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Created: Monday, May 14 2007 12:28.20 CDT

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Kerberos API Tracing

Author: c1de0x

# Views: 2892

Hi all,

I'm looking into API tracing, and heard about kerberos (Rustem Fasihov) in this thread: https://www.openrce.org/forums/posts/274.

As I've been grappling with my own hooking engine for a couple of days, I couldn't resist looking under the hood.

I discovered that at it's core, kerberos seems to use 5-byte hotpatch hooks, which isn't surprising, as it's the mechanism I was using and also - to some degree - that favored by Detours.

What is interesting is that there don't seem to be any 'trampoline' functions. In other words, in both my solution and Detours, it is necessary to generate a 'jump function' (which acts as the target of the 5-byte JMP patch) for each API being hooked. The 'jump'/'trampoline' function typically has the address of the instruction immediately following the patch hard-coded into a JMP so that execution can continue.

It seems that kerberos takes a different approach though, all patches, for all hooked functions, jump to the same method, which begins:



    pushf

spin:

    cmp    g_inHookSemaphore, 1

    jz     spin

    mov    g_inHookSemaphore, 1

    popf

    pop    data1

    pop    data2

    pushf

    pusha

    ...

Blog Comments

camus	Posted: Tuesday, May 15 2007 13:16.42 CDT
Are you sure about that? It's hard to believe :S Here you have an active wait: `spin: cmp g_inHookSemaphore, 1 jz spin` wich is a processor consuming operation. And this is a race condition: `jz spin mov g_inHookSemaphore, 1` Aren't they?

jms	Posted: Wednesday, May 16 2007 12:08.44 CDT
Well it could be that this function trying to determine whether it can use that semaphore, and if not it spinlocks. The next instruction appears to then set the semaphore as locked and continue the instructions, then the next thread comes in it will spin again. Then again, what do I know?

anonymouse Posted: Wednesday, May 16 2007 13:10.31 CDT

g_whatever is a name that was given by you ?

supposedly looking if some other thread is executing the hook it seems
coz when the function finishes it sets the global dword to 0 before exiting



References in ke_core:.text to 10004000..10004003

Address    Disassembly                               Comment

10002547   CMP DWORD PTR DS:[10004000],1             DS:[10004000]=00000000

10002550   MOV DWORD PTR DS:[10004000],1             DS:[10004000]=00000000

10002696   MOV DWORD PTR DS:[10004000],0             DS:[10004000]=00000000

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