Questions[ipc](server)

[Although I'm working in perl I believe that this question relate to Linux System V IPC APIs and limits rather that the anything perl-specific.]

I have two Centos machines, each at CentOS Linux release 7.9.2009 (Core).

I have a program that forks a child and then uses System V IPC messages to communicate with the child, the child prepares answers ands sends them to the parent.

On one machine we see expected behaviour. The child produces batches of messages, the parent consumes them. Occasionally the child works a little faster than the parent, and so may fill the queue, the child then waits until the parent consumes some messages and proceeds.

We can inspect the queue sizes with ipcs -q and see that the default limit of 10 messages per queue is occasionally reached, the child pauses, and then we see the queue empty as expected.

We believe that the queue limits are specified in files in /proc/sys/fs/mqueue/ and for example msg_max is seen to be the expected 10. These values are identical on both machines.

We also can view user ulimits relating to queue using ulimit -q and see a value in excess of 800,000 bytes on both machines.

The puzzle is that on our second machine we see the child write three messages to the queue and attempt to write the fourth and wait - we deliberately set no timeout. It is as though the writer believes that the queue is full, even though ipcs -q shows only 3 items in the queue. At this point in time the parent is not yet attempting to read the messages.

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    
0x0000002a 1474560    dave      600        15020        3  

The question: what, other than the queue being full, would cause msgsnd() to pause? The pause seems to continue indefinitely, the child proceeds when the parent becomes active and reads some messages.

We have many machines running this code without issue. It fails on three new machines. Presumably there is some environment-specific feature that interacts with our code?

The perl code uses a thin library above the system calls (details elided)

$mQueue = msgget(IPC_PRIVATE, IPC_CREAT | S_IRUSR | S_IWUSR);
msgsnd( $mQueue, pack("l! a*", length($msg), $msg);

I have a samba server where I have disabled access to the IPC$ share [1] for security reasons (users and shares enumeration, user info query, ...).

I can access the data shares on this server without problem from any Unix based computer with mount.cifs. However, when I try to access them from Windows computers (tested with Windows 7 and 10), I get an Access denied error just after providing my credentials.

After recording the SMB traffic, I noticed that Windows computers always do a Tree connect request on the IPC$ share before trying to access the share I provided (through map network drive or add network location). My samba server returns Access denied for the IPC$ share and Windows never tries to access the share I want.

So my question is how can I prevent Windows to do this useless request on IPC$ and just access the share I provided? Or at least how can I force Windows to access the real share after the IPC$ one returned Access denied?

Thank you in advance for your help and happy holidays!

Sponge bob

[1] https://www.samba.org/samba/docs/man/Samba-HOWTO-Collection/securing-samba.html#id2618877

I'm investingating nginx's cache mechanism. I set the following in my nginx.conf.

fastcgi_cache_path /run/nginx-cache levels=1:2 keys_zone=PHP_CACHE:4m inactive=5m max_size=1m;

By keys_zone=PHP_CACHE:4m, nginx should allocate 4 MiB of shared memory. To check this, I used ipcs command, but it didn't show anything. Does this mean nginx doesn't really utilize Linux's shared memory mechanism? Or are there other ways to monitor a process's shared memory usage?

I understand semaphores, but what are these semaphore arrays being used on my Linux box?

$ ipcs

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x00000000 327681     root      644        80         2                       
0x00000000 360450     root      644        16384      2                       
0x00000000 393219     root      644        280        2                       
...

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0x4172d4f4 290914305  lazer     660        104       
0x3b87b970 291045378  lazer     660        104       
0xa97eb380 293928963  lazer     660        104       
0x1fde2040 294191108  lazer     660        104       

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    

$

Also, which OS resource are they guarding?

Are there any special permissions needed for domain service account running a windows service which receives messages over named pipes? Hosting environment is Windows Server 2008.

Background:

We have two processes communicating over named pipes. Our windows service exposes WCF service over named pipes binding and a IIS worker process running on the same machine calls that WCF service.

Everything works on our development machines and test server but when we deploy the soution to integration environment the client (IIS worker) is not able to call the service with exception that there is no endpoint listening on the address. The service is up and running and based on logs the service host is listening.

The server is controlled by group policy and low priviledges accounts are used so I think it can be some problem of insufficient permissions.