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Subject[PATCH 1/2] cgroup: let a symlink too be created with a cftype file
Date
From: Angelo Ruocco <angeloruocco90@gmail.com>

This commit enables a cftype to have a symlink (of any name) that
points to the file associated with the cftype.

Signed-off-by: Angelo Ruocco <angeloruocco90@gmail.com>
Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
---
include/linux/cgroup-defs.h | 3 +++
kernel/cgroup/cgroup.c | 33 +++++++++++++++++++++++++++++----
2 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 77258d276f93..2ad9f53ecefe 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -101,6 +101,8 @@ enum {
CFTYPE_WORLD_WRITABLE = (1 << 4), /* (DON'T USE FOR NEW FILES) S_IWUGO */
CFTYPE_DEBUG = (1 << 5), /* create when cgroup_debug */

+ CFTYPE_SYMLINKED = (1 << 6), /* pointed to by symlink too */
+
/* internal flags, do not use outside cgroup core proper */
__CFTYPE_ONLY_ON_DFL = (1 << 16), /* only on default hierarchy */
__CFTYPE_NOT_ON_DFL = (1 << 17), /* not on default hierarchy */
@@ -538,6 +540,7 @@ struct cftype {
* end of cftype array.
*/
char name[MAX_CFTYPE_NAME];
+ char link_name[MAX_CFTYPE_NAME];
unsigned long private;

/*
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 217cec4e22c6..f77dee5eb82a 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -1460,8 +1460,8 @@ struct cgroup *task_cgroup_from_root(struct task_struct *task,

static struct kernfs_syscall_ops cgroup_kf_syscall_ops;

-static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
- char *buf)
+static char *cgroup_fill_name(struct cgroup *cgrp, const struct cftype *cft,
+ char *buf, bool write_link_name)
{
struct cgroup_subsys *ss = cft->ss;

@@ -1471,13 +1471,26 @@ static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,

snprintf(buf, CGROUP_FILE_NAME_MAX, "%s%s.%s",
dbg, cgroup_on_dfl(cgrp) ? ss->name : ss->legacy_name,
- cft->name);
+ write_link_name ? cft->link_name : cft->name);
} else {
- strscpy(buf, cft->name, CGROUP_FILE_NAME_MAX);
+ strscpy(buf, write_link_name ? cft->link_name : cft->name,
+ CGROUP_FILE_NAME_MAX);
}
return buf;
}

+static char *cgroup_file_name(struct cgroup *cgrp, const struct cftype *cft,
+ char *buf)
+{
+ return cgroup_fill_name(cgrp, cft, buf, false);
+}
+
+static char *cgroup_link_name(struct cgroup *cgrp, const struct cftype *cft,
+ char *buf)
+{
+ return cgroup_fill_name(cgrp, cft, buf, true);
+}
+
/**
* cgroup_file_mode - deduce file mode of a control file
* @cft: the control file in question
@@ -1636,6 +1649,9 @@ static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
}

kernfs_remove_by_name(cgrp->kn, cgroup_file_name(cgrp, cft, name));
+ if (cft->flags & CFTYPE_SYMLINKED)
+ kernfs_remove_by_name(cgrp->kn,
+ cgroup_link_name(cgrp, cft, name));
}

/**
@@ -3809,6 +3825,7 @@ static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
{
char name[CGROUP_FILE_NAME_MAX];
struct kernfs_node *kn;
+ struct kernfs_node *kn_link;
struct lock_class_key *key = NULL;
int ret;

@@ -3839,6 +3856,14 @@ static int cgroup_add_file(struct cgroup_subsys_state *css, struct cgroup *cgrp,
spin_unlock_irq(&cgroup_file_kn_lock);
}

+ if (cft->flags & CFTYPE_SYMLINKED) {
+ kn_link = kernfs_create_link(cgrp->kn,
+ cgroup_link_name(cgrp, cft, name),
+ kn);
+ if (IS_ERR(kn_link))
+ return PTR_ERR(kn_link);
+ }
+
return 0;
}

--
2.20.1
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