I am trying to write a BPF program that examines the session ID of any process that calls the tty_write kernel function. I am trying to do this by retrieving a field from the current task_struct struct. My code is as follows:
SEC("kprobe/tty_write")
int kprobe__tty_write(struct pt_regs *ctx)
{
struct task_struct *task;
struct task_struct *group_leader;
struct pid_link pid_link;
struct pid pid;
int sessionid;
// get current sessionid
task = (struct task_struct *)bpf_get_current_task();
bpf_probe_read(&group_leader, sizeof(group_leader), &task->group_leader);
bpf_probe_read(&pid_link, sizeof(pid_link), group_leader->pids + PIDTYPE_SID);
bpf_probe_read(&pid, sizeof(pid), pid_link.pid);
sessionid = pid.numbers[0].nr;
// do stuff with sessionid
return 0;
}
Note that I am compiling my BPF program using clang into an ELF file and loading it with gobpf's ELF package. Unfortunately, the value of sessionid is always 0. Why is this? I don't think I'm accessing the session ID incorrectly as I have done this before using bcc on a 4.11 kernel (due to the how bcc rewrite's BPF programs, I cannot simply use the same code when I want to compile the program myself). The equivalent working bcc code for accessing the sessionid is as follows. Note that this only works on a 4.11 kernel, the following code did not work on a 4.13 kernel. The code above works on neither kernel however.
#!/usr/bin/python
from bcc import BPF
import ctypes as ct
import os
import threading
import time
import sys
prog=r"""
#include <uapi/linux/ptrace.h>
#include <linux/sched.h>
#include <linux/fs.h>
#include <linux/nsproxy.h>
#include <linux/ns_common.h>
#define BUFSIZE 256
struct tty_write_t {
int count;
char buf[BUFSIZE];
unsigned int sessionid;
};
// define maps
BPF_PERF_OUTPUT(tty_writes);
int kprobe__tty_write(struct pt_regs *ctx, struct file *file,
const char __user *buf, size_t count)
{
struct task_struct *task;
struct pid_link pid_link;
struct pid pid;
int sessionid;
// get current sessionid
task = (struct task_struct *)bpf_get_current_task();
bpf_probe_read(&pid_link, sizeof(pid_link), (void *)&task->group_leader->pids[PIDTYPE_SID]);
bpf_probe_read(&pid, sizeof(pid), (void *)pid_link.pid);
sessionid = pid.numbers[0].nr;
// bpf_probe_read() can only use a fixed size, so truncate to count
// in user space:
struct tty_write_t tty_write = {};
bpf_probe_read(&tty_write.buf, BUFSIZE, (void *)buf);
if (count > BUFSIZE) {
tty_write.count = BUFSIZE;
} else {
tty_write.count = count;
}
// add sessionid to tty_write structure and submit
tty_write.sessionid = sessionid;
tty_writes.perf_submit(ctx, &tty_write, sizeof(tty_write));
return 0;
}
"""
b = BPF(text=prog)
BUFSIZE = 256
class TTYWrite(ct.Structure):
_fields_ = [
("count", ct.c_int),
("buf", ct.c_char * BUFSIZE),
("sessionid", ct.c_int)
]
# process tty_write
def print_tty_write(cpu, data, size):
tty_write = ct.cast(data, ct.POINTER(TTYWrite)).contents
print(str(tty_write.sessionid))
b["tty_writes"].open_perf_buffer(print_tty_write)
while 1:
b.kprobe_poll()
4.11 kernel:
uname -a:Linux ubuntu16 4.11.0-14-generic #20~16.04.1-Ubuntu SMP Wed Aug 9 09:06:22 UTC 2017 x86_64 x86_64 x86_64 GNU/Linux
4.13 kernel:
uname -a: Linux ubuntu1710 4.13.0-32-generic #35-Ubuntu SMP Thu Jan 25 09:13:46 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
