So i have an assignment that I am having some difficulty wrapping my head around. The assignment is to create a system call called "mysyscall3" that takes no input and produces
./a.outPID COMM STATE1 systemd INT2 kthreadd INT3 rcu_gp OTHER4 rcu_par_gp OTHER6 kworker/0:0H OTHER7 kworker/u4:0 OTHER8 mm_percpu_wq OTHER9 ksoftirqd/0 INT10 rcu_sched OTHER11 migration/0 INT13 cpuhp/0 INT14 cpuhp/1 INT15 migration/1 INT16 ksoftirqd/1 INT18 kworker/1:0H OTHER20 kdevtmpfs INT21 netns OTHER22 kauditd INT24 khungtaskd INT25 oom_reaper INT26 writeback OTHER27 kcompactd0 INT28 ksmd INT29 khugepaged INT49 cryptd OTHER126 kintegrityd OTHER127 kblockd OTHER128 blkcg_punt_bio OTHER129 tpm_dev_wq OTHER130 md OTHER131 edac-poller OTHER132 watchdogd INT158 kswapd0 INT161 kthrotld OTHER162 acpi_thermal_pm OTHER163 kmpath_rdacd OTHER164 kaluad OTHER166 ipv6_addrconf OTHER226 zswap-shrink OTHER242 kworker/u5:0 OTHER456 nvme-wq OTHER461 nvme-reset-wq OTHER464 nvme-delete-wq OTHER484 ena OTHER507 xfsalloc OTHER513 xfs_mru_cache OTHER517 xfs-buf/nvme0n1 OTHER518 xfs-conv/nvme0n OTHER519 xfs-cil/nvme0n1 OTHER520 xfs-reclaim/nvm OTHER521 xfs-eofblocks/n OTHER522 xfs-log/nvme0n1 OTHER523 xfsaild/nvme0n1 INT524 kworker/0:1H OTHER609 systemd-journal INT639 systemd-udevd INT657 auditd INT701 dbus-daemon INT702 irqbalance INT705 rngd INT709 chronyd INT710 polkitd INT721 sssd INT746 kworker/1:1H OTHER756 sssd_be INT765 sssd_nss INT775 systemd-logind INT858 NetworkManager INT867 tuned INT1010 systemd-resolve INT1063 rsyslogd INT1072 sshd INT1074 agetty INT1075 crond INT1076 agetty INT1188 kworker/0:2 OTHER1535 kworker/0:3 OTHER1546 sshd INT1550 systemd INT1554 (sd-pam) INT1560 sshd INT1561 bash INT1691 kworker/u4:1 OTHER1734 kworker/1:1 OTHER1746 kworker/1:2 OTHER1748 a.out RUNNING86 PIDS 1 RUNNING 43 INT 0 UNINT 0 STOPPED 0 TRACED 42 OTHERThe following is the test code that will be used to test out our system call
/* test.c * * test syscall3 442 * * Compile: * gcc test.c * Run: * ./a.out */#define _GNU_SOURCE#include <unistd.h>#include <sys/syscall.h>#include <stdio.h>#define SYS_mysyscall3 442/* test.c * * gcc test.c # compile * * Use dmesg to verify mysyscall3 was received by the kernel, i.e. * * dmesg | grep mysyscall3 * */#define _GNU_SOURCE#include <unistd.h>#include <sys/syscall.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#define SYS_mysyscall3 442int main(int argc, char **argv){ int maxp; long ret; char *buf; buf = (char*) malloc(512*2048); if ( buf == NULL ) { fprintf(stderr, "Error malloc buf\n"); return 1; } if (argc != 1) { printf("mysyscall3 requires no input.\n"); return -1; } ret = syscall(SYS_mysyscall3, buf); if ( ret ) { // non-zero return value is an error fprintf(stderr, "Error: mysyscall3 returned %ld.\n", ret); return 1; } printf("%s\n", buf); free(buf); return 0;}And below is what i have so far in my /kernel/sys.c file
SYSCALL_DEFINE(mysyscall3) { struct task_struct *task; int numPid = 0; char *buf; buf = (char *)kmalloc(*buf,GFP_KERNEL); if (buf == NULL) { printk(KERN_INFO "mysyscall3 kmalloc\n"); return -EFAULT; } for_each_process(task) { buf = ("%d\t%s\t%ld\n", task->pid,task->comm,task->state); numPid++; } pr_info("%d PIDS\n",numPid); return 0;}So i want to make sure of a few things... since in the test code (2nd snippet) my professor is passing the buf parameter to the system call would it be correct to assume that my actual definition should be SYSCALL_DEFINE1?
Also, i have made the appropriate changes to my systables_64 file to add the system call , and yet on a fresh kernel rebuild and restart if i run my test code nothing is printed to the screen. Should i be using something like copy_to_user() , and if so, should i do that in the for_each_process(3rd code document). I would like to just get something printed to my screen but thats where i keep getting stuck, maybe someone could point me in the right direction ?