ipc: introduce message queue copy feature

This patch is required for checkpoint/restore in userspace.

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.

Signed-off-by: Stanislav Kinsbursky <skinsbursky@parallels.com>
Cc: Serge Hallyn <serge.hallyn@canonical.com>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: Pavel Emelyanov <xemul@parallels.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Michael Kerrisk <mtk.manpages@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/ipc/util.h b/ipc/util.h
index a61e0ca2bffd..eeb79a1fbd83 100644
--- a/ipc/util.h
+++ b/ipc/util.h
@@ -140,6 +140,7 @@ 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 *);