Re: [PATCH v12 2/3] ipc: Conserve sequence numbers in ipcmni_extend mode

[Waiman Long]

On 03/16/2019 02:52 PM, Manfred Spraul wrote:
> From edee319b2d5c96af14b8b8899e5dde324861e4e4 Mon Sep 17 00:00:00 2001
> From: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> Date: Sat, 16 Mar 2019 10:18:53 +0100
> Subject: [PATCH] ipc: Conserve sequence numbers in ipcmni_extend mode
>
> Rewrite, based on the patch from Waiman Long:
>
> The mixing in of a sequence number into the IPC IDs is probably to
> avoid ID reuse in userspace as much as possible. With ipcmni_extend
> mode, the number of usable sequence numbers is greatly reduced leading
> to higher chance of ID reuse.
>
> To address this issue, we need to conserve the sequence number space
> as much as possible. Right now, the sequence number is incremented for
> every new ID created. In reality, we only need to increment the sequence
> number when new allocated ID is not greater than the last one allocated.
> It is in such case that the new ID may collide with an existing one.
> This is being done irrespective of the ipcmni mode.
>
> In order to avoid any races, the index is first allocated and
> then the pointer is replaced.
>
> Changes compared to the initial patch:
> - Handle failures from idr_alloc().
> - Avoid that concurrent operations can see the wrong
>   sequence number.
> (This is achieved by using idr_replace()).
> - IPCMNI_SEQ_SHIFT is not a constant, thus renamed to
> 	ipcmni_seq_shift().
> - IPCMNI_SEQ_MAX is not a constant, thus renamed to
> 	ipcmni_seq_max().
>
> Suggested-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Original-patch-from: Waiman Long <longman@xxxxxxxxxx>
> Signed-off-by: Manfred Spraul <manfred@xxxxxxxxxxxxxxxx>
> ---
>  include/linux/ipc_namespace.h |  1 +
>  ipc/util.c                    | 35 ++++++++++++++++++++++++++++++-----
>  ipc/util.h                    |  8 ++++----
>  3 files changed, 35 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
> index 6ab8c1bada3f..c309f43bde45 100644
> --- a/include/linux/ipc_namespace.h
> +++ b/include/linux/ipc_namespace.h
> @@ -19,6 +19,7 @@ struct ipc_ids {
>  	struct rw_semaphore rwsem;
>  	struct idr ipcs_idr;
>  	int max_idx;
> +	int last_idx;	/* For wrap around detection */
>  #ifdef CONFIG_CHECKPOINT_RESTORE
>  	int next_id;
>  #endif
> diff --git a/ipc/util.c b/ipc/util.c
> index 07ae117ccdc0..6e0fe3410423 100644
> --- a/ipc/util.c
> +++ b/ipc/util.c
> @@ -120,6 +120,7 @@ void ipc_init_ids(struct ipc_ids *ids)
>  	rhashtable_init(&ids->key_ht, &ipc_kht_params);
>  	idr_init(&ids->ipcs_idr);
>  	ids->max_idx = -1;
> +	ids->last_idx = -1;
>  #ifdef CONFIG_CHECKPOINT_RESTORE
>  	ids->next_id = -1;
>  #endif
> @@ -193,6 +194,10 @@ static struct kern_ipc_perm *ipc_findkey(struct ipc_ids *ids, key_t key)
>   *
>   * The caller must own kern_ipc_perm.lock.of the new object.
>   * On error, the function returns a (negative) error code.
> + *
> + * To conserve sequence number space, especially with extended ipc_mni,
> + * the sequence number is incremented only when the returned ID is less than
> + * the last one.
>   */
>  static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
>  {
> @@ -216,17 +221,37 @@ static inline int ipc_idr_alloc(struct ipc_ids *ids, struct kern_ipc_perm *new)
>  	 */
>  
>  	if (next_id < 0) { /* !CHECKPOINT_RESTORE or next_id is unset */
> -		new->seq = ids->seq++;
> -		if (ids->seq > IPCID_SEQ_MAX)
> -			ids->seq = 0;
> -		idx = idr_alloc(&ids->ipcs_idr, new, 0, 0, GFP_NOWAIT);
> +
> +		/* allocate the idx, with a NULL struct kern_ipc_perm */
> +		idx = idr_alloc(&ids->ipcs_idr, NULL, 0, 0, GFP_NOWAIT);
> +
> +		if (idx >= 0) {
> +			/*
> +			 * idx got allocated successfully.
> +			 * Now calculate the sequence number and set the
> +			 * pointer for real.
> +			 */
> +			if (idx <= ids->last_idx) {
> +				ids->seq++;
> +				if (ids->seq >= ipcid_seq_max())
> +					ids->seq = 0;
> +			}
> +			ids->last_idx = idx;
> +
> +			new->seq = ids->seq;
> +			/* no need for smp_wmb(), this is done
> +			 * inside idr_replace, as part of
> +			 * rcu_assign_pointer
> +			 */
> +			idr_replace(&ids->ipcs_idr, new, idx);
> +		}
>  	} else {
>  		new->seq = ipcid_to_seqx(next_id);
>  		idx = idr_alloc(&ids->ipcs_idr, new, ipcid_to_idx(next_id),
>  				0, GFP_NOWAIT);
>  	}
>  	if (idx >= 0)
> -		new->id = (new->seq << IPCMNI_SEQ_SHIFT) + idx;
> +		new->id = (new->seq << ipcmni_seq_shift()) + idx;
>  	return idx;
>  }
>  
> diff --git a/ipc/util.h b/ipc/util.h
> index 9746886757de..8c834ed39012 100644
> --- a/ipc/util.h
> +++ b/ipc/util.h
> @@ -34,13 +34,13 @@
>  extern int ipc_mni;
>  extern int ipc_mni_shift;
>  
> -#define IPCMNI_SEQ_SHIFT	ipc_mni_shift
> +#define ipcmni_seq_shift()	ipc_mni_shift
>  #define IPCMNI_IDX_MASK		((1 << ipc_mni_shift) - 1)
>  
>  #else /* CONFIG_SYSVIPC_SYSCTL */
>  
>  #define ipc_mni			IPCMNI
> -#define IPCMNI_SEQ_SHIFT	IPCMNI_SHIFT
> +#define ipcmni_seq_shift()	IPCMNI_SHIFT
>  #define IPCMNI_IDX_MASK		((1 << IPCMNI_SHIFT) - 1)
>  #endif /* CONFIG_SYSVIPC_SYSCTL */
>  
> @@ -123,8 +123,8 @@ struct pid_namespace *ipc_seq_pid_ns(struct seq_file *);
>  #define IPC_SHM_IDS	2
>  
>  #define ipcid_to_idx(id)  ((id) & IPCMNI_IDX_MASK)
> -#define ipcid_to_seqx(id) ((id) >> IPCMNI_SEQ_SHIFT)
> -#define IPCID_SEQ_MAX	  (INT_MAX >> IPCMNI_SEQ_SHIFT)
> +#define ipcid_to_seqx(id) ((id) >> ipcmni_seq_shift())
> +#define ipcid_seq_max()	  (INT_MAX >> ipcmni_seq_shift())
>  
>  /* must be called with ids->rwsem acquired for writing */
>  int ipc_addid(struct ipc_ids *, struct kern_ipc_perm *, int);

Acked-by: Waiman Long <longman@xxxxxxxxxx>

I am fine with this patch replacing mine.

Cheers,
Longman