Category Archives: linux

How to disable NVIDIA card in X server

20.10.2018 – 22:16

I actively using CUDA capable cards from NVIDIA for machine learning applications.
When GPU is under heavy load by ml tasks it’s almost impossible to do anything in graphical interface because screen refreshes too slow.
So I switched to integrated video controller. It’s not solved the issue with low refresh rate right away because X server still using NVIDIA card.
After some trials and errors I found a quick and dirty solution: disable graphics card driver during boot to prevent X server to use a card.
To accomplish this you need to blacklist driver by adding a following line:

install nvidia /bin/false

1	install nvidia /bin/false

to /etc/modprobe.d/blacklist.conf
And don’t forget to update initramfs:

sudo update-initramfs -u

1	sudo update-initramfs -u

After system boot you can load driver manually by following command:

sudo insmod /lib/modules/`uname -r`/updates/dkms/nvidia.ko

1	sudo insmod /lib/modules/`uname -r`/updates/dkms/nvidia.ko

Yuriy Nazarov | Permalink | Comment (0)

Space efficient source code storage

23.03.2016 – 23:28

During implementation of DistTest I faced with necessity of building a lot of different linux kernel versions. As a first solution I chose downloading archives from kernel.org for each used version. But I soon realized that about 1000 versions of sources with size 0.5-1GB each would consume a lot of disk space. It’s also impossible to build kernel with exact commit precision using this approach.
Set of base versions with corresponding patches can save disk space, but uses a lot of random I/O during applying patches, so it’s slow on HDD and consume finite rewrite resource of SSD. Temporary nature of sources leads to conclusion “use tmpfs”. But aufs offers much less RAM consuming method – store in RAM only diffs.

Yuriy Nazarov | Permalink | Comment (0)

linux

Yet another security note. Hardlinks.

21.03.2014 – 03:08

Let’s imagine that you have a bunch of files(with default mode “rw-r–r–“) and you configured automatic or performed manual hardlink based backup of them.
Well…
Then you moved one of these files to “secured” folder, that have strict rights (“drwx——” for example).
Before adding some confidential information to this file it was good idea to change file permissions to more strict one. But it is not clear how important it is because no one but owner can access file located at “secured/file” when “secured” folder have “drwx——” rights.
Well… Let’s preserve old permissions if changing of them is not necessary.
But what about hardlink to file saved in usual folder? Oh yes, file located at “usual/file” still can be opened by everyone.
Conclusion:
– You must remember about all hardlinks of your files when you think about security.
– Creating hardlinks by inode and opening file by inode denied by security reasons.

Yuriy Nazarov | Permalink | Comment (0)

bash linux

Web server on bash

04.09.2012 – 00:17

web server on bash, nc, awk and sed

#!/bin/sh
rm /tmp/f
mkfifo /tmp/f
#runloop
while true
do
	nc -l 8080 < /tmp/f | (
	#read request
	read line

	#parse request
	args=$(echo -n "$line" | awk '{print $2}' | sed s~'?'~' '~ | awk '{print $2}')
	page=$(echo -n "$line" | awk '{print $2}' | sed s~'?'~' '~ | awk '{print $1}' | sed s~'/'~''~)
	user=$(echo -n "$args" | sed s~','~"\n"~ | sed s~'='~' '~ | awk '{if($1 == "user") print $2}')

	#some logic
	if [ "$page" != 'index.sh' ]
	then
		conttype="application/octet-stream"
		len=$(stat -c%s $page)
		echo "HTTP/1.0 200 OK"
		echo "Server: MadnessBASH"
		echo "Content-Length: $len"
		echo "Content-Type: $conttype"
		echo ""
		cat $page

	else
		conttype="text/html"

		if [ -z "$user" ]
		then
			data=`cat login.htm`
		else
			data=`cat index.htm`
			data=$(echo -n "$data" | sed s~_user_~"$user"~)
		fi
		len=`echo $data | wc -c`

		echo "HTTP/1.0 200 OK"
		echo "Server: MadnessBASH"
		echo "Content-Length: $len"
		echo "Content-Type: $conttype"
		echo ""
		echo $data
	fi

) > /tmp/f
done

#!/bin/sh

rm /tmp/f

mkfifo /tmp/f

#runloop

while true

nc -l 8080 < /tmp/f | (

#read request

read line

#parse request

args=$(echo -n "$line" | awk '{print $2}' | sed s~'?'~' '~ | awk '{print $2}')

page=$(echo -n "$line" | awk '{print $2}' | sed s~'?'~' '~ | awk '{print $1}' | sed s~'/'~''~)

user=$(echo -n "$args" | sed s~','~"\n"~ | sed s~'='~' '~ | awk '{if($1 == "user") print $2}')

#some logic

if [ "$page" != 'index.sh' ]

then

conttype="application/octet-stream"

len=$(stat -c%s $page)

echo "HTTP/1.0 200 OK"

echo "Server: MadnessBASH"

echo "Content-Length: $len"

echo "Content-Type: $conttype"

echo ""

cat $page

else

conttype="text/html"

if [ -z "$user" ]

then

data=`cat login.htm`

else

data=`cat index.htm`

data=$(echo -n "$data" | sed s~_user_~"$user"~)

len=`echo $data | wc -c`

echo "HTTP/1.0 200 OK"

echo "Server: MadnessBASH"

echo "Content-Length: $len"

echo "Content-Type: $conttype"

echo ""

echo $data

) > /tmp/f

done

Yuriy Nazarov | Permalink | Comment (0)

linux

Remote access without “white” ip

03.09.2012 – 00:19

How to get access to shell on the remote computer that don’t have “white”(“real” or “public”) IP without any additional software in linux?
Quick and dirty solution:
on server side:

mkfifo /tmp/p0.0
mkfifo /tmp/p0.1
screen sh -c 'cat < /tmp/p0.1 & cat > /tmp/p0.0'
#now you can detach this from current console and attach it back when you want it

mkfifo /tmp/p0.0

mkfifo /tmp/p0.1

screen sh -c 'cat < /tmp/p0.1 & cat > /tmp/p0.0'

#now you can detach this from current console and attach it back when you want it

on client side:

mkfifo /tmp/localpipe
ssh -p 22 my.host.name 'cat < /tmp/p0.0 & cat > /tmp/p0.1' < /tmp/localpipe | bash > /tmp/localpipe

1 2	mkfifo /tmp/localpipe ssh -p 22 my.host.name 'cat < /tmp/p0.0 & cat > /tmp/p0.1' < /tmp/localpipe \| bash > /tmp/localpipe

pipe ‘/tmp/localpipe’ used to redirect bash output back to ssh
note: use exit command to stop redirection

Yuriy Nazarov | Permalink | Comments (2)

From idea to profit

Category Archives: linux

How to disable NVIDIA card in X server

Space efficient source code storage

Yet another security note. Hardlinks.

Web server on bash

Remote access without “white” ip

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