cgroup: fix subsystem file accesses on the root cgroup (0bfb4aa6) · Commits · 戴 / test

("cgroup: make cgroup_file_open() rcu_read_lock() around cgroup_css() and add cfent->css") added cfent->css to cache the associted cgroup_subsys_state across file operations. A cfent is associated with single css throughout its lifetime and the origimal commit initialized the cache pointer during cgroup_add_file() and verified that it matches the actual one in cgroup_file_open(). While this works fine for !root cgroups, it's broken for root cgroups as files in a root cgroup are created before the css's are associated with the cgroup and thus cgroup_css() call in cgroup_add_file() returns NULL associating all cfents in the root cgroup with NULL css. This makes cgroup_file_open() trigger WARN and fail with -ENODEV for all !core subsystem files in the root cgroups. There's no reason to initialize cfent->css separately from cgroup_add_file(). As the association never changes, cgroup_file_open() can set it unconditionally every time and containing the logic in cgroup_file_open() makes more sense anyway as the only reason it's necessary is file->private_data being already occupied. Fix it by setting cfent->css unconditionally from cgroup_file_open(). Signed-off-by:

Tejun Heo <tj@kernel.org> Acked-by:

Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

+10 −14

Original line number	Diff line number	Diff line
		@@ -2490,10 +2490,18 @@ static int cgroup_file_open(struct inode inode, struct file file)
		}
		rcu_read_unlock();

		/* css should match @cfe->css, see cgroup_add_file() for details */
		if (!css \|\| WARN_ON_ONCE(css != cfe->css))
		if (!css)
		return -ENODEV;

		/*
		* @cfe->css is used by read/write/close to determine the
		* associated css. @file->private_data would be a better place but
		* that's already used by seqfile. Multiple accessors may use it
		* simultaneously which is okay as the association never changes.
		*/
		WARN_ON_ONCE(cfe->css && cfe->css != css);
		cfe->css = css;

		if (cft->read_map \|\| cft->read_seq_string) {
		file->f_op = &cgroup_seqfile_operations;
		err = single_open(file, cgroup_seqfile_show, cfe);
		@@ -2772,18 +2780,6 @@ static int cgroup_add_file(struct cgroup cgrp, struct cftype cft)
		dentry->d_fsdata = cfe;
		simple_xattrs_init(&cfe->xattrs);

		/*
		* cfe->css is used by read/write/close to determine the associated
		* css. file->private_data would be a better place but that's
		* already used by seqfile. Note that open will use the usual
		* cgroup_css() and css_tryget() to acquire the css and this
		* caching doesn't affect css lifetime management.
		*/
		if (cft->ss)
		cfe->css = cgroup_css(cgrp, cft->ss->subsys_id);
		else
		cfe->css = &cgrp->dummy_css;

		mode = cgroup_file_mode(cft);
		error = cgroup_create_file(dentry, mode \| S_IFREG, cgrp->root->sb);
		if (!error) {

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