[PATCH] mountinfo: implement show_path for kernfs and cgroup
Michael Kerrisk (man-pages)
mtk.manpages at gmail.com
Fri May 6 13:39:43 UTC 2016
Hi Serge,
I'll add my own notes below, as much as anything in order to convince
myself that I understand what's going on.
On 05/05/2016 05:20 PM, Serge E. Hallyn wrote:
> Short explanation:
>
> When showing a cgroupfs entry in mountinfo, show the path of the mount
> root dentry relative to the reader's cgroup namespace root.
As part of the commit message, I think it would be useful to add a
sentence here explain why this is needed / which applications need it.
> Long version:
>
> When a uid 0 task which is in freezer cgroup /a/b, unshares a new cgroup
> namespace, and then mounts a new instance of the freezer cgroup, the new
> mount will be rooted at /a/b. The root dentry field of the mountinfo
> entry will show '/a/b'.
So, the point is that if we create a new cgroup namespace,
then we want both /proc/self/cgroup and /proc/self/mountinfo
to show cgroup paths that are correctly virtualized with
respect to the cgroup mount point. Previous to this patch,
/proc/self/cgroup shows the right info, but
/proc/self/mountinfo does not. (Walk through in a moment.)
Is the above a correct summary?
> cat > /tmp/do1 << EOF
> mount -t cgroup -o freezer freezer /mnt
> grep freezer /proc/self/mountinfo
> EOF
>
> unshare -Gm bash /tmp/do1
Should that not be:
unshare -Gm bash /tmp/do1
(i.e., s/G/C/)?
> > 330 160 0:34 / /sys/fs/cgroup/freezer rw,nosuid,nodev,noexec,relatime - cgroup cgroup rw,freezer
> > 355 133 0:34 /a/b /mnt rw,relatime - cgroup freezer rw,freezer
>
> The task's freezer cgroup entry in /proc/self/cgroup will simply show
> '/':
>
> grep freezer /proc/self/cgroup
> 9:freezer:/
>
> If instead the same task simply bind mounts the /a/b cgroup directory,
> the resulting mountinfo entry will again show /a/b for the dentry root.
> However in this case the task will find its own cgroup at /mnt/a/b,
> not at /mnt:
>
> mount --bind /sys/fs/cgroup/freezer/a/b /mnt
> 130 25 0:34 /a/b /mnt rw,nosuid,nodev,noexec,relatime shared:21 - cgroup cgroup rw,freezer
>
> In other words, there is no way for the task to know, based on what is
> in mountinfo, which cgroup directory is its own.
So, my walk through and commentary to the above...
First, a little script to save some typing and verbiage:
# cat cgroup_info.sh
#!/bin/sh
echo -e "\t/proc/self/cgroup:\t$(cat /proc/self/cgroup | grep freezer)"
cat /proc/self/mountinfo | grep freezer |
awk '{print "\tmountinfo:\t\t" $4 "\t" $5}'
#
Create cgroup, place this shell into the cgroup, and look at the state of
the /proc files:
# mkdir -p /sys/fs/cgroup/freezer/a/b
# echo $$ > /sys/fs/cgroup/freezer/a/b/cgroup.procs
# echo $$
2653
# cat /sys/fs/cgroup/freezer/a/b/cgroup.procs
2653 # Our shell
14254 # cat(1)
# ./cgroup_info.sh
/proc/self/cgroup: 10:freezer:/a/b
mountinfo: / /sys/fs/cgroup/freezer
Create a shell in new cgroup and mount namespaces. The act of creating
a new cgroup namespace causes the process's current cgroups directories
to become its cgroup root directories. (Here, I'm using my own version
of the "unshare" utility, which takes the same options as the util-linux
version):
# ~mtk/tlpi/code/ns/unshare -Cm bash
Look at the state of the /proc files:
# ./cgroup_info.sh
/proc/self/cgroup: 10:freezer:/
mountinfo: / /sys/fs/cgroup/freezer
The third entry in /proc/self/info (the pathname of the cgroup inside the
hierarchy) is correctly virtualized w.r.t. the cgroup namespace, which is
rooted at /a/b in the outer namespace.
However, the info in /proc/self/mountinfo is not for this cgroup
namespace, since we are seeing a duplicate of the mount from the
old mount namespace, and the info there does not correspond to the
new cgroup namespace. However, trying to create a new mount still
doesn't show us the right information in mountinfo:
# mount --make-rslave / # Prevent our mount operations
# propagating to other mountns
# mkdir -p /mnt/freezer # Create a new mount point
# umount /sys/fs/cgroup/freezer # Discard old mount
# mount -t cgroup -o freezer freezer /mnt/freezer/
# ./cgroup_info.sh
/proc/self/cgroup: 7:freezer:/
mountinfo: /a/b /mnt/freezer
The act of creating a new cgroup namespace caused the process's
current freezer directory, "/a/b", to become its cgroup freezer root
directory. In other words, the pathname directory of the directory
within the newly mounted cgroup filesystem should be "/",
but mountinfo wrongly shows us "/a/b". The consequence of this is
that the process in the cgroup namespace cannot correctly construct
the pathname of its cgroup root directory from the information in
/proc/PID/mountinfo. (The current patch fixes exactly this problem.)
> With this patch, the dentry root field in mountinfo is shown relative
> to the reader's cgroup namespace. I.e.:
>
> unshare -Gm bash /tmp/do1
> > 330 160 0:34 / /sys/fs/cgroup/freezer rw,nosuid,nodev,noexec,relatime - cgroup cgroup rw,freezer
> > 355 133 0:34 / /mnt rw,relatime - cgroup freezer rw,freezer
>
> This way the task can correlate the paths in /proc/pid/cgroup to
> /proc/self/mountinfo, and determine which cgroup directory (in any
> mount which the reader created) corresponds to the task.
So, I applied your patch against a current (i.e., 4.6-rc6) kernel.
Same steps as before, and here's what I see:
# mkdir -p /sys/fs/cgroup/freezer/a/b
# echo $$ > /sys/fs/cgroup/freezer/a/b/cgroup.procs
# ./cgroup_info.sh
/proc/self/cgroup: 10:freezer:/a/b
mountinfo: / /sys/fs/cgroup/freezer
# ~mtk/tlpi/code/ns/unshare -Cm bash
# ./cgroup_info.sh
/proc/self/cgroup: 10:freezer:/
mountinfo: /../.. /sys/fs/cgroup/freezer
# mount --make-rslave /
# mkdir -p /mnt/freezer
# umount /sys/fs/cgroup/freezer
# mount -t cgroup -o freezer freezer /mnt/freezer/
# ./cgroup_info.sh
/proc/self/cgroup: 10:freezer:/
mountinfo: / /mnt/freezer
Now the root directory path shown by mountinfo is correct,
and when we look inside the mount point, we see that things
look "right" (i.e., a cgroup root directory with no
subdirectories, and the PID of the shell run by unshare is
in the cgroup.procs file of this cgroup):
# ls /mnt/freezer/
cgroup.clone_children freezer.parent_freezing freezer.state tasks
cgroup.procs freezer.self_freezing notify_on_release
# echo $$
3164
# cat /mnt/freezer/cgroup.procs
2653 # First shell that placed in this cgroup
3164 # Shell started by 'unshare'
14197 # cat(1)
All makes sense to me.
Tested-by: Michael Kerrisk <mtk.manpages at gmail.com>
Acked-by: Michael Kerrisk <mtk.manpages at gmail.com>
(I did no review of the patch itself though.)
Cheers,
Michael
> Signed-off-by: Serge Hallyn <serge.hallyn at ubuntu.com>
> ---
> fs/kernfs/mount.c | 14 +++++++++++
> include/linux/kernfs.h | 2 ++
> kernel/cgroup.c | 63 ++++++++++++++++++++++++++++++++++++++++++++++++++
> 3 files changed, 79 insertions(+)
>
> diff --git a/fs/kernfs/mount.c b/fs/kernfs/mount.c
> index f73541f..3b78724 100644
> --- a/fs/kernfs/mount.c
> +++ b/fs/kernfs/mount.c
> @@ -15,6 +15,7 @@
> #include <linux/slab.h>
> #include <linux/pagemap.h>
> #include <linux/namei.h>
> +#include <linux/seq_file.h>
>
> #include "kernfs-internal.h"
>
> @@ -40,6 +41,18 @@ static int kernfs_sop_show_options(struct seq_file *sf, struct dentry *dentry)
> return 0;
> }
>
> +static int kernfs_sop_show_path(struct seq_file *sf, struct dentry *dentry)
> +{
> + struct kernfs_node *node = dentry->d_fsdata;
> + struct kernfs_root *root = kernfs_root(node);
> + struct kernfs_syscall_ops *scops = root->syscall_ops;
> +
> + if (scops && scops->show_path)
> + return scops->show_path(sf, node, root);
> +
> + return seq_dentry(sf, dentry, " \t\n\\");
> +}
> +
> const struct super_operations kernfs_sops = {
> .statfs = simple_statfs,
> .drop_inode = generic_delete_inode,
> @@ -47,6 +60,7 @@ const struct super_operations kernfs_sops = {
>
> .remount_fs = kernfs_sop_remount_fs,
> .show_options = kernfs_sop_show_options,
> + .show_path = kernfs_sop_show_path,
> };
>
> /**
> diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
> index c06c442..30f089e 100644
> --- a/include/linux/kernfs.h
> +++ b/include/linux/kernfs.h
> @@ -152,6 +152,8 @@ struct kernfs_syscall_ops {
> int (*rmdir)(struct kernfs_node *kn);
> int (*rename)(struct kernfs_node *kn, struct kernfs_node *new_parent,
> const char *new_name);
> + int (*show_path)(struct seq_file *sf, struct kernfs_node *kn,
> + struct kernfs_root *root);
> };
>
> struct kernfs_root {
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 909a7d3..afea39e 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -1215,6 +1215,41 @@ static void cgroup_destroy_root(struct cgroup_root *root)
> cgroup_free_root(root);
> }
>
> +/*
> + * look up cgroup associated with current task's cgroup namespace on the
> + * specified hierarchy
> + */
> +static struct cgroup *
> +current_cgns_cgroup_from_root(struct cgroup_root *root)
> +{
> + struct cgroup *res = NULL;
> + struct css_set *cset;
> +
> + lockdep_assert_held(&css_set_lock);
> +
> + rcu_read_lock();
> +
> + cset = current->nsproxy->cgroup_ns->root_cset;
> + if (cset == &init_css_set) {
> + res = &root->cgrp;
> + } else {
> + struct cgrp_cset_link *link;
> +
> + list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
> + struct cgroup *c = link->cgrp;
> +
> + if (c->root == root) {
> + res = c;
> + break;
> + }
> + }
> + }
> + rcu_read_unlock();
> +
> + BUG_ON(!res);
> + return res;
> +}
> +
> /* look up cgroup associated with given css_set on the specified hierarchy */
> static struct cgroup *cset_cgroup_from_root(struct css_set *cset,
> struct cgroup_root *root)
> @@ -1593,6 +1628,33 @@ static int rebind_subsystems(struct cgroup_root *dst_root, u16 ss_mask)
> return 0;
> }
>
> +static int cgroup_show_path(struct seq_file *sf, struct kernfs_node *kf_node,
> + struct kernfs_root *kf_root)
> +{
> + int len = 0, ret = 0;
> + char *buf = NULL;
> + struct cgroup_root *kf_cgroot = cgroup_root_from_kf(kf_root);
> + struct cgroup *ns_cgroup;
> +
> + buf = kmalloc(PATH_MAX, GFP_KERNEL);
> + if (!buf)
> + return -ENOMEM;
> +
> + spin_lock_bh(&css_set_lock);
> + ns_cgroup = current_cgns_cgroup_from_root(kf_cgroot);
> + len = kernfs_path_from_node(kf_node, ns_cgroup->kn, buf, PATH_MAX);
> + spin_unlock_bh(&css_set_lock);
> +
> + if (len >= PATH_MAX)
> + len = -ERANGE;
> + else if (len > 0) {
> + seq_escape(sf, buf, " \t\n\\");
> + len = 0;
> + }
> + kfree(buf);
> + return len;
> +}
> +
> static int cgroup_show_options(struct seq_file *seq,
> struct kernfs_root *kf_root)
> {
> @@ -5433,6 +5495,7 @@ static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
> .mkdir = cgroup_mkdir,
> .rmdir = cgroup_rmdir,
> .rename = cgroup_rename,
> + .show_path = cgroup_show_path,
> };
>
> static void __init cgroup_init_subsys(struct cgroup_subsys *ss, bool early)
>
--
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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