I'm wondering about the security of UNIX signals.  SIGKILL will kill the process. So, what happens when a non root user's process sends a signal to a root user's process? Does the process still carry out the signal handler?I follow the accepted answer (gollum's), and I type man capabilites, and I find a lot of things about the Linux kernel. From man capabilities:NAME   capabilities - overview of Linux capabilitiesDESCRIPTION   For the purpose of performing permission checks, traditional UNIX   implementations distinguish two categories of processes: privileged   processes (whose effective user ID is 0, referred to as superuser or   root), and unprivileged processes (whose effective UID is nonzero).   Privileged processes bypass all kernel permission checks, while   unprivileged processes are subject to full permission checking based   on the process's credentials (usually: effective UID, effective GID,   and supplementary group list).   Starting with kernel 2.2, Linux divides the privileges traditionally   associated with superuser into distinct units, known as capabilities,   which can be independently enabled and disabled.  Capabilities are a   per-thread attribute.
What happens when a non-root user sends signals to root user's process?
security;signals
On Linux it depends on the file capabilities.Take the following simple mykill.c source:#include <stdio.h>#include <sys/types.h>#include <signal.h>#include <stdlib.h>void exit_usage(const char *prog) {        printf(usage: %s -<signal> <pid>\n, prog);        exit(1);}int main(int argc, char **argv) {        pid_t pid;        int sig;        if (argc != 3)                exit_usage(argv[0]);        sig = atoi(argv[1]);        pid = atoi(argv[2]);        if (sig >= 0 || pid < 2)                exit_usage(argv[0]);        if (kill(pid, -sig) == -1) {                perror(failed);                return 1;        }        printf(successfully sent signal %d to process %d\n, -sig, pid);        return 0;}build it:gcc -Wall mykill.c -o /tmp/mykillNow as user root start a sleep process in background:root@horny:/root# /bin/sleep 3600 &[1] 16098Now as normal user try to kill it:demouser@horny:/home/demouser$ ps aux | grep sleeproot     16098  0.0  0.0  11652   696 pts/20   S    15:06   0:00 sleep 500demouser@horny:/home/demouser$ /tmp/mykill -9 16098failed: Operation not permittedNow as root user change the /tmp/mykill caps:root@horny:/root# setcap cap_kill+ep /tmp/mykillAnd try again as normal user:demouser@horny:/home/demouser$ /tmp/mykill -9 16098successfully sent signal 9 to process 16098Finally please delete /tmp/mykill for obvious reasons ;)