Re: Document POSIX MQ /proc/sys/fs/mqueue files

[Davidlohr Bueso]

Hi Michael,

Cc'ing Manfred.

On Mon, 2014-09-29 at 11:10 +0200, Michael Kerrisk (man-pages) wrote:
> Hello Doug, David,
> 
> I think you two were the last ones to make significant 
> changes to the semantics of the files in /proc/sys/fs/mqueue,
> so I wonder if you (or anyone else who is willing) might
> take a look at the man page text below that I've written
> (for the mq_overview(7) page) to describe past and current
> reality, and let me know of improvements of corrections.

Over the years posix mqueues have increasingly become a mess *sigh* :-/
Thanks for doing this and untangling some of the historic changes.

> From mq_overview(7) draft:
> 
>    /proc interfaces
>        The following interfaces can be used to limit the amount of  kerâ
>        nel  memory  consumed  by  POSIX  message  queues  and to set the
>        default attributes for new message queues:
> 
>        /proc/sys/fs/mqueue/msg_default (since Linux 3.5)
>               This file  defines  the  value  used  for  a  new  queue's
>               mq_maxmsg setting when the queue is created with a call to
>               mq_open(3) where attr is specified as NULL.   The  default
>               value for this file is 10.  The minimum and maximum are as
>               for /proc/sys/fs/mqueue/msg_max.  If  msg_default  exceeds
>               msg_max,  a  new queue's default mq_maxmsg value is capped
>               to the msg_max limit. 

I think rephrasing this would read easier. Basically the behavior is
this:

		info->attr.mq_maxmsg = min(ipc_ns->mq_msg_max,
					   ipc_ns->mq_msg_default);

Something like:
"a new queue's default mq_maxmsg value will be the smallest of msg_default and msg_max"

>  Up until Linux 2.6.28, the  default
>               mq_maxmsg  was  10;  from  Linux  2.6.28 to Linux 3.4, the
>               default was the value defined for the msg_max limit.
> 
>        /proc/sys/fs/mqueue/msg_max
>               This file can be used to view and change the ceiling value
>               for the maximum number of messages in a queue.  This value
>               acts as a ceiling on the attr->mq_maxmsg argument given to
>               mq_open(3).   The  default  value  for msg_max is 10.  The
>               minimum value is 1 (10 in  kernels  before  2.6.28).   The
>               upper  limit is HARD_MSGMAX.  The msg_max limit is ignored
>               for  privileged  processes  (CAP_SYS_RESOURCE),  but   the
>               HARD_MSGMAX ceiling is nevertheless imposed.

Note that the HARD_MSGMAX check is done *only* for privileged processes,
regular processes only check against namespace values. This is a pretty
fundamental difference. The same goes of course for msgsize:

	if (capable(CAP_SYS_RESOURCE)) {
		if (attr->mq_maxmsg > HARD_MSGMAX ||
		    attr->mq_msgsize > HARD_MSGSIZEMAX)
			return -EINVAL;
	} else {
		if (attr->mq_maxmsg > ipc_ns->mq_msg_max ||
		    attr->mq_msgsize > ipc_ns->mq_msgsize_max)
			return -EINVAL;
	}


>               The  definition  of  HARD_MSGMAX has changed across kernel
>               versions:
> 
>               *  Up to Linux 2.6.32: 131072 / sizeof(void *)
> 
>               *  Linux 2.6.33 to 3.4: (32768 * sizeof(void *) / 4)
> 
>               *  Since Linux 3.5: 65,536
> 
>        /proc/sys/fs/mqueue/msgsize_default (since Linux 3.5)

You might want to mention the units (bytes) when refering to limits.

>               This file defines the value used for a new queue's mq_msgâ
>               size  setting  when  the  queue  is created with a call to
>               mq_open(3) where attr is specified as NULL.   The  default
>               value  for this file is 8192.  The minimum and maximum are
>               as   for   /proc/sys/fs/mqueue/msgsize_max.     If    msgâ
>               size_default  exceeds  msgsize_max,  a new queue's default
>               mq_msgsize value is capped to the msgsize_max  limit.   Up
>               until  Linux 2.6.28, the default mq_msgsize was 8192; from
>               Linux 2.6.28 to Linux  3.4,  the  default  was  the  value
>               defined for the msgsize_max limit.
> 
>        /proc/sys/fs/mqueue/msgsize_max

Ditto here.

>               This  file  can  be used to view and change the ceiling on
>               the maximum message size.  This value acts as a ceiling on
>               the  attr->mq_msgsize  argument  given to mq_open(3).  The
>               default value for msgsize_max is 8192 bytes.  The  minimum
>               value  is  128 (8192 in kernels before 2.6.28).  The upper
>               limit for msgsize_max has varied across kernel versions:
> 
>               *  Before Linux 2.6.28, the upper limit is INT_MAX.
> 
>               *  From Linux 2.6.28 to 3.4, the limit is 1,048,576.
> 
>               *  Since Linux 3.5, the limit is 16,777,216 (HARD_MSGSIZEâ
>                  MAX).
>               The  msgsize_max  limit  is ignored for privileged process
>               (CAP_SYS_RESOURCE), but, since Linux  3.5,  the  HARD_MSGâ
>               SIZEMAX ceiling is enforced for privileged processes.
> 
>        /proc/sys/fs/mqueue/queues_max
>               This  file  can be used to view and change the system-wide
>               limit on the number of message queues that can be created.
>               The default value for queues_max is 256.  The semantics of
>               this limit have changed across kernel versions as follows:
> 
>               *  Before Linux 3.5, this limit could be  changed  to  any
>                  value  in  the  range 0 to INT_MAX, but privileged proâ
>                  cesses (CAP_SYS_RESOURCE) can exceed the limit.
> 
>               *  Since Linux 3.5, there is a ceiling for this  limit  of
>                  1024     (HARD_QUEUESMAX).      Privileged    processes
>                  (CAP_SYS_RESOURCE) can exceed the queues_max limit, but
>                  the  HARD_QUEUESMAX  limit  is enforced even for priviâ
>                  leged processes.
> 
>               *  Starting with Linux 3.14, the HARD_QUEUESMAX ceiling is
>                  removed: no ceiling is imposed on the queues_max limit,
>                  and privileged processes (CAP_SYS_RESOURCE) can  exceed
>                  the limit.

Given that this was treated as a bug that breaks user-space, I don't
think we really want to document the behavior between 3.5 and 3.14 (all
three bullets). Stable kernels back to 3.5 now have the default
behavior, so its as nothing ever changed(?). Now, if you explicitly want
to document such bug, I would agree, but just not mentioning it as
intentional differences in behavior. Does that make sense?

Thanks,
Davidlohr


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