A blind debugging session.
The website using PHP is down.
What we have to work with
The source code. (No)
Knowledge of the system. (No)
Familiarity with the programming language. (No)
SSH Access. (Yes)
Logging in the real world is often useless.
Find a pid. ps aux | grep apache
sudo strace -ff -s 2048 -p pid
How to read strace output
write(l, "hi\n", 3) = 3
write -> function name
arguments are l, "hi\n" and 3
return value is 3
To learn more about system calls: man 2 write
strace gives lot of output. Work backwards and find the failure. Look for the error message in the strace output. Find the cause by reading the strace output. Find the offender and write down your hypothesis. Prove your hypothesis. Find the offender. Fix the bug.
0. Forget everything you think you know.
1. Get a third party opinion.
2. Refer the 'Linux Performance Tools' diagram for a list of third-party tools.
You can also use strace to start a process.
sudo strace -ff apt-get update
Work backwards, find failure. Find the cause. Confirm your hypothesis. Locate a hook. Stare at the code. Confirm your hypothesis.
2. Locate the correct source code.
3. Identify a hard-coded string to grep for.
4. Stare at the code until it makes sense.
5. Fix whatever is broken.
0. Forget everything you think you know.
1. Get a third party opinion.
2. Locate the correct source code.
3. Identify a hard-coded string to grep for.
4. Stare at the code until it makes sense.
5. Fix whatever is broken.
puts Process.ppid
t = Thread.new do
sleep 10
end
# Grabbing the pid.
pid = Process.pid
puts pid
# Get the child pids.
pipe = IO.popen("ps -ef | grep #{pid}")
child_pids = pipe.readlines.map do |line|
puts line
parts = line.split(/\s+/)
# puts parts
# parts[2] if parts[3] == pid.to_s and parts[2] != pipe.pid.to_s
end.compact
# Show the child processes.
# puts child_pids
#
#
# q = Queue.new
# # q.pop
#
t.join
