How can I get more information (apart from PID) of the sender process when a signal is received ? Like process name and it's args

Question

I am running an Embedded Linux OS with systemd as init. Sometimes I see systemd manager does a log dump during early bootup upon receiving SIGUSR2 signal. I found the sender PID using signalfd() but by the time I try to print using cat /proc/pid/cmdline there seems to be no trace of it.

Answer 1

I added a small piece of code in kill() system call in kernel/signal.c to execute a script to get more details about the sender process (cmdline, its parent, etc.,)

Code in kill() system call:

 // We are concerned only about SIGUSR2 to init
 if (17 == sig && 1 == pid)
 {
     printk("PID %d sent SIGUSR2 to systemd\n", info.si_pid, pid);
     char *envp[] = { "HOME=/", NULL };
     char *argv[] = { "/bin/sh", "-c", "/etc/getSenderInfo.sh", NULL };
     call_usermodehelper(argv[0], argv, envp, UMH_WAIT_PROC);
 }

/etc/getSenderInfo.sh

#!/bin/sh
pid=$(dmesg | grep -w "sent SIGUSR2 to systemd" | awk '{print $2}')

while [ $pid -ne 0 ]
do
    echo "ps -o ppid= -o cmd= -p $pid"
    ppid=$(ps -o ppid= -o cmd= -p $pid)
    echo $ppid
    pid=$(echo $ppid | awk '{print $1}')
done

I am not sure if this is the most right way but this did the trick for me