cgroup: implement cftype->write()

During the recent conversion to kernfs, cftype's seq_file operations
are updated so that they are directly mapped to kernfs operations and
thus can fully access the associated kernfs and cgroup contexts;
however, write path hasn't seen similar updates and none of the
existing write operations has access to, for example, the associated
kernfs_open_file.

Let's introduce a new operation cftype->write() which maps directly to
the kernfs write operation and has access to all the arguments and
contexts.  This will replace ->write_string() and ->trigger() and ease
manipulation of kernfs active protection from cgroup file operations.

Two accessors - of_cft() and of_css() - are introduced to enable
accessing the associated cgroup context from cftype->write() which
only takes kernfs_open_file for the context information.  The
accessors for seq_file operations - seq_cft() and seq_css() - are
rewritten to wrap the of_ accessors.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

1 files changed, 8 insertions, 6 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 671d8a6dae37..a16f91d12f4e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -283,11 +283,10 @@ static inline bool cgroup_is_dead(const struct cgroup *cgrp)
         return test_bit(CGRP_DEAD, &cgrp->flags);
 }
 
-struct cgroup_subsys_state *seq_css(struct seq_file *seq)
+struct cgroup_subsys_state *of_css(struct kernfs_open_file *of)
 {
-        struct kernfs_open_file *of = seq->private;
         struct cgroup *cgrp = of->kn->parent->priv;
-        struct cftype *cft = seq_cft(seq);
+        struct cftype *cft = of_cft(of);
 
         /*
          * This is open and unprotected implementation of cgroup_css().
@@ -302,7 +301,7 @@ struct cgroup_subsys_state *seq_css(struct seq_file *seq)
         else
                 return &cgrp->dummy_css;
 }
-EXPORT_SYMBOL_GPL(seq_css);
+EXPORT_SYMBOL_GPL(of_css);
 
 /**
  * cgroup_is_descendant - test ancestry
@@ -1035,8 +1034,8 @@ static umode_t cgroup_file_mode(const struct cftype *cft)
         if (cft->read_u64 || cft->read_s64 || cft->seq_show)
                 mode |= S_IRUGO;
 
-        if (cft->write_u64 || cft->write_s64 || cft->write_string ||
-            cft->trigger)
+        if (cft->write_u64 || cft->write_s64 || cft->write ||
+            cft->write_string || cft->trigger)
                 mode |= S_IWUSR;
 
         return mode;
@@ -2726,6 +2725,9 @@ static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
         struct cgroup_subsys_state *css;
         int ret;
 
+        if (cft->write)
+                return cft->write(of, buf, nbytes, off);
+
         /*
          * kernfs guarantees that a file isn't deleted with operations in
          * flight, which means that the matching css is and stays alive and