limit the syscalls to track on examples/systrack.lua

* see #96

README.md

@@ -1030,94 +1030,21 @@ hello kernel!
 
 [systrack](examples/systrack.lua)
 is a kernel script that implements a device driver to monitor system calls.
-It prints the amount of times each system call was called since the driver has been installed.
+It prints the amount of times each [system call](examples/systrack.lua#L29)
+was called since the driver has been installed.
 
 #### Usage
 
 ```
 sudo make examples_install         # installs examples
 sudo lunatik run examples/systrack # runs systracker
 cat /dev/systrack
-timerfd_settime: 121
-mprotect: 44
-geteuid: 7
-fchmod: 1
-munmap: 43
-close: 812
-getgid: 7
-rt_sigaction: 221
-getuid: 15
-nanosleep: 59
-sendmsg: 5
-futex: 160
-socket: 6
-gettid: 139
-prctl: 1
-epoll_pwait: 229
-syslog: 1
-pread64: 17
-epoll_ctl: 2
-fcntl: 95
-brk: 16
-statx: 33
-unlinkat: 4
-waitid: 3
-sched_getaffinity: 10
-ioctl: 10314
-openat: 806
-clone: 8
-inotify_add_watch: 48
-prlimit64: 3
-getdents64: 20
-signalfd4: 1
-bind: 1
-write: 198
-writev: 51
-getpid: 45
-symlinkat: 1
-getppid: 1
-fadvise64: 3
-readlinkat: 38
-dup3: 25
-epoll_create1: 1
-getsockname: 1
-getxattr: 1
-wait4: 17
-rt_sigprocmask: 285
-setpgid: 14
-timerfd_create: 3
-recvmsg: 374
-rt_sigreturn: 9
-umask: 2
-rseq: 3
-getrandom: 15
-set_tid_address: 3
-execve: 3
-kill: 1
-setitimer: 71
-statfs: 3
-getsockopt: 6
-faccessat: 22
-ppoll: 444
-recvfrom: 15
-clock_nanosleep: 47
-setsockopt: 7
-sendto: 7
-pselect6: 76
-pipe2: 12
-ftruncate: 2
-fsync: 1
-renameat: 2
-getegid: 7
-exit_group: 10
-getrusage: 2
-newfstatat: 1141
-mmap: 67
-uname: 1
-utimensat: 2
-lseek: 21
-read: 1269
-set_robust_list: 11
+writev: 0
+close: 1927
+write: 1085
+openat: 2036
+read: 4131
+readv: 0
 ```
 
 ## References

examples/systrack.lua

@@ -21,18 +21,19 @@
 -- SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 --
 
-local linux   = require("linux")
-local probe   = require("probe")
-local device  = require("device")
-local syscall = require("syscall.table")
+local linux  = require("linux")
+local probe  = require("probe")
+local device = require("device")
+local systab = require("syscall.table")
 
-local track = {}
+local syscalls = {"openat", "read", "write", "readv", "writev", "close"}
 
 local function nop() end -- do nothing
 
 local s = linux.stat
 local driver = {name = "systrack", open = nop, release = nop, mode = s.IRUGO}
 
+local track = {}
 local toggle = true
 function driver:read()
 	local log = ""
@@ -45,9 +46,12 @@ function driver:read()
 	return log
 end
 
-for symbol, address in pairs(syscall) do
+for _, symbol in ipairs(syscalls) do
+	local address = systab[symbol]
+	track[symbol] = 0
+
 	local function handler()
-		track[symbol] = (track[symbol] or 0) + 1
+		track[symbol] = track[symbol] + 1
 	end
 
 	probe.new(address, {pre = handler, post = nop})