[clue] [TECH] reading semaphore data using /proc/kcore?
Will
will.sterling at gmail.com
Thu May 12 12:25:37 MDT 2011
You can write a small C program that takes the id and log file as arguments.
The utilities I'm aware that let you view shared memory segments are
specific to the application using the shared memory.
http://www.cs.cf.ac.uk/Dave/C/node27.html#SECTION002761000000000000000
On Thu, May 12, 2011 at 12:10 PM, Jim Ockers <ockers at ockers.net> wrote:
> Hi CLUEbies,
>
> I have some processes that maintain a 340 byte data structure in memory as
> a shared memory semaphore. I'd like to be able to grab the bytes out of
> memory from time to time, without the cooperation or knowledge of the
> processes that are reading & writing the data. We suspect some sort of
> weird data corruption and I'd like to be able to produce sort of a
> time-stamped "packet trace" of the semaphore data so we can compare the
> values in memory with the expected values. Also what happens is one of the
> processes crashes from time to time for no apparent reason and we want to
> read the semaphore data out of memory at the moment it crashes.
>
> This might be a n00b question but a google search wasn't helpful and so I
> thought someone out there in CLUE-land might know.
>
> Here is ipcs output:
>
> [root /usr/sbin]# ipcs
>
> ------ Shared Memory Segments --------
> key shmid owner perms bytes nattch status
> 0x00000000 0 root 600 1808 3 dest
> 0x00000000 32769 root 600 340 3 dest
> 0x00000000 65538 root 600 1728012 3 dest
> 0x00000000 98307 root 600 1220 2 dest
>
> ------ Semaphore Arrays --------
> key semid owner perms nsems status
> 0x00000000 0 root 600 7
>
> ------ Message Queues --------
> key msqid owner perms used-bytes messages
>
> We want shmid 32769 with 340 bytes. How can I get it?
>
> [ userid ]$ cd /proc/sysvipc/
> [ userid /proc/sysvipc]$ ls -al
> total 0
> dr-xr-xr-x 2 root root 0 May 12 12:00 .
> dr-xr-xr-x 54 root root 0 May 11 09:35 ..
> -r--r--r-- 1 root root 0 May 12 12:00 msg
> -r--r--r-- 1 root root 0 May 12 12:00 sem
> -r--r--r-- 1 root root 0 May 12 12:00 shm
> [logger at mlogger /proc/sysvipc]$ cat sem
> key semid perms nsems uid gid cuid cgid
> otime ctime
> 0 0 600 7 0 0 0 0 1305223243
> 1305149770
> [logger at mlogger /proc/sysvipc]$ cat shm
> key shmid perms size cpid lpid nattch uid gid
> cuid cgid atime dtime ctime
> 0 0 1600 1808 580 670 3 0 0
> 0 0 1305150844 1305151313 1305149770
> 0 32769 1600 340 580 670 3 0 0
> 0 0 1305150844 1305151313 1305149770
> 0 65538 1600 1728012 580 670 3 0 0
> 0 0 1305150844 1305151313 1305149770
> 0 98307 1600 1220 580 1104 2 0 0
> 0 0 1305150844 1305150844 1305149770
> [logger at mlogger /proc/sysvipc]$ cat msg
> key msqid perms cbytes qnum lspid lrpid uid gid
> cuid cgid stime rtime ctime
>
> It seems I might just be able to dd the relevant bytes out of /proc/kcore
> but I don't know where to find these bytes. Yes I suppose I could do it the
> hard way and try to dump the whole 512MB kcore to a file and then xxd and
> grep through it but that is inelegant. Is there an elegant way to get
> NUMBEROFBYTESTOSKIP given what I know about the semaphore from the SYSVIPC
> (system V inter-process communication) information so I can do something
> like this:
>
> dd if=/proc/kcore skip=NUMBEROFBYTESTOSKIP bs=340 count=1 of=/tmp/dump
>
> Thanks,
> Jim
>
> --
> Jim Ockers, P.Eng. (ockers at ockers.net)
> Contact info: http://www.ockers.net/msi.html
>
>
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