Subject: Re: [PATCH v6 09/10] IPC: message queue copy feature introduced

From: Serge Hallyn <[email-masked]>
Date: Tue, 23 Oct 2012 11:39:48 -0500

Quoting Stanislav Kinsbursky ([email-masked]):
> This patch is required for checkpoint/restore in userspace.
> IOW, c/r requires some way to get all pending IPC messages without deleting
> them from the queue (checkpoint can fail and in this case tasks will be resumed,
> so queue have to be valid).
> To achive this, new operation flag MSG_COPY for sys_msgrcv() system call was
> introduced. If this flag was specified, then mtype is interpreted as number of
> the message to copy.
> If MSG_COPY is set, then kernel will allocate dummy message with passed size,
> and then use new copy_msg() helper function to copy desired message (instead of
> unlinking it from the queue).
> 
> Notes:
> 1) Return -ENOSYS if MSG_COPY is specified, but CONFIG_CHECKPOINT_RESTORE is
> not set.

How much could it clean things up if a new ipc/cr.c contained

#ifdef CONFIG_CHECKPOINT_RESTORE
struct msg_msg *copy_msg(struct msg_msg *src, struct msg_msg *dst)
{
...
}
#else
struct msg_msg *copy_msg(struct msg_msg *src, struct msg_msg *dst)
{
	return -ENOSYS;
}
#endif

and you went on from there to try to remove all the new #ifdefs from the
existing files?

> Signed-off-by: Stanislav Kinsbursky <[email-masked]>
> ---
>  include/uapi/linux/msg.h |    1 +
>  ipc/msg.c                |   50 ++++++++++++++++++++++++++++++++++++++++++++--
>  ipc/msgutil.c            |   38 +++++++++++++++++++++++++++++++++++
>  ipc/util.h               |    1 +
>  4 files changed, 88 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/msg.h b/include/uapi/linux/msg.h
> index 76999c9..c1af84a 100644
> --- a/include/uapi/linux/msg.h
> +++ b/include/uapi/linux/msg.h
> @@ -11,6 +11,7 @@
>  /* msgrcv options */
>  #define MSG_NOERROR     010000  /* no error if message is too big */
>  #define MSG_EXCEPT      020000  /* recv any msg except of specified type.*/
> +#define MSG_COPY        040000  /* copy (not remove) all queue messages */
>  
>  /* Obsolete, used only for backwards compatibility and libc5 compiles */
>  struct msqid_ds {
> diff --git a/ipc/msg.c b/ipc/msg.c
> index 9808da8..6f52c6b 100644
> --- a/ipc/msg.c
> +++ b/ipc/msg.c
> @@ -788,19 +788,48 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp,
>  	struct msg_msg *msg;
>  	int mode;
>  	struct ipc_namespace *ns;
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +	struct msg_msg *copy = NULL;
> +	unsigned long copy_number = 0;
> +#endif
>  
>  	if (msqid < 0 || (long) bufsz < 0)
>  		return -EINVAL;
> +	if (msgflg & MSG_COPY) {
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +
> +		if (msgflg & MSG_COPY) {
> +			copy_number = msgtyp;
> +			msgtyp = 0;
> +		}
> +
> +		/*
> +		 * Create dummy message to copy real message to.
> +		 */
> +		copy = load_msg(buf, bufsz);
> +		if (IS_ERR(copy))
> +			return PTR_ERR(copy);
> +		copy->m_ts = bufsz;
> +#else
> +		return -ENOSYS;
> +#endif
> +	}
>  	mode = convert_mode(&msgtyp, msgflg);
>  	ns = current->nsproxy->ipc_ns;
>  
>  	msq = msg_lock_check(ns, msqid);
> -	if (IS_ERR(msq))
> +	if (IS_ERR(msq)) {
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +		if (msgflg & MSG_COPY)
> +			free_msg(copy);
> +#endif
>  		return PTR_ERR(msq);
> +	}
>  
>  	for (;;) {
>  		struct msg_receiver msr_d;
>  		struct list_head *tmp;
> +		long msg_counter = 0;
>  
>  		msg = ERR_PTR(-EACCES);
>  		if (ipcperms(ns, &msq->q_perm, S_IRUGO))
> @@ -820,8 +849,16 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp,
>  				if (mode == SEARCH_LESSEQUAL &&
>  						walk_msg->m_type != 1) {
>  					msgtyp = walk_msg->m_type - 1;
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +				} else if (msgflg & MSG_COPY) {
> +					if (copy_number == msg_counter) {
> +						msg = copy_msg(walk_msg, copy);
> +						break;
> +					}
> +#endif
>  				} else
>  					break;
> +				msg_counter++;
>  			}
>  			tmp = tmp->next;
>  		}
> @@ -834,6 +871,10 @@ long do_msgrcv(int msqid, void __user *buf, size_t bufsz, long msgtyp,
>  				msg = ERR_PTR(-E2BIG);
>  				goto out_unlock;
>  			}
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +			if (msgflg & MSG_COPY)
> +				goto out_unlock;
> +#endif
>  			list_del(&msg->m_list);
>  			msq->q_qnum--;
>  			msq->q_rtime = get_seconds();
> @@ -917,8 +958,13 @@ out_unlock:
>  			break;
>  		}
>  	}
> -	if (IS_ERR(msg))
> +	if (IS_ERR(msg)) {
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +		if (msgflg & MSG_COPY)
> +			free_msg(copy);
> +#endif
>  		return PTR_ERR(msg);
> +	}
>  
>  	bufsz = msg_handler(buf, msg, bufsz);
>  	free_msg(msg);
> diff --git a/ipc/msgutil.c b/ipc/msgutil.c
> index 26143d3..b281f5c 100644
> --- a/ipc/msgutil.c
> +++ b/ipc/msgutil.c
> @@ -100,7 +100,45 @@ out_err:
>  	free_msg(msg);
>  	return ERR_PTR(err);
>  }
> +#ifdef CONFIG_CHECKPOINT_RESTORE
> +struct msg_msg *copy_msg(struct msg_msg *src, struct msg_msg *dst)
> +{
> +	struct msg_msgseg *dst_pseg, *src_pseg;
> +	int len = src->m_ts;
> +	int alen;
> +
> +	BUG_ON(dst == NULL);
> +	if (src->m_ts > dst->m_ts)
> +		return ERR_PTR(-EINVAL);
> +
> +	alen = len;
> +	if (alen > DATALEN_MSG)
> +		alen = DATALEN_MSG;
> +
> +	dst->next = NULL;
> +	dst->security = NULL;
>  
> +	memcpy(dst + 1, src + 1, alen);
> +
> +	len -= alen;
> +	dst_pseg = dst->next;
> +	src_pseg = src->next;
> +	while (len > 0) {
> +		alen = len;
> +		if (alen > DATALEN_SEG)
> +			alen = DATALEN_SEG;
> +		memcpy(dst_pseg + 1, src_pseg + 1, alen);
> +		dst_pseg = dst_pseg->next;
> +		len -= alen;
> +		src_pseg = src_pseg->next;
> +	}
> +
> +	dst->m_type = src->m_type;
> +	dst->m_ts = src->m_ts;
> +
> +	return dst;
> +}
> +#endif
>  int store_msg(void __user *dest, struct msg_msg *msg, int len)
>  {
>  	int alen;
> diff --git a/ipc/util.h b/ipc/util.h
> index 271bded..027f507 100644
> --- a/ipc/util.h
> +++ b/ipc/util.h
> @@ -142,6 +142,7 @@ int ipc_parse_version (int *cmd);
>  
>  extern void free_msg(struct msg_msg *msg);
>  extern struct msg_msg *load_msg(const void __user *src, int len);
> +extern struct msg_msg *copy_msg(struct msg_msg *src, struct msg_msg *dst);
>  extern int store_msg(void __user *dest, struct msg_msg *msg, int len);
>  
>  extern void recompute_msgmni(struct ipc_namespace *);
> 
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