fix: make BPF probe to compile on CentOS 8 and RHEL 8 again

The distro backported the process type functionalities and structs
from 4.19 to 4.18, that made our checks against 4.19 ineffective.

This patch adds some RHEL specific checks to short circuit RHEL
backports before the 4.19 checks are done.

Co-Authored-By: Leonardo Di Donato <leodidonato@gmail.com>
Signed-off-by: Lorenzo Fontana <fontanalorenz@gmail.com>

driver/bpf/fillers.h

@@ -20,10 +20,12 @@ or GPL2.txt for full copies of the license.
 //#define COS_73_WORKAROUND
 
 #include "../ppm_flag_helpers.h"
+#include "../ppm_version.h"
 
 #include <linux/tty.h>
 #include <linux/audit.h>
 
+
 /*
  * Linux 5.6 kernels no longer include the old 32-bit timeval
  * structures. But the syscalls (might) still use them.
@@ -1389,7 +1391,9 @@ static __always_inline int bpf_ppm_get_tty(struct task_struct *task)
 
 static __always_inline struct pid *bpf_task_pid(struct task_struct *task)
 {
-#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 19, 0)
+#if (PPM_RHEL_RELEASE_CODE > 0 && PPM_RHEL_RELEASE_CODE >= PPM_RHEL_RELEASE_VERSION(8, 0))
+	return _READ(task->thread_pid);
+#elif LINUX_VERSION_CODE < KERNEL_VERSION(4, 19, 0)
 	return _READ(task->pids[PIDTYPE_PID].pid);
 #else
 	return _READ(task->thread_pid);
@@ -1426,7 +1430,7 @@ static __always_inline pid_t bpf_pid_nr_ns(struct pid *pid,
 	return nr;
 }
 
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 19, 0)
+#if ((PPM_RHEL_RELEASE_CODE > 0 && PPM_RHEL_RELEASE_CODE >= PPM_RHEL_RELEASE_VERSION(8, 0))) || LINUX_VERSION_CODE >= KERNEL_VERSION(4, 19, 0)
 static __always_inline struct pid **bpf_task_pid_ptr(struct task_struct *task,
 						     enum pid_type type)
 {
@@ -1445,7 +1449,9 @@ static __always_inline pid_t bpf_task_pid_nr_ns(struct task_struct *task,
 	if (!ns)
 		ns = bpf_task_active_pid_ns(task);
 
-#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 19, 0)
+#if (PPM_RHEL_RELEASE_CODE > 0 && PPM_RHEL_RELEASE_CODE >= PPM_RHEL_RELEASE_VERSION(8, 0))
+	nr = bpf_pid_nr_ns(_READ(*bpf_task_pid_ptr(task, type)), ns);
+#elif LINUX_VERSION_CODE < KERNEL_VERSION(4, 19, 0)
 	if (type != PIDTYPE_PID) {
 		if (type == __PIDTYPE_TGID)
 			type = PIDTYPE_PID;
@@ -1468,7 +1474,9 @@ static __always_inline pid_t bpf_task_pid_vnr(struct task_struct *task)
 
 static __always_inline pid_t bpf_task_tgid_vnr(struct task_struct *task)
 {
-#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 19, 0)
+#if (PPM_RHEL_RELEASE_CODE > 0 && PPM_RHEL_RELEASE_CODE >= PPM_RHEL_RELEASE_VERSION(8, 0))
+	return bpf_task_pid_nr_ns(task, PIDTYPE_TGID, NULL);
+#elif LINUX_VERSION_CODE < KERNEL_VERSION(4, 19, 0)
 	return bpf_task_pid_nr_ns(task, __PIDTYPE_TGID, NULL);
 #else
 	return bpf_task_pid_nr_ns(task, PIDTYPE_TGID, NULL);