 34 insertions(+), 11 deletions(-)

diff --git a/fs/namei.c b/fs/namei.c
index a85deb55d0c9..4932b5467285 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -2232,20 +2232,43 @@ static const char *path_init(struct nameidata *nd, unsigned flags)
if (!f.file)
return ERR_PTR(-EBADF);

- dentry = f.file->f_path.dentry;
+ if (is_container_file(f.file)) {
+ struct container *c = f.file->private_data;
+ unsigned seq;

- if (*s && unlikely(!d_can_lookup(dentry))) {
- fdput(f);
- return ERR_PTR(-ENOTDIR);
- }
+ if (!*s)
+ return ERR_PTR(-EINVAL);

- nd->path = f.file->f_path;
- if (flags & LOOKUP_RCU) {
- nd->inode = nd->path.dentry->d_inode;
- nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
+ if (flags & LOOKUP_RCU) {
+ do {
+ seq = read_seqcount_begin(&c->seq);
+ nd->path = c->root;
+ nd->inode = nd->path.dentry->d_inode;
+ nd->seq = __read_seqcount_begin(&nd->path.dentry->d_seq);
+ } while (read_seqcount_retry(&c->seq, seq));
+ } else {
+ spin_lock(&c->lock);
+ nd->path = c->root;
+ path_get(&nd->path);
+ spin_unlock(&c->lock);
+ nd->inode = nd->path.dentry->d_inode;
+ }
} else {
- path_get(&nd->path);
- nd->inode = nd->path.dentry->d_inode;
+ dentry = f.file->f_path.dentry;
+
+ if (*s && unlikely(!d_can_lookup(dentry))) {
+ fdput(f);
+ return ERR_PTR(-ENOTDIR);
+ }
+
+ nd->path = f.file->f_path;
+ if (flags & LOOKUP_RCU) {
+ nd->inode = nd->path.dentry->d_inode;
+ nd->seq = read_seqcount_begin(&nd->path.dentry->d_seq);
+ } else {
+ path_get(&nd->path);
+ nd->inode = nd->path.dentry->d_inode;
+ }
}
fdput(f);
return s;