20
It's the wrong question, really. AH is the exception.
Now the real question is, why is AH such an exception? It's an old register, from the 8086 era. It exists to facilitate moving over code from the 8080.
The 8080 has different registers from the 8086, so you can't move over code directly. In particular, it didn't have an AL,AH or AX register. It did ...
17
Is it possible to access the higher part of the 32-bit and 64-bit registers? If so, which ones?
It is impossible to access the higher parts of the EAX and RAX registers, or of any other 32 and 64-bit registers, directly. You'll have to use indirect instruction sequences if you're interested in doing that. This is because there are no encodings to access ...
7
The shr rax, 3 is an unsigned divide by 8 with truncation towards zero. The inclusion of the adc rax, 0 makes the division round to nearest instead. (Though 0.5 will always be rounded up)
So this operation sets RAX to
1 if RAX is in the range of [4-11] (8*1 ±4)
2 if RAX is in the range of [12-20] (8*2 ±4)
3 if RAX is in the range of [20-27] (8*3 ±4)
You ...
7
I believe the best tool for rewriting assembly to C is IDA Graph View, which is toggled with space.
It let you see the function as Basic Blocks, connected by control flow instructions. In this specific function, I cannot spot any jumps so you will see one long block.
The first thing you usually see in a function is the function prologue which sets up the ...
5
Update: Thanks to Nirlzr, I see I missed the emphasized point at the bottom of the post. This answer, though not what OP is looking for, may serve useful for anyone down the road who is looking for a way to access those bits. Apologies for missing OPs full intention!
I actually wrote an in-depth article on this topic a couple of years ago: Accessing and ...
4
This function computes (in a recursive manner) the sum of the n first integers:
400700: sub rsp,0x18 ; Align the stack
400704: mov QWORD PTR [rsp+0x8],rdi ; Store first argument on stack
400709: cmp QWORD PTR [rsp+0x8],0x0 ; test (n == 0)
+--40070f: jne 400718 <bar+0x18> ; jump to 400718 if (n != 0)
| ...
4
Since you mentioned ELF in the tag. WinMain, DllMain, etc should not be a concern for you. They're name conventions for Windows.
The main function is the first function executed for a C/C++ program. However, it doesn't mean this is the real first function / code executed. You will usually find some initialization code before this function is called.
my ...
4
lea = address
mov = contents
if address 0x401000 contains 0xDeadBeef like ef be ad de
lea MySecretPlace, [401000] MySecretPlace will be 0x401000
Mov MySecretPlace, [401000] MySecretPlace will be DeadBeef
mov MySecretPlace, byte ptr [401000] MySecretPlace will be 0xef 0r 0xef depending on EndianNess
mov MySecretPlace, word ptr [401000] ...
3
Just a quick note in case you would not be aware of this:
$ cat tiny.c
#include <unistd.h>
void _start() {
_exit(42);
}
on x86-64, here is what I get (you need a static libc: libc.a):
$ gcc -static -ffreestanding -nostartfiles -s -o tiny tiny.c
$ ./tiny || echo $?
42
Pay attention that:
$ file tiny
tiny: ELF 64-bit LSB executable, x86-64, ...
3
It is possible to do what you want. There are some difficulties you may encounter though. I'll show you a short instruction how it can be done in several different cases.
Have a look at ELF specification to know how every file in that format looks like, which sections it consists of and how they are located in the process image at the runtime. Even when ...
3
If you use -dr instead of plain -d, objdump will list relocation info next to the instructions. E.g. here's a random sample I just compiled
00000000 <main>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 83 e4 f0 and $0xfffffff0,%esp
6: 83 ec 20 sub $0x20,%...
3
I believe the discrepancy between 40 and actual sum of 48 is mostly an error, however there are many other registers used for handling hardware, memory management, and control of different features of the CPU.
The answer you linked to covers all the commonly used registers in the following image (taken from there):
There are, however plenty of less ...
3
Wikipedia has a page about the x86 architecture and all its known registers. Here is a small picture gathering all what we know about it.
In fact, not all these registers are officially documented. But, all the registers we know (and we know how to use) are listed in the picture. And, it makes much more than 40.
And, I do not know what are these 'hardware' ...
3
These two symbols aren't exported in the usual way (i.e. via the export table). Instead, they are public symbols inside the run-time library itself. The startup code that runs before _main() performs the command-line resolution, assigning parameters into the __wargv array, and storing the count in __argc. The relative addresses are fixed for the file, but ...
3
You can use Ida's Appcall functionality:
Appcall is a mechanism to call functions inside the debugged program
from the debugger or your script as if it were a built-in function.
Such a mechanism can be used for debugging, fuzzing and testing
applications. Appcall mechanism highly depends on the type information
of the function to be called. For ...
2
Actually, there is no lea edx, [0x80490c8] for 64-bits addressing modes. Since, afaik, in all 64-bits addressing modes lea is a register relative opcode.
LEA - Load Effective Address
Computes the effective address of the second operand (the
source operand) and stores it in the first operand (destination
operand). The source operand is a memory ...
2
Windows loader does not care about section names, so the name does not really matter, but usually this section contains the pointer to the indirect call guard check (___guard_check_icall_fptr). The pointer to it is stored in the GuardCFCheckFunctionPointer field of the load configuration directory.
I guess in the newer files it may contain some additional ...
2
The most useful reference I've seen about this section is in this Adobe source comment:
//
// Allocate xyz in the .00cfg so that it is implicitly merged into the
// import section of the binary (which is read-only).
// Indirect calls using this pointer do not have the guard check function,
// and so will not fail the CFG check.
//
The code appears to add ...
2
Mainly, this is because the program did enough pop previously to get rsp just in front the saved rip. Then, a simple push will overwrite the saved rip.
As you can notice on the first picture, your saved rip is at 0x5561dca8 and, your rsp as well. Therefore, a push operation will store the address of the touch3 function as return address of the stack-frame.
...
2
It seems to be %rip (the address of the ud2):
#include <signal.h>
#include <stdio.h>
#include <ucontext.h>
void handler(int signo, siginfo_t *info, void *context) {
ucontext_t *uc = (ucontext_t*)context;
printf("%llx\n", uc->uc_mcontext.gregs[REG_RIP]);
}
compiles to:
handler(int, siginfo_t*, void*):
movq 168(%rdx),...
2
You have several options here, but I like this one.
Since the code is already inside the executable, so you have to patch the entrypoint to jump to a new function.
To do so, I suggest to use a shared object (.so). In this module you will both: the new function and a 'patcher function'. The latter function must be defined with __attribute__((constructor)), ...
2
For this case, here I compile the MS Docs example in 64 bit PE binary with gcc -ggdb command (or use gcc -S for the assembly file). Here is the assembly section of SetCommState function in Intel syntax:
mov dword ptr [rbp-44], 57600 ; dcb.BaudRate = CBR_57600
mov byte ptr [rbp-30], 8 ; dcb.ByteSize = 8
mov byte ptr [rbp-29], 0 ...
2
Pretty much, they will all eventually read the PEPROCESS object directory.
Maybe not, depends on what you're trying to hide from. If it's just some basic usermode process iteration then you're probably fine. There's quite a few ways to iterate the process object directory though, such as ZwGetNextProcess.
Your best bet would probably be ObRegisterCallbacks ...
2
When does the PE loader fill in the IAT?
The IAT is updated at load time by the PE Loader [1], this is called Load-time dynamic linking, as opposed to Run-time dynamic linking, where LoadLibrary/GetProcAddress are necessary.
Hooking an API not included in Import Address Table
There are several ways to do that actually, and Export Address Table hooking is ...
2
I would like to extend the .text section and insert [code]. If there's a better method of inserting new functionality into an ELF I'm all ears.
Techniques for adding arbitrary code to ELF files were pioneered by Linux virus writers beginning all the way back in the 1990s. In comparison with the methods they developed, as well as with more modern techniques,...
2
I've found out how this works. I had the right idea, but I had made the mistake of assuming it was n bytes ahead, failing to factor in the size of the actual value at hand.
Since the specific type of v1 was a single, this means I had to multiply the 1818 by 4 bytes. Looking at the corresponding ASM, movss dword ptr [rdi+1C68h], xmm1, I can see that rdi+...
2
Details of such low-level registers are often disclosed only to Intel's trusted partners under NDA since they're not intended to be accessed by the general application programmer but only by writers of BIOS or other system-level code. That said, sometimes you can find info in unexpected places...
#define MSR_POWER_CTL 0x1FC
#define ...
2
movsxd moves the dword by sign extending the dword into qword
so for this example rdx will be eax+edx
C:\>python -c "print( hex(0x26d1+0x40d))
0x2ade
you can use some emulators like unicorn
or use a debugger and patch this instructions some place and loop
or compile a small source as below to get an understanding
(the code below sign extends a 16bit ...
2
It's called x86 but you have the option to pick size - 64.
1
Welcome to the wonderful world of AT&T assembly! The 9 in 9(%rax, %rdx) is commonly called displacement or sometimes base, and you should indeed just add all three values:
9+rax+rdx = 9+0x100+0x3 = 0x10C
This address is then dereferenced so the value 0x11 is loaded from the address 0x10C.
I would recommend reading the Solaris x86 Assembly Language ...
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