Storing SYSCALL functions in array of function pointers

Question

I'm working on a project in which I have to hook 80%-90% of the system call functions on OSX (10.10.5). I'm doing this from a kernel extension. Since I have to (un)hook many functions, I want to store the original kernel function into an array of function pointers, to that I can do a quick lookup into the array in order to restore the original function upon unhooking.

    int (*kern_open)(struct proc *, struct open_args *, int *);
    int mon_open(struct proc *p, struct open_args *uap, int *retval) {
    kern_open = sysent[SYS_open].sy_call;
    sysent[SYS_open].sy_call = mon_open;

This works, the kern_open function is used to store the original kernel function that s called upon a system call. mon_open is my hooking function.

What I want to achieve is the following; so that upon unhooking, I can just iterate thru the KernSysCall array and restore the functions.

    // global array of function pointers that all have the same func def.
    static int (*KernSysCall[SYS_MAXSYSCALL])(struct proc *, struct args *, int *);
    KernSysCall[SYS_open] = sysent[SYS_open].sy_call;
    sysent[SYS_open].sy_call = mon_open;

Restoring: sysent[SYS_open].sy_call = KernSysCall[SYS_open];

However, storing the original kernel function inside the array of function pointers is causing a kernel panic. I haven't been able to attach the lldb yet, due to a error: KDP_REATTACH failed error. I'm hoping someone knows what is causing the kernel panic.

Below is a log of the kernel panic.

Anonymous UUID:       052D64D2-A43C-99F8-D221-B591991E54AF

Wed Nov 11 12:55:06 2015

*** Panic Report ***
panic(cpu 0 caller 0xffffff80093f0024): Kernel trap at 0x0000000000000000, type 14=page fault, registers:
CR0: 0x0000000080010033, CR2: 0x0000000000000000, CR3: 0x00000000769bb018, CR4: 0x00000000001606e0
RAX: 0x0000000000000000, RBX: 0xffffff80115e3fc0, RCX: 0x0000000000000001, RDX: 0xffffff80115e3fc0
RSP: 0xffffff8068dabaf8, RBP: 0xffffff8068dabf50, RSI: 0xffffff80115e3f80, RDI: 0xffffff8010059cf0
R8:  0xffffff7f8afaccdf, R9:  0xffffff8009ae2a18, R10: 0xffffff8009939740, R11: 0x0000000000000000
R12: 0xffffff8010059cf0, R13: 0x0000000000000005, R14: 0xffffff80115e3f80, R15: 0xffffff801188b480
RFL: 0x0000000000010282, RIP: 0x0000000000000000, CS:  0x0000000000000008, SS:  0x0000000000000010
Fault CR2: 0x0000000000000000, Error code: 0x0000000000000010, Fault CPU: 0x0 VMM

Backtrace (CPU 0), Frame : Return Address
0xffffff8068dab790 : 0xffffff80092e4ed1 mach_kernel : _panic + 0xd1
0xffffff8068dab810 : 0xffffff80093f0024 mach_kernel : _kernel_trap + 0x664
0xffffff8068dab9e0 : 0xffffff800940de53 mach_kernel : trap_from_kernel + 0x26
0xffffff8068daba00 : 0x0 
0xffffff8068dabf50 : 0xffffff800982c0c1 mach_kernel : _unix_syscall64 + 0x2f1
0xffffff8068dabfb0 : 0xffffff800940e656 mach_kernel : _hndl_unix_scall64 + 0x16

BSD process name corresponding to current thread: xpcproxy
Boot args: debug=0x14e kext-dev-mode=1 -v keepsyms=1 kmem=1

Mac OS version:
14F27

Kernel version:
Darwin Kernel Version 14.5.0: Wed Jul 29 02:26:53 PDT 2015; root:xnu-2782.40.9~1/DEVELOPMENT_X86_64
Kernel UUID: C75BDFDD-9F27-3694-BB80-73CF991C13D8
Kernel slide:     0x0000000009000000
Kernel text base: 0xffffff8009200000
__HIB  text base: 0xffffff8009100000
System model name: VMware7,1 (Mac-66F35F19FE2A0D05)

System uptime in nanoseconds: 251264993940
last loaded kext at 249789197520: my.kext   1 (addr 0xffffff7f8afa9000, size 57344)
last unloaded kext at 116769666233: com.apple.driver.AppleFileSystemDriver  3.0.1 (addr 0xffffff7f8aed3000, size 16384)
loaded kexts:
my.kext 1

[more kexts here]

In request, the code for mon_open():

int
mon_open(struct proc *p, struct open_args *uap, int *r) {
    int error;
    char processname[MAXCOMLEN+1];
    char intercepted_path[MAXPATHLEN];

    pid_t pid = proc_pid(p);

    proc_name(pid, processname, sizeof(processname));

    size_t dummy = 0;
    error = copyinstr((void *)uap->path, (void *)intercepted_path, MAXPATHLEN, &dummy);
    if (!error) {
        printf("[MYKEXT] open called with path: %s, PID: %d, processname: %s\n", intercepted_path, pid, processname);
    }

    return kern_open(p, uap, r);
}

Thank you very much in advance!

Answer 1

Amazingly stupid, I forgot to point kern_open (the return value of mon_open) to the function pointer in the array. kern_open was NULL, so this caused the NULL-pointer exception. Now the array of function pointers is working correctly.

@pmdj, thank you very much for your help.