问题
I'm working on system call interception (for open() system call) and I got one problem: I have two kernel modules (mod1 and mod2) and both of them are trying to intercept open() syscall. I've loaded mod1 first and then mod2. The mod1 intercepted open() by:
original_open1 = sys_call_table[__NR_open];
sys_call_table[__NR_open] = mod1_open;
Here original_open1 would be sys_open.
After this, mod2 intercepted open() by:
original_open2 = sys_call_table[__NR_open];
sys_call_table[__NR_open] = mod2_open;
Here, original_open2 would be mod1_open() since mod1 was loaded first.
Now, the problem is: Suppose I unload mod1 first and open() system call gets executed, then mod2_open() would get called, which ultimately calls mod1_open().
Since mod1 is already unloaded, calling mod1_open() would cause panic (since the function pointer is no longer a valid memory region).
I need some mechanism to avoid this problem. Basically, I want a solution which facilitates loading/unloading the modules (which intercept same syscall) in any random order without causing any panic.
来源：https://stackoverflow.com/questions/44983399/multiple-kernel-modules-intercepting-same-system-call-and-crash-during-unload