Then the answer (at least on 32-bit system) is straightforward: we have to overwrite the MSB (most significant byte). Listing12.6. You cannot increment or decrement the address stored in a function pointer or perform any other arithmetic operations. Another common technique for runtime dispatching is through function pointers. ASN.1 BER representation for length: (a) 1 byte; (b) multibyte. This section looks at assembly to implement a switch efficiently for different types of x. Function pointer is a concept where a pointer variable can point to the memory address of a function. As we can see from the snippet the function ends up hitting [1] an indirect CALL using the pointer stored at [HalDispatchTable + 4] (the second entry of the HalDispatchTable). Since function name itself pointing to the function address, we can omit '&' operator for initializing function pointer as well as we can omit '*' from function pointer while invoking a function using function pointer.See the below example. The address is overwritten so that the vtable address points into our own buffer. XDR represents variable-length arrays by first specifying an unsigned integer (4 bytes) that gives the number of elements in the array, followed by that many elements of the appropriate type. You can find more information and program guidelines in the GitHub repository. This technique was originally used by Ruben Santamarta and described in his excellent paper, Exploiting Common Flaws in Drivers.5 This technique has been chosen among the others mainly for a few reasons: it doesn't need a mandatory recovery, it is stable, and at the time of writing it can also be successfully used on the x64 Windows platform. Regular pointers can be used with an array of function pointers in the same manner that regular pointers can. ASN.1 represents each data item with a triple of the form. In this example, two class objects are instantiated on the heap. This means we can use bits 9 to 11 of this product as our hash function. For both arrays and structures, the size of each element/component is represented in a multiple of 4 bytes. Smaller data types are padded out to 4 bytes with 0s. Park, in Advances in GPU Research and Practice, 2017. Function pointer points to executable code not to data. The stack offers a very simple method for changing the execution of code, and hence these buffer overflow scenarios are pretty well understood. That means the function now has some address. In the taxonomy just introduced, XDR, Supports the entire C-type system with the exception of function pointers, Does not use tags (except to indicate array lengths). In the above declarations, the function pointer IP contains the address of the fun function. In this example, *IP is a pointer that points to a function that returns an integer value and accepts an integer value as an argument. A very useful technique in these situations is to use a hashing function. The add() function is called by the phrase funcptr(7,10), and the result is put in the sum variable. We have also changed function call by removing *, the program still works. Adapted from: First, the HalDispatchTable is located in the Kernel Executive and owns a corresponding exported symbol that can be found using the method presented in the Kernel Information Gathering section. To perform the switch we apply the hashing function and then use the optimized switch code of Section 6.8.1 on the hash value y. The code switch_relative is slightly slower compared to switch_absolute, but it is position independent: There is one final optimization you can make. Then you can use code of the form. The conclusion is: the split constructor is used every time there is task stealing, to create a brand new concurrent task executed by another thread. We just need to take care of two more things: the inter-procedure calling convention and the return value. For example, in below program, user is asked for a choice between 0 and 2 to do different tasks. Accessibility StatementFor more information contact us atinfo@libretexts.orgor check out our status page at Figure 7.10 illustrates the 4-byte architecture definition tag that is included at the front of each NDR-encoded message. 3)A functions name can also be used to get functions address. If we overwrite the MSB with the 0x01 value we end up having 0x01A79A1E. declares a function named fn that returns a pointer to an integer. This is the partial dump of the HalDispatchTable: The second entry is 0x80A79A1E. If you're currently enrolled in a Computer Science related field of study and are interested in participating in the program, please complete this form Internally, this function calls KeQueryIntervalProfile(), which is shown in the next code snippet (taken from the 32-bit version of Windows): 809a1b27 ff157c408980calldword ptr [nt!HalDispatchTable+0x4] [1], 809a1b37 8b45fcmoveax,dword ptr [ebp-4][3]. By continuing you agree to the use of cookies. What we have to do is simply overwrite this function pointer, replacing it with the address of our payload. The following switch_hash assembly uses this hash function to perform the switch. Your email address will not be published. One of the claims to fame of ASN.1 BER is that it is used by the Internet standard Simple Network Management Protocol (SNMP). The value itself, in the case of an integer, is represented in twos complement notation and big-endian form, just as in XDR. Unlike other pointers, a function pointer points to code rather than data. Suppose we want to call method_k when x = 2k for eight possible methods. We obviously need to take care of a few specific details: Just like in the Solaris case, we need to find suitable objects for our purposes. We cannot allocate or deallocate memory for a function pointer. Its as if we are declaring a function called *fun_ptr which takes int and returns void. int result = (*IP)(10, 20); // Calling a function using function pointer. The pointer variable which holds the address of a function is called a function pointer. By trying different multipliers we find that 15 31 x has a different value in bits 9 to 11 for each of the eight switch values. ASN.1 supports the C-type system without, The good old approach of exhausting the slabs (partial slabs) until a new one is allocated to, then placing a target object with some sensible data (e.g., a. Back to: C Tutorials For Beginners and Professionals. Here, in this article, I try to explain the Function Pointer in C Language with Examples. J. Lee, J. Each type of data item would contain a function pointer to the appropriate function. There is no concept of saved EIP in relation to a heap, but there are other important things that often get stored there. The runtime for SnuCL Single keeps the ICD dispatch tables of all available OpenCL platforms under a single operating system instance. I hope you enjoy this Function Pointer in C Language with Examples article. The first method uses a table of function addresses. Dawe is passionate about innovation, new technology and software development. That is, XDR uses big-endian format for integers. The switch_absolute code performs a switch using an inlined table of function pointers: The code works because the pc register is pipelined. ScienceDirect is a registered trademark of Elsevier B.V. ScienceDirect is a registered trademark of Elsevier B.V. Encyclopedia of Physical Science and Technology (Third Edition), Stairway to Successful Kernel Exploitation, Hack Proofing Your Network (Second Edition), The basic trick to heap overflows is to corrupt a, illustrates the example. A hashing function is any function y = f(x) that maps the values we are interested in into a continuous range of the form 0 y < N. Instead of a switch on x, we can use a switch on y = f(x). But, it is recommended to use the indirection operator as it makes it clear to the user that we are using a function pointer. This compiler takes a description of a program written in the Interface Definition Language (IDL) and generates the necessary stubs. This section will show how to overwrite the HalDispatchTable to execute code at Ring 0. With this settled, writing an exploit using the overwrite-into-the-next-object technique is not different from the Solaris or SLAB case, and we will not describe it yet another time. At this point you may be asking why a byte can specify a length of up to 127 bytes rather than 256. 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One example is to prevent code redundancy, which is what most programmers seek to achieve. In that case, the code of that function always resides in the memory. Its stubs can be either interpreted or compiled. We then have to create a function that will wrap our payload. The example C function ref_switch performs different actions according to the value of x. The Linux counterpart (for tracking the allocator) of the Solaris kstat framework is a simple text file, exported inside the /proc filesystem: /proc/slabinfo. We are setting the address of the printname() function to ptr with the expression ptr=printname. Kaare Christian, in Encyclopedia of Physical Science and Technology (Third Edition), 2003. How do we perform the switch efficiently, without having to test x against each possible value in turn? This entry is used by an undocumented system call (NtQueryIntervalProfile()) that is not frequently used. The first field, IntegrRep, defines the format for all integers contained in the message. In other words, routines like xdr_item are used on both the client and the server. Let us see, how to assign the address of a function to a function pointer in C Language. The downside to this is that heap object addresses may contain a NULL character, limiting what we can do. Use the switch value to index a table of function pointers or to branch to short sections of code at regular intervals. The steps to trigger such a scenario (in the common LIFO free objects situation) are as follows: Force the allocation of a new page for the cache. If you recall the example we provided in the Controlling Heap Allocator's Behavior subsection, it basically means to overflow into C by writing past A. To get the address of a function, we must first state the functions name. In the program above, were displaying the address of our main() function. If the cache is not implemented with a LIFO approach for free lists, you need to substitute steps 25 with whatever algorithm is necessary to have two adjacent objects so that your victim object gets allocated once the target object has already been allocated. Next, the FloatRep byte defines which floating-point representation is being used: 0 means IEEE 754, 1 means VAX, 2 means Cray, and 3 means IBM. A new, additional constructor, different from the copy constructor, is needed by the algorithm. For different values of x we need to perform different actions. We simply use the sched_setaffinity() call to bind our user-land process to the first CPU (CPU 0), thus ensuring that all SLUB operations will be carried on/from the same CPU cache, the one associated to the first CPU. For our purposes a good hashing function is easy to compute and does not suffer from many collisions. 1)Unlike normal pointers, a function pointer points to code, not data. If you want to have a function pointer to a function, then the type of function pointer must match the type of the function.That means the return type and the arguments list of function pointer must match that of the function. Larry L. Peterson, Bruce S. Davie, in Computer Networks (Fifth Edition), 2012. In the main() method, the Function1 method is called in which we pass the address of Function2. This page titled 12.7: Function Pointer in C is shared under a CC BY-SA license and was authored, remixed, and/or curated by Patrick McClanahan. This is the same as what we have already learned about pointersjust that we are now pointing at functions. 5)Function pointer can be used in place of switch case. We already know how to call a function in C Language. Kernel dispatch tables usually hold function pointers. In that section, it was mentioned that the declaration. It should now be clearer why having multiple objects of the same size packed inside the same slab cache helps here. The obvious (and ideal) option is for C to hold either a function pointer so that we end in the case we described in the Overwriting Global Structures' Function Pointers subsection, or a data pointer that later will be used in a write operation so that we end in the case we described in the Arbitrary Memory Overwrite section. Suppose we declare a function as follows: We can call the above function as follows: We can also call the above function using a function pointer as follows: Passing a functions address as an argument to another function in C: In the next article, I am going to discuss. The start of executable code is commonly stored in a function pointer. We need to guarantee that once a new slab is created and allocated to the specific CPU, all our allocations/frees will go through it. Class objects and structs are often stored on the heap, so there are usually many opportunities to do this. It basically involves overwriting the object adjacent to the overflowing object. Dispatching via function pointers. Here fptr can point to any of the three (add,multiply,divide) functions,since they are of same type.Address of these functions are passed to the function operation at runtime and will be called from there. Make sure the switch value is in the range 0 x < N for some small N. To do this you may have to use a hashing function.