linux-kernel - Re: [PATCH] IPC: message queue stealing feature introduced

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Date:	Wed, 15 Feb 2012 10:40:13 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Stanislav Kinsbursky <skinsbursky@...allels.com>
Cc:	akpm@...ux-foundation.org, criu@...nvz.org,
	lucas.demarchi@...fusion.mobi, dhowells@...hat.com,
	linux-kernel@...r.kernel.org, serge.hallyn@...onical.com
Subject: Re: [PATCH] IPC: message queue stealing feature introduced

Le mercredi 15 février 2012 à 13:08 +0400, Stanislav Kinsbursky a
écrit :
> This patch is required for checkpoint/restore in userspace project.
> C/r requires some way to get all pending IPC messages without deleting them for
> 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_STEAL for sys_msgrcv() system call
> introduced.
> If this flag is set, then passed struct msgbuf pointer will be used for storing
> array of structures:
> 
> struct msgbuf_a {
> 	long mtype;         /* type of message */
> 	size_t msize;       /* size of message */
> 	char mtext[0];      /* message text */
> };
> 

This structure is not 32/64 bit safe and will need compat tricks.

+                       ret = do_steal_msg(mtext, arrsz, msg);
+                       if (ret < 0) {
+                               msg = ERR_PTR(ret);
+                               goto out_unlock;
+                       }
+                       mtext += ret + sizeof(struct msgbuf_a);
+                       arrsz -= ret + sizeof(struct msgbuf_a);

I cant see where alignement requirements are met.

(ret should be rounded up to a multiple of alignof(struct msgbuf_a)



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