Strcmp assembly x86

So, as promised, I start here a series of articles dedicated to the world of exploitation : at the end of this one, I will go deeper with the x86 Assembly series. It could be described as a piece of code or a series of commands as well that takes advantage of a software vulnerability in order to execute arbitrary code mostly arbitrary code execution, privilege escalation and others. The one kind I will describe here is the one applied to buffer overflowsi. Well, buffer overflow happens.

Why should I care about a larger string copied into a shorter buffer? Why should I bother? First necessary step: disable the ASLR feature on the Linux machine ASLR is a protection mechanism that randomized the addresses of the data areas of the program with the following command:.

With the help of GDB it is possible inion pain print the stack status at the desired time by placing a breakpoint at the right point of code:. So, the password variable address is 0xFFFFD what happens if we start writing from here a string longer than the space it was reserved for the variable itself? Well, we would go over 0xFFFFD25F and starting overwriting the address of the previous stack frame and the address of the next instruction to be executed after the current function is over.

That is the vulnerability! Trying is very easy:. But this totally means that we can put inside EIP whatever value we want: even the cookie function one. That totally deserves a try, but we need the address of the cookie function before. So, we practically executed the cookie function!

But this is cool enough: we changed the execution flow! Not enough! Obviously harmonium apps shell code should be the shortest possible, in order to fit into the variable space; also, as usually the writing operation on the variable is performed with a string copy function, the shellcode MUST be in such situation NULL-free, i.

Finally, a shellcode has to rely only on the executable section, without referencing data stored on the data section. How could I do this? Well, first thing I need the assembly code of such operation. The shortest way to do this, as far as I know, is by calling directly the write system call of the Linux kernel, which needs the following information:. For now it is sufficient to know that executing INT 0x80 on a Linux machine means calling the kernel and executing the desired system call.

At the end of the shell code we MUST exit from the process, otherwise we would execute meaningless code outside the variable containing the shellcode. Doing this is easy, as there is the exit system call provided by the Linux kernel. In the end, I come up with the following code:. The precedent commands generated the pwned executables: by executing it, you will realize that it does exactly what we wanted to do.

Now we need to get the machine code corresponding to these x86 instructions. Even worse, it uses values from the data section.

One problem at a time. Why the 0 values? And the executable still works. But there is still the string issue. That can be solved with antigravity nice trick:. What does this code do? Well, the CALL instruction pushes onto the stack the address of the next instruction to be executed after that a RET instruction is executed: this means that the string address is pushed onto the stack, as I wrote its declaration right after the CALL instruction!

Popping it from the stack to the ECX register is enough to have the same result of the previous versions. Objdump, please:. Thank you objdump for trying to parse my string as x86 instructions. The shellcode, at the moment, is 36 characters long, definitely shorter than our 64 bytes buffer.C has a reputation for being inflexible. But did you know you can change the argument order of C functions if you don't like them? It's a variable that points to DoubleToTheIntbut with a type that says it likes its integer arguments to come before its doubles.

You might expect the IntPowerOfDouble to take its arguments in the same order as DoubleToTheIntbut cast the arguments to a different type, or something like that. But that's not what happens. Now try changing all the int arguments to float — you'll see that FloatPowerOfDouble does something even stranger. That is. What's going on? The above code examples represent a kind of type punning of functions — a dangerous form of "assembly without assembly" that should never be used on the job, in the vicinity of heavy machinery, or in conjunction with prescription drugs.

Assembly Register Calling Convention Tutorial

The code examples will make perfect sense to anyone who understands code at the assembly level — but is likely to be baffling to everyone else.

I cheated a little bit above — I assumed you're running code on a bit x86 PC. If you're on another architecture, the trick above might not work. In spite of C's reputation for having an infinite number of dark corners, the int-double-argument-order behavior is certainly not a part of the C standard.

It's a result of how functions are called on today's x86 machines, and can be used for some neat programming tricks. If you took a C class in college, you might remember that arguments are passed to functions on the stack.

The caller puts arguments on the stack in reverse order, and the called function the callee reads its arguments off of the stack memory. That's how I was taught it, anyway — but most computer nowadays pass the first several arguments directly in CPU registers.

SSE string functions for x64 Linux

That way the function never has to hit stack memory, which is slow to access compared to the registers. The number and location of registers used for function arguments depend on something called the calling convention. Windows has one convention — it has four registers set aside for floating-point values, and four registers for integer and pointer values. Unix has another convention, called the System V convention. It has eight registers set aside for floating-point values, and six registers for integer and pointer values.

If arguments don't fit into registers, then they go into stack memory the old-fashioned way. In C, header files really just exist to tell the compiler where to put the function arguments, often a combination of registers and the stack. Each calling convention has its own algorithm for allocating those arguments into registers and onto the stack.

Unix, for example, is very aggressive about breaking up structs and trying to fit all of the fields into registers, whereas Windows is a bit lazier and just passes a pointer to a large struct parameter. When the compiler encounters DoubleToTheInt 0.

Now you might have an inkling of why the little trick at the beginning worked. Consider the function signature:. Called as IntPowerOfDouble0. Because the compiled function has no idea how it was called — only where in registers and stack memory to expect its arguments — we can call a function using a different argument order by casting the function pointer to an incorrect but ABI-compatible function signature. In fact, as long as integer arguments and floating-point arguments appear in the same order, we can interleave integer and floating-point arguments in any way we'd like, and the register layout will be identical.

Note that double-precision and single-precision arguments both occupy the XMM registers — but they are cisco dart download ABI-compatible with each other. So if you recall the second code sample at the beginning, the reason that FloatPowerOfDouble returned zero and not In the ensuing confusion, exponents went around pretending to be significands, significand bits were chopped off or treated as exponents, and the FloatPowerOfDouble function ended up raising a Very Small Number to a Very Large Number, producing a zero.

Mystery solved. Also note the???Find centralized, trusted content and collaborate around the technologies you use most. Connect and share knowledge within a single location that is structured and easy to search.

You are trying to associate strcmp with the assembly instruction cmp. But they are different things. In the processor level assemblythe cmp A, B compares 2 different registers, for example, and set the result in some flags. Flags are bits that can be tested by other instructions like jzjnz and then redirect the flow according to the desired intention. When you do a call strcmp you are actually calling a "high level function" high level, when compared to assembly that will perform a lot of stuff to compare 2 C-Style strings.

And if you understand this, you will see that it doesn't matter in fact, because the paradigm is a little nonton felam. Stack Overflow for Teams — Collaborate and share knowledge with a private group.

Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. What registers does strcmp evaluate? Asked 6 years, 11 months ago. Active 6 years, 11 months ago. Viewed 11k times. It seems like this is common knowledge, but I can't find it written on the internet yet. I'm unsure because with cmp it's stated cmp eax, ebp. I know the result of the comparison is returned in the EAX register if that helps. I updated my question after looking at it in a different debugger.

It's msvcrt's strcmp — user For standard calling conventions on x86, arguments are pushed to the stack before calling the function. On x64, some arguments may be passed in registers instead. Immideately before, you should see some instructions pushing registers on to the stack in the case of x86, or some movement in the registers for x64 may be harder to see.

Again, you cannot correctly identify what values are used and how, without having the exact function signature and system architechture. And even then, the compiler may optimize however it feels. I see the values pushed to the stack now, thanks! Add a comment. Active Oldest Votes. And if you understand this, you will see that it doesn't matter in fact, because the paradigm is a little different: cmp is an assembly instruction. Wagner Patriota Wagner Patriota 5, 23 23 silver badges 44 44 bronze badges.

Thanks, you're answer combined with the comments given already means I need to look to the values being put on the stack. Sign up or log in Sign up using Google. Sign up using Facebook.Monday Super kalyan 28, by Thiago Macieira Comments. You know that every toolkit and language that is more recent than C has its own string class. Some say it's a feature of the C language not to have a string built-in, keeping it lean and simple.

It wasn't until the Standard Template Library was standardised that the language got std::string. But that was too late for Qt: Qt 1. And for a long while, Qt could not rely on there being a sane implementation of STL in the user's compilers.

The first of those innovations was already present in Qt 1. For Qt 2. As soon as you have Unicode strings in an 8-bit source code, a question always comes about: what encoding is your file?

Even today, with the widespread use of UTF-8, we can't rely on that fact text editors in Windows being the worst example. And since Qt must convert from a given 8-bit encoding into the UTF format used by QString, it has to know what you used. That's fine for application developers, but how about library developers? If you want your library to be used by application developers who use other encodings in their applications, how do you solve the problem?

One solution is to restrict yourself to ASCII codepoints only that is, from 0 tohoping that will be the correct thing. In most cases, that will work out just fine, but there are some weird encodings out there that are not fully compatible with ASCII. There's a drawback though: if the application developer set the codec to be something complex, all of your simple ASCII strings will still be parsed using that codec.

You'll then suggest using QString::fromLatin1which appeared in Qt 2. That is, actually, a very simple operation because the ISO encoding matches exactly the first codepoints in Unicode. Therefore, that's a fast operation and you won't have a performance penalty due to slow codecs.

The use of QString::fromLatin1 is widespread in Qt 2 and 3 applications and libraries, as well as Qt itself. In fact, you're likely to find code like the following in many source files:. That doesn't eliminate all performance penalty, though. It creates a QString in any case and that could be a waste of processor time for some simple operations, especially on temporaries.

For example, a comparison like:. Seems like a waste, right? Well, we thought so too. For Qt 4. I can summarise it like this:. You can look up the complete version in Qt's source code of today. But you can readily see that it's small and almost trivial.

This allows us to rewrite the comparison above as:. This is the best we can squeeze out of it. Well, unfortunately, there are two problems with current Qt 4. Take, for example, QString::replace. What that means is that a QString temporary is created if you were to call that function with a QLatin1String argument.

We're addressing this issue in Qt 4. The other issue is the signature of those functions. Let me take the simplest example from qstring.However, there is an exception at the exact line of jne zeroFlag saying : 0xC Access violation writing location 0x It seems your debugger stops at the last instruction before the one where an exception occurred.

The error is actually at the next line:. That tries to increment a value stored at address esi. Since esi is 0, incrementing a value at address 0 results in an access violation.

With that said, there's no need to copy C-strings into temporary arrays, you can compare them in-place and you can optimize the process by comparing in dwords, falling back to a byte compare in the last chunk. So, you fault on the jne instruction with address NULL. This is literally impossible. Remembering how the processor works, it actually faulted on the previous instruction and IP points to the next one; if the debugger doesn't adjust for this it faulted on the cmp instruction, which is equally impossible.

Only one possibility. The code your running is not the code you see in the debugger. Rebuild all, and fix your compilation errors, and try again. The error is actually at holden paint code 568 next line: inc [esi] That tries to increment a value stored at address esi. To increment esi itself, simply write: inc esi With that said, there's no need to copy C-strings into temporary arrays, you can compare them in-place and you can optimize the process by comparing in dwords, falling back to a byte compare in the last chunk.But what does it compare exactly, and how does it relate to strcmp?

Register eax will contain the return code from strcmpafter the call. The test eax, eax is the same as and eax, eax bitwise and except that it doesn't store the result in eax. So eax isn't affected by the test, but the zero-flag is, for example. The test eax, eax is necessary to make the jne work in the first place. And jne is the same as jnzjust as je is the same as jz. Both act based on the ZF zero-flag value. Conversely if eax contains zero upon return from strcmpthe jump via jne will not happen.

You might be missing the fact that call strcmp will not set ZF for you - it returns the result in the EAX register. If you want a way to remember what does test eax, eax it can be translated like this in C:. This is my first program i am trying to reverse and my intro to this field. The C program will test if two strings match, and it will printf a message for each occasion. What i do not understand is what's up with this line: test eax,eax What caused this line?

How does it relate with strcmp? EAX is the register used by IA32 calling conventions to either return an interger value or a memory address to the calling routine. By design, strcpy can return either -1,0 or 1 in EAX with 0 indicating both strings match. And what does test eax,eax do?

Testing for NULL vs. length of string pointer x86 vs x64?

Checks to see if what? How does it work? I mean, it does a logical AND. But what exactly does it checks? How will the jne, not fire? What would have to be the value of eax for jne to not jump?

What are the conditions for jne jumping and not jumping? And how does this relates to strcmp? Anding a value with itself gives the same value, so test eax, eax sets the flags based on whatever eax contains.

ZF is set when the result of an operation is zero. So the jump will be taken when strcmp returns nonzero, meaning the strings are unequal. I recommend reading some easy book on x86 assembly, it will help you a lot. Edheldil 1 2. Is that an interpreter program? Private Sep 3 at Sign up or log in StackExchange. Post as a guest Name.

Popular posts from this blog Algorithm pseudocode in markdown. Use LaTeX inside the markdown to Tcpa settlement check formulas. Then I convert the markdown file to PDF with pandoc.

All works like a charm.Abstract: Using new Intel Core i7 instructions to speed up string manipulation. SSE 4. Intel had published the description for new instruction formatsbut no sample code nor high-level guidelines. This article tries to provide them.

PcmpIstrI is one of the new string-handling instructions comparing their operands. The second operand can be a memory location. Note the immediate operandwhich consists of several bit fields controlling operation modes.

The heart of a string-processing instruction is the aggregation operation immediate bits []. This operation compares two strings think of strcmp or memcmp. The result of comparison is a bit mask 1 if the corresponding bytes are equal, 0 if not equal.

For example:. The first operand is a character set, the second is a string think of strspn or strcspn. The bit mask includes 1 if the character belongs to a set, 0 if not:. The first operand consists of ranges, for example, "azAZ" means "all characters from a to z and all characters from A to Z":. Substring search strstr. The first operand contains a string to search for, the second is a string to search in.

The bit mask includes 1 if the substring is found at the corresponding position:. After computing the aggregation function, IntRes1 can be complemented, expanded into byte mask or shrinked into index. The result is written into xmm0 or ECX registers.

Intel manual explains these details well, so there is no need to repeate them here. The following strcmp and strlen functions were written and published when the processors with SSE 4. Later they were tested on real hardware and found to be correct. The first processor with SSE 4. Typo in Intel manual: on figure"imm8[]" near Optional boolean negation should be "imm8[]".

Peter is the developer of Aba Search and Replace, a tool for replacing text in multiple files. Ten recent comments are shown below. Show all comments. Note that the inner loop in the above code gets slightly bigger in case GCC chooses e. This can be avoided by explicitly choosing a register, e. Thank you very much for sharing this! Fitting the loop into 16 bytes is a great idea; you have done a lot of work on the lowest level to achieve this.

You are right that equality is tested more often than the alphabetic order, but the comparison to zero is often missed:. It's certainly a matter of taste, and you might prefer the standard behavior returning zero for equal strings. The strcmp() and strncmp() functions return an integer less than, equal to, or greater than zero if s1 (or the first n bytes thereof) is found. You are trying to associate strcmp with the assembly instruction cmp.

But they are different things. In the processor level (assembly), the mt6739wa twrp A. › topic. example: mov edi,example_test mov esi,edi mov edx,esi ; ESI == EDI == EDX == example_test call strcmp cmp dword ecx,00 ; if ECX is not. I have never managed to memorize all of x86 Assembly's string instructions strncmp displays a nice usage of string instructions which makes it a nice. strcmp needs two strings (one in eax, second in ebx) and returns 0 if strings are equal or 1 / -1 if they are not into ecx. Here is the assembly code for x86 architecture. strcmp: pusq %rbp.

Strlen masm

movq %rsp, %rbp. Implement the following functions in x86 assembly: strlen; strchr; memcpy; memset; strcmp; strset. Here is the C prototype for strlen. To practice, I am trying to rewrite a basic libc in assembly (NASM Intel syntax).But I'm stuck on the strcmp function:;; Compare two C-s. The strcmp function performs an ordinal comparison of string1 and string2 and returns a value that indicates their relationship. wcscmp and. //. // This file contains routines where functionality is inlined in assembly language.

//. // by Alan Wernick. #include. Strlen assembly x strlen and strcmp implementation in x86 FASM assembly, needs string in eax and is returning length of that string into ebx. strcmp needs. Finding a String Length in Assembly Language, The C library provides a strlen() function to find the length of the string. The function is usually written in C. x86 Assembly Tutorial.

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A great guide to the basics of x86 assembly strlen. ASCII string length A basic Windows x86 ASM example. Name: ; ; Build: g++ -c -m32 -o main.o ; nasm -f elf32 -o Source: Modern x86 Assembly Language Programming p. asm_minilibc/x86/ Filename: ;; Author: chapui_s strcmp: push ebp. mov ebp, esp. pushfd. push esi. push edi. I've programmed in x86 assembly language before as a teenager, so that it is identical (and thus comparable) to glibc's strcmp.

x86 Assembly/SSE SSE stands for Streaming SIMD Extensions. It is essentially the floating-point equivalent of the MMX instructions. The SSE registers are Ok. I have a strcmp function that I wrote in assembly that More info on cmps: 5 AMD and Intel x86 Processors – bit Mode grams directly in machine code, but he writes them in “Assembly language” passing *t_str to strlen.