Tag: Linux

Listing Modules In Dynamically Linked Shared Object Libraries

We had to rebuild a server over the weekend — it’s a lot harder to get Apache and PHP set up when you don’t have root access to just install things from the yum repository. And, unlike the servers where I built httpd and php from source … we basically relayed requests to the Unix admin to have packages installed. One of the confusions during the whole process was that we didn’t know what to use as the module name for PHP to load in the httpd.conf file. The line from our old server (LoadModule php5_module /etc/httpd/modules/libphp5.so) produced an error that there was no such thing to load.

When a library fails to load with some error, I know to use ldd … but I didn’t know there was a way to list out the modules in a library. Fortunately, one of my coworkers had already run nm and listed out the modules — nm -D –defined-only sharedLibraryFile | grep module — and we were able to identify that the libphp5.so that we had wasn’t anything like the one on the old server. By listing the modules for each of the shared object libraries installed by the php package, we got the proper module name for httpd.conf

What Can I sudo?

Some 90% of my Linux experience is on servers where I have root or root-equivalent access (i.e. I can sudo anything). In those cases, ‘what can I run under sudo’ was never a question. And I’d use something like “sudo less /etc/sudoers” to inspect what someone else was able to run when they questioned their access. In my new position, we have a lot of servers that we own too — the Engineering IT support group lets us spin up our own VMs, do whatever we want (within reason). But we have a few IT-managed servers with very restricted rights. And the commands I would use to perform functions (think systemctl restart httpd) aren’t in my sudoers access list. Luckily you can list out what you can run under sudo:

$ sudo -l
[sudo] password for useraccount:
Matching Defaults entries for useraccount on this host:
syslog=auth, loglinelen=0, syslog_goodpri=info, syslog_badpri=err,
logfile=/var/log/sudo.log

User useraccount may run the following commands on this host:
(ALL) /opt/lampp/lampp start, (ALL) /opt/lampp/lampp stop, (ALL)
/opt/lampp/lampp restart, (ALL) /usr/sbin/apachectl

And that is how I know to use apachectl instead of systemctl.

NVIDIA Driver Installation Issue – Fedora 30

NVIDIA finally released an updated driver for Scott’s laptop — one that should be compatible with the 5.x kernel. Ran through the normal process and got the following error:

     Unable to load the nvidia-drm kernel module

Which … was at least new. Tried running through the installation again but not registering the driver with the kernel. Installation completed successfully, and he’s able to boot the 5.8.100 kernel.

Software Flow Control and vim

In the early 90’s, one of the things I liked about Microsoft’s ecosystem was that they developed a standard for keyboard shortcuts. In most applications, developed by Microsoft or not, you could hit ctrl-p to print or ctrl-x to exit. Or ctrl-s to save. It’s quite convenient when I’m using Windows applications, but hitting ctrl-s to save without really thinking about it hangs vim. Hangs like “go into another shell and kill vim & that ssh session”. This is because ctrl-s, in Linux, means XOFF — the software flow control command that means “hi, I’m a thing from 1968 and my buffer is getting full. chill out for a bit and let me catch up”. Recovery is simple enough, send XON — “hi, that thing from 1968 again, and I’m all caught up. send me some more stuff”. That’s ctrl-q.

But there aren’t many slow anything‘s involved in computing these days, which means XON/XOFF isn’t the most useful of features for most people (* if you’ve got real serial devices attached … you may not be “most people” here). Instead of remembering ctrl-q gets gets vim back without killing it, just disable START/STOP control. Thing is it’s not really vim that’s using flow control — it’s the terminal emulator — so the “fix” isn’t something you’ll have to do in vim. In your ~/.bashrc or ~/.bash_profile (or globally in something like /etc/profile.d/disableFlowControl.sh)

# Disable XON/XOFF flow control so ctrl-s doesn’t hang vim
stty -ixon

If you can add -ixoff to avoid ctrl-q from meaning XON too … but I don’t bother since “start sending me data” doesn’t seem to hang anything.

Un-killable Process

Scott had a Dolphin instance veg out on him. Not much for it other than killing the process. Except it didn’t kill. Now SIGTERM (15) I don’t expect to kill a vegged out process, but SIGKILL (9)? I’ve never seen that fail. A little research later, and I’ve discovered “uninterruptible sleep”.  Which, at 11PM is starting to sound really good to me. But not something I associate with computer programs. Essentially, processes that are waiting on I/O very briefly pop into this state and pop out of the state when the I/O operation completes. Code needs to have timeouts to prevent the application from getting stuck waiting for I/O. And, evidently, Scott has discovered a scenario in which Dolphin does not have a timeout.

How can you tell that your process is stuck in uninterruptible sleep? Use “ps u” (or “ps aux” for all processes) and check the “STAT” column.

From “man ps”:

PROCESS STATE CODES
Here are the different values that the s, stat and state output
specifiers (header "STAT" or "S") will display to describe the state of
a process:

D uninterruptible sleep (usually IO)
I Idle kernel thread
R running or runnable (on run queue)
S interruptible sleep (waiting for an event to complete)
T stopped by job control signal
t stopped by debugger during the tracing
W paging (not valid since the 2.6.xx kernel)
X dead (should never be seen)
Z defunct ("zombie") process, terminated but not reaped by its parent

For BSD formats and when the stat keyword is used, additional
characters may be displayed:

< high-priority (not nice to other users)
N low-priority (nice to other users)
L has pages locked into memory (for real-time and custom IO)
s is a session leader
l is multi-threaded (using CLONE_THREAD, like NPTL pthreads do)
+ is in the foreground process group

Rebooting clears the process (or sorting whatever is blocking the I/O operation). But there are processes that “kill -9” won’t terminate.

Recreating Grub Bootloader After the Windows Install Wipes It

Setting up the dual-boot Windows/Fedora system was straight-forward on my laptop. I installed Windows, then installed Linux and grub mkconfig found Windows and included it in the menu. Scott already had Fedora, and we needed to repair his Windows installation. Which, of course, blew away grub. Easy enough to get back, provided you’ve got a Live USB installation from which to boot.

Boot the Live media and use fdisk to find the Linux partition (in our installations, /boot is contained within the root partition).

[root@fedora02 ~]# fdisk -l
Disk /dev/sda: 10 GiB, 10737418240 bytes, 20971520 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: dos
Disk identifier: 0xd847fbc2

Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 1026047 1024000 500M 83 Linux
/dev/sda2 1026048 20971519 19945472 9.5G 8e Linux LVM

Mount that partition somewhere:

mkdir /mnt/mycomputer
mount /dev/sda2 /mnt/mycomputer

Add bind mounts so /dev and /proc are in there

mount –bind /dev /mnt/mycomputer/dev
mount –bind /proc /mnt/mycomputer/proc

Chroot yourself into the mount point

chroot /mnt/mycomputer

Now you can reinstall grub. You don’t want the partition (e.g. /dev/sda2) but the disk. The following commands install the grub2 bootloader and reboot.

grub2-install /dev/sda
reboot

If you forget to pay attention on boot and thus inadvertently end up in the default operating system (<G>), edit /etc/default/grub and increase “GRUB_TIMEOUT=5”. Build the grub config — this should identify your Windows partition and include it in the menu

grub2-mkconfig -o /boot/grub2/grub.cfg

Reboot again, and you’ll be able to select between Windows and Linux.

Using grub rescue to boot machine and repair MBR

Using grub rescue to boot machine and repair MBR

Use “ls” to find your partition list:
ls
(hd0) (hd0,msdos3) (hd0,msdos2) (hd0,msdos1)

Check the content of each to find your Linux partition:
ls (hd0,msdos3)/
ls (hd0,msdos2)/
ls (hd0,msdos1)/

You want the one with the /boot folder. In our case, this is (hd0,msdos3). The following commands will boot your Linux OS.

set prefix=(hd0,msdos3)/boot/grub2
set root=(hd0,msdos3)
insmod normal
normal

<root password for maintenance>

Use “df” or “mount” to figure out which disk holds the Linux partition. In our case, it is /dev/sdb. You don’t want the partition (e.g. /dev/sdb2) but the disk. The following commands install the grub2 bootloader and build a config file.

grub2-install /dev/sdb
grub2-mkconfig -o /boot/grub2/grub.cfg