linux-kernel - Re: [RFC][PATCH] /proc/pid/maps doesn't match "ipcs -m" shmid

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Message-ID: <46689B55.1020905@us.ibm.com>
Date:	Thu, 07 Jun 2007 16:57:09 -0700
From:	Badari Pulavarty <pbadari@...ibm.com>
To:	"Serge E. Hallyn" <serge@...lyn.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Albert Cahalan <acahalan@...il.com>,
	lkml <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, ebiederm@...ssion.com,
	torvalds@...ux-foundation.org
Subject: Re: [RFC][PATCH] /proc/pid/maps doesn't match "ipcs -m" shmid



Serge E. Hallyn wrote:

>Quoting Serge E. Hallyn (serge@...lyn.com):
>
>>Quoting Badari Pulavarty (pbadari@...ibm.com):
>>
>>>On Thu, 2007-06-07 at 15:37 -0500, Serge E. Hallyn wrote:
>>>
>>>>Quoting Badari Pulavarty (pbadari@...ibm.com):
>>>>
>>>>>On Thu, 2007-06-07 at 12:48 -0700, Andrew Morton wrote:
>>>>>
>>>>>>On Thu, 07 Jun 2007 10:06:37 -0700
>>>>>>Badari Pulavarty <pbadari@...ibm.com> wrote:
>>>>>>
>>>>>>>On Thu, 2007-06-07 at 12:43 -0400, Albert Cahalan wrote:
>>>>>>>
>>>>>>>>On 6/7/07, Badari Pulavarty <pbadari@...ibm.com> wrote:
>>>>>>>>
>>>>>>>>>BTW, I agree with Eric that its would be nice to use shmid as part
>>>>>>>>>of name instead of forcing to be as inode number. It should be
>>>>>>>>>possible for pmap to workout shmid from "key" or name. Isn't it ?
>>>>>>>>>
>>>>>>>>It is not at all nice.
>>>>>>>>
>>>>>>>>1. it's incompatible ABI breakage
>>>>>>>>2. where will you put the key then, in the inode? :-)
>>>>>>>>
>>>>>>>Nope. Currently "key" is part of the name (but its not unique).
>>>>>>>
>>>>>>>>Changing to "SYSVID%d" is no good either. Look, people
>>>>>>>>are ***parsing*** this stuff in /proc. The /proc filesystem
>>>>>>>>is not some random sandbox to be playing in.
>>>>>>>>
>>>>>>>>Before you go messing with it, note that the device number
>>>>>>>>also matters. (it's per-boot dynamic, but that's OK)
>>>>>>>>That's how one knows that /SYSV00000000 is not just
>>>>>>>>a regular file; sadly these didn't get a non-/ prefix.
>>>>>>>>(and no you can't fix that now; it's way too late)
>>>>>>>>
>>>>>>>>Next time you feel like breaking an ABI, mind putting
>>>>>>>>"LET'S BREAK AN ABI!" in the subject of your email?
>>>>>>>>
>>>>>>>I am not breaking ABI. Its already broken in the current
>>>>>>>mainline. I am trying to fix it by putting back the ino#
>>>>>>>as shmid. Eric had a suggestion that, instead of depending
>>>>>>>on the inode# to be shmid, we could embed shmid into name
>>>>>>>(instead of "key" which is currently not unique).
>>>>>>>
>>>>>>>>BTW, I suspect this kind of thing also breaks:
>>>>>>>>a. fuser, lsof, and other resource usage display tools
>>>>>>>>b. various obscure emulators (similar to valgrind)
>>>>>>>>
>>>>>>>If you strongly feel that "old" behaviour needs to be retained, 
>>>>>>>
>>>>>>yup, we should put it back.  The change was, afaik, accidental.
>>>>>>
>>>>>>>here is the patch I originally suggested.
>>>>>>>
>>>>>>Confused.  Will this one-liner fix all the userspace breakage to which
>>>>>>Albert refers?
>>>>>>
>>>>>Yes. Albert, please correct me if I am wrong.
>>>>>
>>>>It will, but could lead to two different inodes with the same i_ino,
>>>>right?
>>>>
>>>Only if we generate same ID in two different namespaces. Is it currently
>>>possible ? 
>>>
>>Should be nothing stopping it.
>>
>
>(just to be more certain, a quick test showed I can get id 0 for
>different keys, and different ids for the same key 0xff, in different
>ipc namespaces)
>
Funny. I played with it and decided that it can happen :)

Thanks,
Badari



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