[Devel] [PATCH RH9 1/5] ms/cgroup: let a symlink too be created with a cftype file

[Kirill Tkhai]

On 05.10.2021 20:54, Cyrill Gorcunov wrote:
> From: Angelo Ruocco <angeloruocco90 at 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 at gmail.com>
> Signed-off-by: Paolo Valente <paolo.valente at linaro.org>
> Signed-off-by: Jens Axboe <axboe at kernel.dk>
> 
> https://jira.sw.ru/browse/PSBM-101019
> (cherry-picked from 54b7b868e826b294687c439b68ec55fe20cafe5b)
> Signed-off-by: Andrey Ryabinin <aryabinin at virtuozzo.com>
> Signed-off-by: Cyrill Gorcunov <gorcunov at virtuozzo.com>
> ---
>  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 583ce2bce98c..0b2222b884ce 100644
> --- a/include/linux/cgroup-defs.h
> +++ b/include/linux/cgroup-defs.h
> @@ -125,6 +125,8 @@ enum {
>  	 */
>  	CFTYPE_VE_WRITABLE	= (1 << 15),
>  
> +	CFTYPE_SYMLINKED	= (1 << 6),	/* pointed to by symlink too */

We already have:

        CFTYPE_PRESSURE         = (1 << 6),     /* only if pressure feature is enabled */  


> +
>  	/* 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 */
> @@ -552,6 +554,7 @@ 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 08b7cff7a1c3..74d0b503e696 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -1471,8 +1471,8 @@ 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;
>  
> @@ -1482,13 +1482,26 @@ 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
> @@ -1647,6 +1660,9 @@ 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));
>  }
>  
>  /**
> @@ -4012,6 +4028,7 @@ 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;
>  
> @@ -4042,6 +4059,14 @@ 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;
>  }
>  
>