I'm trying to understand a small binary using gdb but there is something I can't find a way to achieve : how can I find the list of jumps that point to a specified address?
I have a small set of instructions in the disassembled code and I want to know where it is called.
I first thought about searching the corresponding instruction in .text, but since there are many kind of jumps, and address can be relative, this can't work.
Is there a way to do that?
Alternatively, if I put a breakpoint on this address, is there a way to know the address of the previous instruction (in this case, the jump)?
If this is some subroutine being called from other places, then it must respect some ABI while it's called.
Depending on a CPU used, the return address (and therefore a place from where it was called) will be stored somewhere (on stack or in some registers). If you replace original code with the one that examines this, you can create a list of return addresses. Or simpler, as you suggested, if you use gdb and put a breakpoint at that routine, you can see from where it was called by using a bt command.
If it was actual jump (versus a "jump to subroutine") that led you there (which I doubt, if it's called from many places, unless it's a kind of longjmp/setjmp), then you will probably not be able to determine where this was called from, unless the CPU you are using allows you to trace the execution in some way.
Related
I have an MCU (say an STM32) running, and I would like to 'pass' it a separately compiled binary file over UART/USB and use it like calling a function, where I can pass it data and collect its output? After its complete, a second, different binary would be sent to be executed, and so on.
How can I do this? Does this require an OS be running? I'd like to avoid that overhead.
Thanks!
It is somewhat specific to the mcu what the exact call function is but you are just making a function call. You can try the function pointer thing but that has been known to fail with thumb (on gcc)(stm32 uses the thumb instruction set from arm).
First off you need to decide in your overall system design if you want to use a specific address for this code. for example 0x20001000. or do you want to have several of these resident at the same time and want to load them at any one of multiple possible addresses? This will determine how you link this code. Is this code standalone? with its own variables or does it want to know how to call functions in other code? All of this determines how you build this code. The easiest, at least to first try this out, is a fixed address. Build like you build your normal application but based in a ram address like 0x20001000. Then you load the program sent to you at that address.
In any case the normal way to "call" a function in thumb (say an stm32). Is the bl or blx instruction. But normally in this situation you would use bx but to make it a call need a return address. The way arm/thumb works is that for bx and other related instructions the lsbit determines the mode you switch/stay in when branching. Lsbit set is thumb lsbit clear is arm. This is all documented in the arm documentation which completely covers your question BTW, not sure why you are asking...
Gcc and I assume llvm struggles to get this right and then some users know enough to be dangerous and do the worst thing of ADDing one (rather than ORRing one) or even attempting to put the one there. Sometimes putting the one there helps the compiler (this is if you try to do the function pointer approach and hope the compiler does all the work for you *myfun = 0x10000 kind of thing). But it has been shown on this site that you can make subtle changes to the code or depending on the exact situation the compiler will get it right or wrong and without looking at the code you have to help with the orr one thing. As with most things when you need an exact instruction, just do this in asm (not inline please, use real) yourself, make your life 10000 times easier...and your code significantly more reliable.
So here is my trivial solution, extremely reliable, port the asm to your assembly language.
.thumb
.thumb_func
.globl HOP
HOP:
bx r0
I C it looks like this
void HOP ( unsigned int );
Now if you loaded to address 0x20001000 then after loading there
HOP(0x20001000|1);
Or you can
.thumb
.thumb_func
.globl HOP
HOP:
orr r0,#1
bx r0
Then
HOP(0x20001000);
The compiler generates a bl to hop which means the return path is covered.
If you want to send say a parameter...
.thumb
.thumb_func
.globl HOP
HOP:
orr r1,#1
bx r1
void HOP ( unsigned int, unsigned int );
HOP(myparameter,0x20001000);
Easy and extremely reliable, compiler cannot mess this up.
If you need to have functions and global variables between the main app and the downloaded app, then there are a few solutions and they involve resolving addresses, if the loaded app and the main app are not linked at the same time (doing a copy and jump and single link is generally painful and should be avoided, but...) then like any shared library you need to have a mechanism for resolving addresses. If this downloaded code has several functions and global variables and/or your main app has several functions and global variables that the downloaded library needs, then you have to solve this. Essentially one side has to have a table of addresses in a way that both sides agree on the format, could be as a simple array of addresses and both sides know which address is which simply from position. Or you create a list of addresses with labels and then you have to search through the list matching up names to addresses for all the things you need to resolve. You could for example use the above to have a setup function that you pass an array/structure to (structures across compile domains is of course a very bad thing). That function then sets up all the local function pointers and variable pointers to the main app so that subsequent functions in this downloaded library can call the functions in the main app. And/or vice versa this first function can pass back an array structure of all the things in the library.
Alternatively a known offset in the downloaded library there could be an array/structure for example the first words/bytes of that downloaded library. Providing one or the other or both, that the main app can find all the function addresses and variables and/or the caller can be given the main applications function addresses and variables so that when one calls the other it all works... This of course means function pointers and variable pointers in both directions for all of this to work. Think about how .so or .dlls work in linux or windows, you have to replicate that yourself.
Or you go the path of linking at the same time, then the downloaded code has to have been built along with the code being run, which is probably not desirable, but some folks do this, or they do this to load code from flash to ram for various reasons. but that is a way to resolve all the addresses at build time. then part of the binary in the build you extract separately from the final binary and then pass it around later.
If you do not want a fixed address, then you need to build the downloaded binary as position independent, and you should link that with .text and .bss and .data at the same address.
MEMORY
{
hello : ORIGIN = 0x20001000, LENGTH = 0x1000
}
SECTIONS
{
.text : { *(.text*) } > hello
.rodata : { *(.rodata*) } > hello
.bss : { *(.bss*) } > hello
.data : { *(.data*) } > hello
}
you should obviously do this anyway, but with position independent then you have it all packed in along with the GOT (might need a .got entry but I think it knows to use .data). Note, if you put .data after .bss with gnu at least and insure, even if it is a bogus variable you do not use, make sure you have one .data then .bss is zero padded and allocated for you, no need to set it up in a bootstrap.
If you build for position independence then you can load it almost anywhere, clearly on arm/thumb at least on a word boundary.
In general for other instruction sets the function pointer thing works just fine. In ALL cases you simply look at the documentation for the processor and see the instruction(s) used for calling and returning or branching and simply use that instruction, be it by having the compiler do it or forcing the right instruction so that you do not have it fail down the road in a re-compile (and have a very painful debug). arm and mips have 16 bit modes that require specific instructions or solutions for switching modes. x86 has different modes 32 bit and 64 bit and ways to switch modes, but normally you do not need to mess with this for something like this. msp430, pic, avr, these should be just a function pointer thing in C should work fine. In general do the function pointer thing then see what the compiler generates and compare that to the processor documentation. (compare it to a non-function pointer call).
If you do not know these basic C concepts of function pointer, linking a bare metal app on an mcu/processor, bootstrap, .text, .data, etc. You need to go learn all that.
The times you decide to switch to an operating system are....if you need a filesystem, networking, or a few things like this where you just do not want to do that yourself. Now sure there is lwip for networking and some embedded filesystem libraries. And multithreading then an os as well, but if all you want to do is generate a branch/jump/call instruction you do not need an operating system for that. Just generate the call/branch/whatever.
Loading and execution a fully linked binary and loading and calling a single function (and returning to the caller) are not really the same thing. The latter is somewhat complicated and involves "dynamic linking", where the code effectively and secures in the same execution environment as the caller.
Loading a complete stand-alone executable in the other hand is more straightforward and is the function of a bootloader. A bootloader loads and jumps to the loaded executable which then establishes it's own execution environment. Returning to the bootloader requires a processor reset.
In this case it would make sense to have the bootloader load and execute code in RAM if you are going to be frequently loading different code. However be aware that on Harvard Architecture devices like STM32, RAM execution may slow down execution because data and instruction fetch share the same bus.
The actual implementation of a bootloader will depend on the target architecture, but for Cortex-M devices is fairly straightforward and dealt with elsewhere.
STM32 actually includes an on-chip bootloader (you need to configure the boot source pins to invoke it), which I believe can load and execute code in RAM. It is normally used to load a secondary bootloader to load and program flash, but it can be used for loading any code.
You do need to build and link your code to run from RAM at the address tle loader locates it, or if supported build position-indeoendent code that can run from anywhere.
The mechanism that allows gdb to perform backtrace 1 is well explained.
Starting from the current frame, look at the return address
Look for a function whose code section contains that address.
Theoretically, there might be hundreds of thousands of functions to consider.
I was wondering if there are any inherent limitations that prevent gdb
from creating a lookup table with return address -> function name.
What makes you think GDB does a straight search through all functions? This isn't what happens. GDB organises symbols into a couple of different data structures that allow for more efficient mapping between addresses and the enclosing function.
A good place to start might be here: https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=blob;f=gdb/blockframe.c;h=d9c28e0a0176a1d91fec1df089fdc4aa382e8672;hb=HEAD#l118
The mechanism that allows gdb to perform backtrace 1 is well explained.
This isn't at all how GDB performs a backtrace.
The address stored in the rip register points to the current instruction, and has nothing to do with return address.
The return address is stored on the stack, or possibly in another register. To find where it is stored (on x86_64, and assuming e.g. Linux/ELF/DWARF file format), GDB looks up unwind descriptor that covers the current value of RIP. The unwind descriptor also tells GDB how to restore other registers to the state they were just before the current function was called.
You can see unwind descriptors with e.g. readelf -wf a.out command.
Once GDB knows how to find return address and restore registers, it can effectively perform an up command, stepping from current (called) frame into previous (caller) frame.
Now this process repeats, until either GDB finds a special unwind descriptor which says "I am the last, don't try to unwind past me", or some error occurs (e.g. restored RIP is 0).
Notably, nowhere in this process does GDB have to consider thousands of functions.
Is it possible without any file.out and source code, but just the binary?
Is it possible, knowing the name of a var, found and read at runtime the value?
Is it possible, knowing the name of a var, found and read at runtime the value
It depends.
If the variable is a global, and the binary is not stripped, then you should be able to examine its value with a simple
x/gx &var
print var
The latter may print the variable as if it were of type int (if the binary has no debug info), which may not be what you are looking for.
If the variable is local (automatic), then you can print it only while inside the routine in which it is declared (obviously).
If the binary has debug info, then simple print var in correct context should work.
If the binary doesn't, you'll have to figure out the in-memory address of the variable (usually at fixed offset from stack pointer of frame pointer register), and examine that address. You can often figure out a lot about the given routine by disassembling it.
Update:
if I strip the binary, is harder to do the reverse engineering?
Sure: the less info you provide to the attacker, the harder you make his job.
But you also make your job harder: when your binary doesn't work, often your end-user will know more about his system than you do. Often he will load your binary into GDB, and tell you exactly where your bug is. With a stripped executable, he likely wouldn't be able to do that, so you'll guess back and forth, and after a week of trying will lose that customer.
And there is nothing you can do to prevent a sufficiently determined and sufficiently skilled hacker with root access to his system and hardware from reverse engineering your program.
In the end, in my experience, anti-circumvention techniques are usually much more trouble than they are worth.
Im trying to get the engine version of a game from a global pointer, but I am fairly new to this. Here is a very small example I found...
http://ampaste.net/mb42243
And this is the disassembly for what I am trying to get, the pointer (gpszVersionString) is the highlighted line (line 5)
http://ampaste.net/m2a8f8887
So what I need to find out is basically using the example approach I found to get it, would I need to basically sig out the first part of the function and find the offset to that line?
Like...
Memory signature - /x56/x8B/x35/x74/xD5/x29/x10/x68/x00/xA8/x38/x10
Then an offset to reach that line? (not sure how to find the offset)
You can't directly do this. Process address space is completely unique to your process -- 0xDEADBEEF can point to "Dog" in one process, while 0xDEADBEEF can point to "Cat" in another. You would have to make operating system calls that allow you to access another process' address space, and even then you'd have to guess. Many times that location will be different each run of the application -- you can't generally predict what the runtime layout of a process will be in all cases.
Assuming you're on Windows you'll need to (EDIT: You don't need A and B in all cases but you usually need them) A. be an administrator, B. take the SeDebugPrivilege for your process, C, open a handle to the process, and then D. use ReadProcessMemory/WriteProcessMemory to do what you want.
Hope that helps :)
EDIT 2: It looks like you're looking at an address taken from a disassembler. If that's the case, then you can't use that value of the address -- the image can be re-based at runtime and the value there would be completely different. Particularly on recent versions of Windows which support Address Space Layout Randomization.
I've been looking around but was unable to figure out how one could print out in GDB the result of an evaluation. For example, in the code below:
if (strcmp(current_node->word,min_node->word) > 0)
min_node = current_node;
(above I was trying out a possible method for checking alphabetical order for strings, and wasn't absolutely certain it works correctly.)
Now I could watch min_node and see if the value changes but in more involved code this is sometimes more complicated. I am wondering if there is a simple way to watch the evaluation of a test on the line where GDB / program flow currently is.
There is no expression-level single stepping in gdb, if that's what you are asking for.
Your options are (from most commonly to most infrequently used):
evaluate the expression in gdb, doing print strcmp(current_node->word,min_node->word). Surprisingly, this works: gdb can evaluate function calls, by injecting code into the running program and having it execute the code. Of course, this is fairly dangerous if the functions have side effects or may crash; in this case, it is so harmless that people typically won't think about potential problems.
perform instruction-level (assembly) single-stepping (ni/si). When the call instruction is done, you find the result in a register, according to the processor conventions (%eax on x86).
edit the code to assign intermediate values to variables, and split that into separate lines/statements; then use regular single-stepping and inspect the variables.
you may simply try to type in :
call "my_funtion()"
as far as i rember, though it won't work when a function is inlined.