[Devel] [PATCH RH7 v3] cgroup: add export_operations to cgroup super block

[Vasily Averin]

On 7/28/20 2:59 PM, Andrey Zhadchenko wrote:
> criu uses fhandle from fdinfo to dump inotify objects. cgroup super block has
> no export operations, but .encode_fh and .fh_to_dentry are needed for
> inotify_fdinfo function and open_by_handle_at syscall in order to correctly
> open files located on cgroupfs by fhandle.
> Add hash table as a storage for inodes with exported fhandle.
> 
> v3: use inode->i_gen to protect from i_ino reusage. increase fhandle size to
> 2 * u32.
> Add an option to take reference of inode in cgroup_find_inode, so no one can
> delete recently found inode.
> 
> https://jira.sw.ru/browse/PSBM-105889
> Signed-off-by: Andrey Zhadchenko <andrey.zhadchenko at virtuozzo.com>
> ---
>  kernel/cgroup.c | 137 +++++++++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 136 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/cgroup.c b/kernel/cgroup.c
> index 9fdba79..267a5a4 100644
> --- a/kernel/cgroup.c
> +++ b/kernel/cgroup.c
> @@ -62,6 +62,8 @@
>  #include <linux/kthread.h>
>  #include <linux/ve.h>
>  #include <linux/stacktrace.h>
> +#include <linux/exportfs.h>
> +#include <linux/time.h>
>  
>  #include <linux/atomic.h>
>  
> @@ -765,6 +767,7 @@ static struct inode *cgroup_new_inode(umode_t mode, struct super_block *sb)
>  
>  	if (inode) {
>  		inode->i_ino = get_next_ino();
> +		inode->i_generation = get_seconds();
>  		inode->i_mode = mode;
>  		inode->i_uid = current_fsuid();
>  		inode->i_gid = current_fsgid();
> @@ -1390,9 +1393,140 @@ out:
>  }
>  #endif
>  
> +/*
> + * hashtable for inodes that have exported fhandles.
> + * When we export fhandle, we add it's inode into
> + * hashtable so we can find it fast
> + */
> +
> +#define CGROUP_INODE_HASH_BITS 10
> +static DEFINE_HASHTABLE(cgroup_inode_table, CGROUP_INODE_HASH_BITS);
> +static DEFINE_SPINLOCK(cgroup_inode_table_lock);
> +
> +struct cg_inode_hitem {
> +	struct inode *inode;
> +	struct hlist_node hlist;
> +};
> +
> +static inline unsigned long cgroup_inode_get_hash(unsigned int i_ino)
> +{
> +	return hash_32(i_ino, CGROUP_INODE_HASH_BITS);
> +}
> +
> +static struct cg_inode_hitem *cgroup_find_inode(unsigned long fhandle[2],
> +							char take_ref)
> +{
> +	struct cg_inode_hitem *i;
> +	struct hlist_head *head = cgroup_inode_table
> +		+ cgroup_inode_get_hash(fhandle[1]);
> +	struct cg_inode_hitem *found = NULL;
> +
> +	spin_lock(&cgroup_inode_table_lock);
> +	hlist_for_each_entry(i, head, hlist) {
> +		if (i->inode->i_generation == fhandle[0] &&
> +		    i->inode->i_ino == fhandle[1]) {
> +			/*
> +			 * If we need to increase refcount, we should be aware
> +			 * of possible deadlock. Another thread may have
> +			 * started deleting this inode: iput->iput_final->
> +			 * ->cgroup_delete_inode->cgroup_find_inode. if we
> +			 * just call igrab, it will try to take i_lock and
> +			 * this will result in deadlock, because deleting
> +			 * thread has already taken it and waits on
> +			 * cgroup_inode_table_lock to find inode in hashtable.
> +			 * If i_count is zero, someone is deleting it -> skip.
> +			 */
> +			if (take_ref)
> +				if (!atomic_inc_not_zero(&i->inode->i_count))
> +					continue;
> +			found = i;
> +			break;
> +		}
> +	}
> +	spin_unlock(&cgroup_inode_table_lock);
> +
> +	return found;
> +}
> +
> +static struct dentry *cgroup_fh_to_dentry(struct super_block *sb,
> +		struct fid *fid, int fh_len, int fh_type)
> +{
> +	struct cg_inode_hitem *item;
> +	struct dentry *dentry = ERR_PTR(-ENOENT);
> +	unsigned long fhandle[2] = {fid->raw[0], fid->raw[1]};
> +
> +	if (fh_len < 2)
> +		return NULL;
> +
> +	item = cgroup_find_inode(fhandle, 1);
> +	if (item) {
> +		dentry = d_find_alias(item->inode);
> +		iput(item->inode);
> +	}
> +	return dentry;
> +}
> +
> +static int cgroup_encode_fh(struct inode *inode, __u32 *fh, int *len,
> +				struct inode *parent)
> +{
> +	struct cg_inode_hitem *item;
> +	struct hlist_head *head = cgroup_inode_table
> +		+ cgroup_inode_get_hash(inode->i_ino);
> +	unsigned long fhandle[2] = {inode->i_generation, inode->i_ino};
> +
> +	if (*len < 2) {
> +		*len = 2;
> +		return FILEID_INVALID;
> +	}
> +
> +	if (!cgroup_find_inode(fhandle, 0)) {
> +		item = kmalloc(sizeof(struct cg_inode_hitem),
> +			GFP_KERNEL);
> +		/*
> +		 * encode_fh is expected to return 255 (FILEID_INVALID)
> +		 * in case of failure. We can't return ENOMEM, so
> +		 * return FILEID_INVALID at least
> +		 */
> +		if (!item)
> +			return FILEID_INVALID;
> +		item->inode = inode;
> +
> +		spin_lock(&cgroup_inode_table_lock);
> +		hlist_add_head(&item->hlist, head);
> +		spin_unlock(&cgroup_inode_table_lock);

I'm understand that it can be correct,
however in general this logic looks racy:
you need both check _and_ add/delete entry in single transaction under taken lock
otherwise "double add" or "double delete" can happen.

> +	}
> +
> +	fh[0] = fhandle[0];
> +	fh[1] = fhandle[1];
> +	*len = 2;
> +	return 1;
> +}
> +
> +static const struct export_operations cgroup_export_ops = {
> +	.encode_fh      = cgroup_encode_fh,
> +	.fh_to_dentry	= cgroup_fh_to_dentry,
> +};
> +
> +static int cgroup_delete_inode(struct inode *inode)
> +{
> +	struct cg_inode_hitem *item = NULL;
> +	unsigned long fhandle[2] = {inode->i_generation, inode->i_ino};
> +
> +	item = cgroup_find_inode(fhandle, 0);
> +	if (item) {
> +		spin_lock(&cgroup_inode_table_lock);
> +		hlist_del(&item->hlist);
> +		spin_unlock(&cgroup_inode_table_lock);

look comment above.

> +
> +		kfree(item);
> +	}
> +
> +	return generic_delete_inode(inode);
> +}
> +
>  static const struct super_operations cgroup_ops = {
>  	.statfs = simple_statfs,
> -	.drop_inode = generic_delete_inode,
> +	.drop_inode = cgroup_delete_inode,
>  	.show_options = cgroup_show_options,
>  #ifdef CONFIG_VE
>  	.show_path = cgroup_show_path,
> @@ -1539,6 +1673,7 @@ static int cgroup_set_super(struct super_block *sb, void *data)
>  	sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
>  	sb->s_magic = CGROUP_SUPER_MAGIC;
>  	sb->s_op = &cgroup_ops;
> +	sb->s_export_op = &cgroup_export_ops;
>  
>  	return 0;
>  }
>