From fc31076c23b9eff091f21e3c708f0b0f587ea91e Mon Sep 17 00:00:00 2001
From: Alexander Morozov <lk4d4@docker.com>
Date: Fri, 17 Jul 2015 11:41:54 -0700
Subject: [PATCH] Substract source mount from cgroup dir

This is needed because for nested containers cgroups. Without this patch
they creating unnecessary intermediate cgroup like:
/sys/fs/cgroup/memory/system.slice/docker-9409d9f0b68fb9e9d7d532d5b3f35e7c7f9cca1312af392ae3b28436f1f2998f.scope/system.slice/docker-9409d9f0b68fb9e9d7d532d5b3f35e7c7f9cca1312af392ae3b28436f1f2998f.scope/docker/908ebcc9c13584a14322ec070bd971e0de62f126c0cd95c079acdb99990ad3a3

It is because in /proc/self/cgroup we see paths from host, and they don't
exist in container.

Signed-off-by: Alexander Morozov <lk4d4@docker.com>
---
 libcontainer/cgroups/fs/apply_raw.go | 12 ++++++++----
 libcontainer/cgroups/utils.go        | 22 ++++++++++++++++++++++
 2 files changed, 30 insertions(+), 4 deletions(-)

diff --git a/libcontainer/cgroups/fs/apply_raw.go b/libcontainer/cgroups/fs/apply_raw.go
index 3d099d86..1954357b 100644
--- a/libcontainer/cgroups/fs/apply_raw.go
+++ b/libcontainer/cgroups/fs/apply_raw.go
@@ -235,16 +235,20 @@ func getCgroupData(c *configs.Cgroup, pid int) (*data, error) {
 	}, nil
 }
 
-func (raw *data) parent(subsystem, mountpoint string) (string, error) {
+func (raw *data) parent(subsystem, mountpoint, src string) (string, error) {
 	initPath, err := cgroups.GetInitCgroupDir(subsystem)
 	if err != nil {
 		return "", err
 	}
-	return filepath.Join(mountpoint, initPath), nil
+	relDir, err := filepath.Rel(src, initPath)
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(mountpoint, relDir), nil
 }
 
 func (raw *data) path(subsystem string) (string, error) {
-	mnt, err := cgroups.FindCgroupMountpoint(subsystem)
+	mnt, src, err := cgroups.FindCgroupMountpointAndSource(subsystem)
 	// If we didn't mount the subsystem, there is no point we make the path.
 	if err != nil {
 		return "", err
@@ -255,7 +259,7 @@ func (raw *data) path(subsystem string) (string, error) {
 		return filepath.Join(raw.root, subsystem, raw.cgroup), nil
 	}
 
-	parent, err := raw.parent(subsystem, mnt)
+	parent, err := raw.parent(subsystem, mnt, src)
 	if err != nil {
 		return "", err
 	}
diff --git a/libcontainer/cgroups/utils.go b/libcontainer/cgroups/utils.go
index f58e717f..92438a07 100644
--- a/libcontainer/cgroups/utils.go
+++ b/libcontainer/cgroups/utils.go
@@ -42,6 +42,28 @@ func FindCgroupMountpoint(subsystem string) (string, error) {
 	return "", NewNotFoundError(subsystem)
 }
 
+func FindCgroupMountpointAndSource(subsystem string) (string, string, error) {
+	f, err := os.Open("/proc/self/mountinfo")
+	if err != nil {
+		return "", "", err
+	}
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		txt := scanner.Text()
+		fields := strings.Split(txt, " ")
+		for _, opt := range strings.Split(fields[len(fields)-1], ",") {
+			if opt == subsystem {
+				return fields[4], fields[3], nil
+			}
+		}
+	}
+	if err := scanner.Err(); err != nil {
+		return "", "", err
+	}
+
+	return "", "", NewNotFoundError(subsystem)
+}
+
 func FindCgroupMountpointDir() (string, error) {
 	mounts, err := mount.GetMounts()
 	if err != nil {