Tracing Objective-C method calls

Linux has this great tool called strace, on OSX there’s a tool called dtruss — based on dtrace. Dtruss is great in functionality, it gives pretty much everything you need. It is just not as nice to use as strace. However, on Linux there is also ltrace for library tracing. That is arguably more useful because you can see much more granular application activity. Unfortunately, there isn’t such a tool on OSX. So, I decided to make one — albeit a simpler version for now. I called it objc_trace.

Objc_trace’s functionality is quite limited at the moment. It will print out the name of the method, the class and a list of parameters to the method. In the future it will be expanded to do more things, however just knowing which method was called is enough for many debugging purposes.

Something about the language

Without going into too much detail let’s look into the relevant parts of the Objective-Cruntime. This subject has been covered pretty well by the hacker community. In Phrack there is a great article covering various internals of the language. However, I will scratch the surface to review some aspects that are useful for this context.

The language is incredibly dynamic. While still backwards compatible to C (or C++), most of the code is written using classes and methods a.k.a. structures and function pointers. A class is exactly what you’re thinking of. It can have static or instance methods or fields. For example, you might have a class 


 with a method 


 that returns the contents. You might call it this way:

    Book* book = [[Book alloc] init];
    Page* pages = [book Pages];



 function is a static method while the others (




) are dynamic. What actually happens is that the system sends messages to the object or the static class. The message contains the class name, the instance, the method name and any parameters. The runtime will resolve which compiled function actually implements this method and call that.

If anything above doesn’t make sense you might want to read the referenced Phrack article for more details.

Message passing is great, though there are all kinds of efficiency considerations in play. For example, methods that you call will eventually get cached so that the resolution process occurs much faster. What’s important to note is that there is some smoke and mirrors going on.

The system is actually not sending messages under the hood. What it is doing is routing the execution using a single library call: 


 [1]. This is due to how the concept of a message is implemented under the hood.

    id objc_msgSend(id self, SEL op, ...)

Let’s take ourselves out of the Objective-C abstractions for a while and think about how things are implemented in C. When a method is called the stack and the registers are configured for the 




 type is kind of like a 

void *

 but restricted to Objective-C class instances. 


 type is actually 


 type and refers to selectors, more specifically the methods names (which include parameters). For example, a method that takes two parameters will have a selector that might look something like this: 


. Colons signal that there should be a parameter there. Really quite confusing but we won’t dwell on that.

The useful part is that a selector is a C-String that contains the method name and its named parameters. A non-obfuscating compiler does not remove them because the names are needed to resolve the implementing function.

Shockingly, the function that implements the method takes in two extra parameters ahead of the user defined parameters. Those are the 


 and the 


. If you look at the disassembly, it looks something like this (taken from Damn Vulnerable iOS App):

__text:100005144 ; YapDatabaseViewState - (id)createGroup:(id) withCapacity:(uint64_t)
__text:100005144 ; Attributes: bp-based frame
__text:100005144 ; id __cdecl -[YapDatabaseViewState createGroup:withCapacity:]
                        ;         (struct YapDatabaseViewState *self, SEL, id, uint64_t)
__text:100005144 __YapDatabaseViewState_createGroup_withCapacity__

Notice that the C function is called 


, the method is called 


 and the class is 


. It takes two parameters: an 


and a 


. However, it also takes a 

struct YapDatabaseViewState *self

 and a 


. This signature essentially matches the signature of 


, except that the latter has variadic parameters.

The existence and the location of the extra parameters is not accidental. The reason for this is that 


 will actually redirect execution to the implementing function by looking up the selector to function mapping within the class object. Once it finds the target it simply jumps there without having to readjust the parameter registers. This is why I referred to this as a routing mechanism, rather than message passing. Of course, I say that due to the implementation details, rather than the conceptual basis for what is happening here.

Quite smart actually, because this allows the language to be very dynamic in nature i.e. I can remap SEL to Function mapping and change the implementation of any particular method. This is also great for reverse engineering because this system retains a lot of the labeling information that the developer puts into the source code. I quite like that.

The plan

Now that we’ve seen how Objective-C makes method calls, we notice that 


 becomes a choke point for all method calls. It is like a hub in a poorly setup network with many many users. So, in order to get a list of every method called all we have to do is watch this function. One way to do this is via a debugger such as LLDB or GDB. However, the trouble is that a debugger is fairly heavy and mostly interactive. It’s not really good when you want to capture a run or watch the process to pin point a bug. Also, the performance hit might be too much. For more offensive work, you can’t embed one of those debuggers into a lite weight implant.

So, what we are going to do is hook the 


 function on an ARM64 iOS Objective-C program. This will allow us to specify a function to get called before 


 is actually executed. We will do this on a Jailbroken iPhone — so no security mechanism bypasses here, the Jailbreak takes care of all of that.

Figure 1: Patching at high level

On the high level the hooking works something like this. 


 instructions are modified in the preamble to jump to another function. This other function will perform our custom tracing features, restore the CPU state and return to a jump table. The jump table is a dynamically generated piece of code that will execute the preamble instructions that we’ve overwritten and jump back to 


 to continue with normal execution.


The implementation of the technique presented can be found in the objc_tracerepository.

The first thing we are going to do is allocate what I call a jump page. It is called so because this memory will be a page of code that jumps back to continue executing the original function.

<span class="n">s_jump_page</span><span class="o">*</span> <span class="n">t_func</span> <span class="o">=</span>
   <span class="p">(</span><span class="n">s_jump_page</span><span class="o">*</span><span class="p">)</span><span class="n">mmap</span><span class="p">(</span><span class="nb">NULL</span><span class="p">,</span> <span class="mi">4096</span><span class="p">,</span>
            <span class="n">PROT_READ</span> <span class="o">|</span> <span class="n">PROT_WRITE</span><span class="p">,</span>
            <span class="n">MAP_ANON</span>  <span class="o">|</span> <span class="n">MAP_PRIVATE</span><span class="p">,</span> <span class="o">-</span><span class="mi">1</span><span class="p">,</span> <span class="mi">0</span><span class="p">);</span>

Notice that the type of the jump page is 


 which is a structure that will represent our soon to be generated code.

<span class="k">typedef</span> <span class="k">struct</span> <span class="p">{</span>
    <span class="n">instruction_t</span>     <span class="n">inst</span><span class="p">[</span><span class="mi">4</span><span class="p">];</span>    
    <span class="n">s_jump_patch</span> <span class="n">jump_patch</span><span class="p">[</span><span class="mi">5</span><span class="p">];</span>
    <span class="n">instruction_t</span>     <span class="n">backup</span><span class="p">[</span><span class="mi">4</span><span class="p">];</span>    
<span class="p">}</span> <span class="n">s_jump_page</span><span class="p">;</span>



 structure contains four instructions that we overwrite (think back to the diagram at step 2). We also keep a backup of these instruction at the end of the structure — not strictly necessary but it makes for easier unhooking. Then there are five structures called jump patches. These are special sets of instructions that will redirect the CPU to an arbitrary location in memory. Jump patches are also represented by a structure.

<span class="k">typedef</span> <span class="k">struct</span> <span class="p">{</span>
    <span class="n">instruction_t</span> <span class="n">i1_ldr</span><span class="p">;</span>
    <span class="n">instruction_t</span> <span class="n">i2_br</span><span class="p">;</span>
    <span class="n">address_t</span> <span class="n">jmp_addr</span><span class="p">;</span>
<span class="p">}</span> <span class="n">s_jump_patch</span><span class="p">;</span>

Using these structures we can build a very elegant and transparent mechanism for building dynamic code. All we have to do is create an inline assembly function in C and cast it to the structure.

<span class="n">__attribute__</span><span class="p">((</span><span class="kr">naked</span><span class="p">))</span>
<span class="kt">void</span> <span class="n">d_jump_patch</span><span class="p">()</span> <span class="p">{</span>
    <span class="n">__asm__</span> <span class="n">__volatile__</span><span class="p">(</span>
        <span class="c1">// trampoline to somewhere else.
</span>        <span class="s">"ldr x16, #8;</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"br x16;</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">".long 0;</span><span class="se">\n</span><span class="s">"</span> <span class="c1">// place for jump address
</span>        <span class="s">".long 0;</span><span class="se">\n</span><span class="s">"</span>
    <span class="p">);</span>
<span class="p">}</span>

This is ARM64 Assembly to load a 64-bit value from address 


 then jump to it. The 


 placeholders are places for the target address.

<span class="n">s_jump_patch</span><span class="o">*</span> <span class="n">jump_patch</span><span class="p">(){</span>
    <span class="k">return</span> <span class="p">(</span><span class="n">s_jump_patch</span><span class="o">*</span><span class="p">)</span><span class="n">d_jump_patch</span><span class="p">;</span>
<span class="p">}</span>

In order to use this we simply cast the code i.e. the 


 function pointer to the structure and set the value of the 


 field. This is how we implement the function that generates the custom trampoline.

<span class="kt">void</span> <span class="nf">write_jmp_patch</span><span class="p">(</span><span class="kt">void</span><span class="o">*</span> <span class="n">buffer</span><span class="p">,</span> <span class="kt">void</span><span class="o">*</span> <span class="n">dst</span><span class="p">)</span> <span class="p">{</span>
    <span class="c1">// returns the pointer to d_jump_patch.
</span>    <span class="n">s_jump_patch</span> <span class="n">patch</span> <span class="o">=</span> <span class="o">*</span><span class="p">(</span><span class="n">jump_patch</span><span class="p">());</span>

    <span class="n">patch</span><span class="p">.</span><span class="n">jmp_addr</span> <span class="o">=</span> <span class="p">(</span><span class="n">address_t</span><span class="p">)</span><span class="n">dst</span><span class="p">;</span>

    <span class="o">*</span><span class="p">(</span><span class="n">s_jump_patch</span><span class="o">*</span><span class="p">)</span><span class="n">buffer</span> <span class="o">=</span> <span class="n">patch</span><span class="p">;</span>
<span class="p">}</span>

We take advantage of the C compiler automatically copying the entire size of the structure instead of using 


. In order to patch the original 


 function we use 


 function and point it to the 


 function. Of course, before we can do that we copy the original instructions to the jump page for later execution and back up.

    <span class="c1">//   Building the Trampoline
</span>    <span class="o">*</span><span class="n">t_func</span> <span class="o">=</span> <span class="o">*</span><span class="p">(</span><span class="n">jump_page</span><span class="p">());</span>
    <span class="c1">// save first 4 32bit instructions
</span>    <span class="c1">//   original -&gt; trampoline
</span>    <span class="n">instruction_t</span><span class="o">*</span> <span class="n">orig_preamble</span> <span class="o">=</span> <span class="p">(</span><span class="n">instruction_t</span><span class="o">*</span><span class="p">)</span><span class="n">o_func</span><span class="p">;</span>
    <span class="k">for</span><span class="p">(</span><span class="kt">int</span> <span class="n">i</span> <span class="o">=</span> <span class="mi">0</span><span class="p">;</span> <span class="n">i</span> <span class="o">&lt;</span> <span class="mi">4</span><span class="p">;</span> <span class="n">i</span><span class="o">++</span><span class="p">)</span> <span class="p">{</span>
        <span class="n">t_func</span><span class="o">-&gt;</span><span class="n">inst</span>  <span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="n">orig_preamble</span><span class="p">[</span><span class="n">i</span><span class="p">];</span>
        <span class="n">t_func</span><span class="o">-&gt;</span><span class="n">backup</span><span class="p">[</span><span class="n">i</span><span class="p">]</span> <span class="o">=</span> <span class="n">orig_preamble</span><span class="p">[</span><span class="n">i</span><span class="p">];</span>
    <span class="p">}</span>

Now that we have saved the original instructions from 


 we have to be aware that we’ve copied four instructions. A lot can happen in four instructions, all sorts of decisions and branches. In particular I’m worried about branches because they can be relative. So, what we need to do is validate that 


 doesn’t have any branches. If it does, they will need to modified to preserve functionality.

This is why 


 has five jump patches:

  1. All four instructions are non branches, so the first jump patch will automatically redirect execution to 

     (skipping the patch).

  2. There are up to four branch instructions, so each of the jump patches will be used to redirect to the appropriate offset into 


Checking for branch instructions is a bit tricky. ARM64 is a RISC architecture and does not present the same variety of instructions as, say, x86-64. But, there are still quite a few [2].

  1. Conditional Branches:
    • B.cond label jumps to PC relative offset.
    • CBNZ Wn|Xn, label jumps to PC relative offset if Wn is not equal to zero.
    • CBZ Wn|Xn, label jumps to PC relative offset if Wn is equal to zero.
    • TBNZ Xn|Wn, #uimm6, label jumps to PC relative offset if bit number uimm6 in register Xn is not zero.
    • TBZ Xn|Wn, #uimm6, label jumps to PC relative offset if bit number uimm6 in register Xn is zero.
  2. Unconditional Branches:
    • B label jumps to PC relative offset.
    • BL label jumps to PC relative offset, writing the address of the next sequential instruction to register X30. Typically used for making function calls.
  3. Unconditional Branches to register:
    • BLR Xm unconditionally jumps to address in Xm, writing the address of the next sequential instruction to register X30.
    • BR Xm jumps to address in Xm.
    • RET {Xm} jumps to register Xm.

We don’t particular care about category three because, register states should not influenced by our hooking mechanism. However, category one and two are PC relative and therefore need to be updated if found in the preamble.

So, I wrote a function that updates the instructions. At the moment it only handles a subset of cases, specifically the 




 instructions. The former is found in 



__text:18DBB41C0  EXPORT _objc_msgSend
__text:18DBB41C0   _objc_msgSend
__text:18DBB41C0     CMP             X0, #0
__text:18DBB41C4     B.LE            loc_18DBB4230
__text:18DBB41C8   loc_18DBB41C8
__text:18DBB41C8     LDR             X13, [X0]
__text:18DBB41CC     AND             X9, X13, #0x1FFFFFFF8

Now, I don’t know about you but I don’t particularly like to use complicated bit-wise operations to extract and modify data. It’s kind of fun to do so, but it is also fragile and hard to read. Luckily for us, C was designed to work at such a low level. Each ARM64 instruction is four bytes and so we use bit fields in C structures to deal with them!

<span class="k">typedef</span> <span class="k">struct</span> <span class="p">{</span>
    <span class="kt">uint32_t</span> <span class="n">offset</span>   <span class="o">:</span> <span class="mi">26</span><span class="p">;</span>
    <span class="kt">uint32_t</span> <span class="n">inst_num</span> <span class="o">:</span> <span class="mi">6</span><span class="p">;</span>
<span class="p">}</span> <span class="n">inst_b</span><span class="p">;</span>

This is the unconditional PC relative jump.

<span class="k">typedef</span> <span class="k">struct</span> <span class="p">{</span>
    <span class="kt">uint32_t</span> <span class="n">condition</span><span class="o">:</span> <span class="mi">4</span><span class="p">;</span>
    <span class="kt">uint32_t</span> <span class="n">reserved</span> <span class="o">:</span> <span class="mi">1</span><span class="p">;</span>
    <span class="kt">uint32_t</span> <span class="n">offset</span>   <span class="o">:</span> <span class="mi">19</span><span class="p">;</span>
    <span class="kt">uint32_t</span> <span class="n">inst_num</span> <span class="o">:</span> <span class="mi">8</span><span class="p">;</span>
<span class="p">}</span> <span class="n">inst_b_cond</span><span class="p">;</span>

And this one is the conditional PC relative jump. Back in the day, I wrote a plugin for IDAPro that gives the details of instruction under the cursor. It is called IdaRef and, for it, I produced an ASCII text file that has all the instruction and their bit fields clearly written out [3]. So the 


 looks like this in memory. Notice right to left bit numbering.

31 30 29 28 27 26 25 24 23                                                              5 4 3            0
0  1  0  1  0  1  0  0                                      imm19                         0     cond

That is what we map our 


 structure to. Doing so allows us very easy abstraction over bit manipulation.

<span class="kt">void</span> <span class="nf">check_branches</span><span class="p">(</span><span class="n">s_jump_page</span><span class="o">*</span> <span class="n">t_func</span><span class="p">,</span> <span class="n">instruction_t</span><span class="o">*</span> <span class="n">o_func</span><span class="p">)</span> <span class="p">{</span>
    <span class="p">...</span>        
        <span class="n">instruction_t</span> <span class="n">inst</span> <span class="o">=</span> <span class="n">t_func</span><span class="o">-&gt;</span><span class="n">inst</span><span class="p">[</span><span class="n">i</span><span class="p">];</span>
        <span class="n">inst_b</span><span class="o">*</span>       <span class="n">i_b</span>      <span class="o">=</span> <span class="p">(</span><span class="n">inst_b</span><span class="o">*</span><span class="p">)</span><span class="o">&amp;</span><span class="n">inst</span><span class="p">;</span>
        <span class="n">inst_b_cond</span><span class="o">*</span>  <span class="n">i_b_cond</span> <span class="o">=</span> <span class="p">(</span><span class="n">inst_b_cond</span><span class="o">*</span><span class="p">)</span><span class="o">&amp;</span><span class="n">inst</span><span class="p">;</span>

        <span class="p">...</span>
        <span class="p">}</span> <span class="k">else</span> <span class="nf">if</span><span class="p">(</span><span class="n">i_b_cond</span><span class="o">-&gt;</span><span class="n">inst_num</span> <span class="o">==</span> <span class="mh">0x54</span><span class="p">)</span> <span class="p">{</span>
            <span class="c1">// conditional branch
            <span class="c1">// save the original branch offset
</span>            <span class="n">branch_offset</span> <span class="o">=</span> <span class="n">i_b_cond</span><span class="o">-&gt;</span><span class="n">offset</span><span class="p">;</span>
            <span class="n">i_b_cond</span><span class="o">-&gt;</span><span class="n">offset</span> <span class="o">=</span> <span class="n">patch_offset</span><span class="p">;</span>
        <span class="p">}</span>

        <span class="p">...</span>
            <span class="c1">// set jump point into the original function,
</span>            <span class="c1">//   don't forget that it is PC relative
</span>            <span class="n">t_func</span><span class="o">-&gt;</span><span class="n">jump_patch</span><span class="p">[</span><span class="n">use_jump_patch</span><span class="p">].</span><span class="n">jmp_addr</span> <span class="o">=</span>
                 <span class="p">(</span><span class="n">address_t</span><span class="p">)(</span>
                    <span class="p">((</span><span class="n">instruction_t</span><span class="o">*</span><span class="p">)</span><span class="n">o_func</span><span class="p">)</span>
                    <span class="o">+</span> <span class="n">branch_offset</span> <span class="o">+</span> <span class="n">i</span><span class="p">);</span>
        <span class="p">...</span>

With some important details removed, I’d like to highlight how we are checking the type of the instruction by overlaying the structure over the instruction integer and checking to see if the value of the instruction number is correct. If it is, then we use that pointer to read the offset and modify it to point to one of the jump patches. In the patch we place the absolute value of the address where the instruction would’ve jumped were it still back in the original 


 function. We do so for every branch instruction we might encounter.

Once the jump page is constructed we insert the patch into 


 and complete the loop. The most important thing is, of course, that the hook function restores all the registers to the state just before CPU enters into 


 otherwise the whole thing will probably crash.

It is important to note that at the moment we require that the function to be hooked has to be at least four instructions long because that is the size of the patch. Other than that we don’t even care if the target is a proper C function.

Do look through the implementation [4], I skip over some details that glues things together but the important bits that I mention should be enough to understand, in great detail, what is happening under the hood.

Interpreting the call

Now that function hooking is done, it is time to level up and interpret the results. This is where we actually implement the 


 functionality. So, the patch to 


 actually redirects execution to one of our functions:

<span class="n">__attribute__</span><span class="p">((</span><span class="kr">naked</span><span class="p">))</span>
<span class="n">id</span> <span class="n">objc_msgSend_trace</span><span class="p">(</span><span class="n">id</span> <span class="n">self</span><span class="p">,</span> <span class="n">SEL</span> <span class="n">op</span><span class="p">)</span> <span class="p">{</span>
    <span class="n">__asm__</span> <span class="n">__volatile__</span> <span class="p">(</span>
        <span class="s">"stp fp, lr, [sp, #-16]!;</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"mov fp, sp;</span><span class="se">\n</span><span class="s">"</span>

        <span class="s">"sub    sp, sp, #(10*8 + 8*16);</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"stp    q0, q1, [sp, #(0*16)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"stp    q2, q3, [sp, #(2*16)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"stp    q4, q5, [sp, #(4*16)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"stp    q6, q7, [sp, #(6*16)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"stp    x0, x1, [sp, #(8*16+0*8)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"stp    x2, x3, [sp, #(8*16+2*8)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"stp    x4, x5, [sp, #(8*16+4*8)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"stp    x6, x7, [sp, #(8*16+6*8)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"str    x8,     [sp, #(8*16+8*8)];</span><span class="se">\n</span><span class="s">"</span>

        <span class="s">"BL _hook_callback64_pre;</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"mov x9, x0;</span><span class="se">\n</span><span class="s">"</span>

        <span class="c1">// Restore all the parameter registers to the initial state.
</span>        <span class="s">"ldp    q0, q1, [sp, #(0*16)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"ldp    q2, q3, [sp, #(2*16)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"ldp    q4, q5, [sp, #(4*16)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"ldp    q6, q7, [sp, #(6*16)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"ldp    x0, x1, [sp, #(8*16+0*8)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"ldp    x2, x3, [sp, #(8*16+2*8)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"ldp    x4, x5, [sp, #(8*16+4*8)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"ldp    x6, x7, [sp, #(8*16+6*8)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"ldr    x8,     [sp, #(8*16+8*8)];</span><span class="se">\n</span><span class="s">"</span>
        <span class="c1">// Restore the stack pointer, frame pointer and link register
</span>        <span class="s">"mov    sp, fp;</span><span class="se">\n</span><span class="s">"</span>
        <span class="s">"ldp    fp, lr, [sp], #16;</span><span class="se">\n</span><span class="s">"</span>

        <span class="s">"BR x9;</span><span class="se">\n</span><span class="s">"</span>       <span class="c1">// call the jump page
</span>    <span class="p">);</span>
<span class="p">}</span>

This function stores all calling convention relevant registers on the stack and calls our, 


, regular C function that can assume that it is the 


 as it was called. In this function we can read parameters as if they were sent to the method call, this includes the class instance and the selector. Once 


 returns our 


 function will restore the registers and branch to the configured jump page which will eventually branch back to the original call.

<span class="kt">void</span><span class="o">*</span> <span class="nf">hook_callback64_pre</span><span class="p">(</span><span class="n">id</span> <span class="n">self</span><span class="p">,</span> <span class="n">SEL</span> <span class="n">op</span><span class="p">,</span> <span class="kt">void</span><span class="o">*</span> <span class="n">a1</span><span class="p">,</span> <span class="kt">void</span><span class="o">*</span> <span class="n">a2</span><span class="p">,</span> <span class="kt">void</span><span class="o">*</span> <span class="n">a3</span><span class="p">,</span> <span class="kt">void</span><span class="o">*</span> <span class="n">a4</span><span class="p">,</span> <span class="kt">void</span><span class="o">*</span> <span class="n">a5</span><span class="p">)</span> <span class="p">{</span>
    <span class="c1">// get the important bits: class, function
</span>    <span class="kt">char</span><span class="o">*</span> <span class="n">classname</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span><span class="o">*</span><span class="p">)</span> <span class="n">object_getClassName</span><span class="p">(</span> <span class="n">self</span> <span class="p">);</span>
    <span class="k">if</span><span class="p">(</span><span class="n">classname</span> <span class="o">==</span> <span class="nb">NULL</span><span class="p">)</span> <span class="p">{</span>
        <span class="n">classname</span> <span class="o">=</span> <span class="s">"nil"</span><span class="p">;</span>
    <span class="p">}</span>
    <span class="kt">char</span><span class="o">*</span> <span class="n">opname</span> <span class="o">=</span> <span class="p">(</span><span class="kt">char</span><span class="o">*</span><span class="p">)</span> <span class="n">op</span><span class="p">;</span>
    <span class="p">...</span>
    <span class="k">return</span> <span class="n">original_msgSend</span><span class="p">;</span>
<span class="p">}</span>

Once we get into the 


 function, things get much simpler since we can use the objc API to do our work. The only trick is the realization that the 


 type is actually a 


 which we cast directly. This gives us the full selector. Counting colons will give us the count of parameters the method is expecting. When everything is done the output looks something like this:

iPhone:~ root# DYLD_INSERT_LIBRARIES=libobjc_trace.dylib /Applications/
objc_msgSend function substrated from 0x197967bc0 to 0x10065b730, trampoline 0x100718000
000000009c158310: [NSStringROMKeySet_Embedded alloc ()]
000000009c158310: [NSSharedKeySet initialize ()]
000000009c158310: [NSStringROMKeySet_Embedded initialize ()]
000000009c158310: [NSStringROMKeySet_Embedded init ()]
000000009c158310: [NSStringROMKeySet_Embedded initWithKeys:count: (0x0 0x0 )]
000000009c158310: [NSStringROMKeySet_Embedded setSelect: (0x1 )]
000000009c158310: [NSStringROMKeySet_Embedded setC: (0x1 )]
000000009c158310: [NSStringROMKeySet_Embedded setM: (0xf6a )]
000000009c158310: [NSStringROMKeySet_Embedded setFactor: (0x7b5 )]


We modify the 


 preamble to jump to our hook function. The hook function then does whatever and restores the CPU state. It then jumps into the jump page which executes the possibly modified preamble instructions and jumps back into 


 to continue execution. We also maintain the original unmodified preamble for restoration when we need to remove the hook. Then we use the parameters that were sent to 


 to interpret the call and print out which method was called with which parameters.

As you can see using function hooking for making objc_trace is but one use case. But this use case is incredibly useful for blackbox security testing. That is particularly true for initial discovery work of learning about the application.

[1] objc-msg-arm64.s

[2] ARM Reference Manual

[3] ARM Instruction Details

[4] objc_trace.m

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