PE - Portable Executable File

Hello, folks!

I'm here again bringing an overview about PE files. I think it's unproductive to write an extensive technical post about PE files since there is a lot of information about that in the web. I linked in the end of the post very good references about it.

So the idea here is just to give a general idea of the PE file, then you will be able to do your own researches, you can use the reference as a start. I think it's more easy to learn when you can grab a general idea then start to digging more into each topic.

Anything that I wrote here is in the reference, so you can use a reference to a more in depth reading. Any questions you are free to ask me I would be glad to help if a can.

Tools used in this post:

• Resource Hacker
• x64dbg
• CFF Explorer
• IDA Free

PE File Format

Let's start thinking about the PE file with this poor analogy. PE file it's some kind of recipe with all the ingredients inside of it. So you have all steps to execute the recipe with all ingredients included in it or with a reference to it.

This format is inherited from the COFF (Common Object File Format) format that came along with VAX/VMS architecture, since Microsoft came from Digital Equipment Corporations that used COFF format file. These formats serves as base for the loaders to read an executable on the system. So to quickly migrate to Windows NT, the developers maintained the original format and enhanced it to PE (Portable Executable).

The PE format is used on Windows to execute programs and is the standard format, i.e. way of organize the data inside a file that make possible to all flavors of Windows to read it, load it and execute it. There aren't almost no difference between 32-bit and 64-bit PE files, the difference resides mostly on field's size.

DLL and EXE files uses the same PE Format and differs just in some values of some fields, mainly in the "File Header->Characteristics". DLL is basically the same for OCX and even CPL files. Once you know the structure of the PE File you know how the executable is disposed on the memory when it's executed, therefore the loader will decide which parts of the file on disk will be mapped into the memory. Let's see a little overview on the pic below:

PE File - File on Disk and In Memory

All the information the loader will be map into the memory will be in the PE file itself. And all the information about how to translate the offset of the file on disk to the file mapped into the memory will be accessible in the file. Next is a SVG image from Wikipedia has a nice view of the PE:

• PE - Portable Executable Structure

When the PE file is loaded into memory it is known as module as all the other PE files is imported by it. The beginning address of a PE file is know as HMODULE as is referenced in Microsoft's API. Differently when the file is on disk, in memory we have the concept of virtual memory, i.e. we don't access the real physical memory of our computer, the OS creates a virtual memory space to allocate everything, it acts mapping/translating the virtual addresses to the real physical memory. The OS then can control better the memory management and security. Some regions of virtual memory space are protected by the Windows Memory Manager (Windows Component) that is specified in the section header of the PE format to read-only, read/write and execute.

MS-DOS Header

Came in handy in the first version of windows, because windows machines isn't so common like in nowadays. So the executable could at least print some messaging asking for Windows to run it. This header and in the executables in general always starts with e_magic field or IMAGE_DOS_SIGNATURE, it's important to remeber this. The most important field here is the e_lfanew that have the offset to NT Header where all the useful information resides.

PE Sections

PE file sections are used to split the data in the file. Some sections represents code and other data. There is some kinds of data like, spaces to read and write information, API import, function export, resources and so on. Every section in the PE file specifies what is in it. Commonly PE file has two type of section, code and data.

Windows Loader grab the information on the section header to properly load the section in memory. There is a code section and other data sections. Each section has it's own attributes like which type of data and if this section is read only or read/write in memory, all specified on the field Characteristics in the Section Header. In some cases the section can be shared between process if it specified.

Section names is just a way to better identify what is within, for the operating system it doesn't matter the name itself just the field Characteristics that is indicating the type of the section.

It's important to remember that since the operating system uses virtual memory protection the Optional Header->SectionAlignment value (space between sections) in memory would be different from the file on disk. In disk the default value is multiple of 200h (hex) (so the offset in disk would be like 200h, 400h, 600h...), but in memory the loader maps the sections in a way that each section starts at the beginning of a memory page (which inherit the security flag read-only or read/write specified in the section header). Windows 32-bit has a page memory size of 4Kb and 64-bit 8kb. So for each architecture this would be the alignment of the section, anyway you can always check in the field in the Optional Header->FileAlignment.

Relative virtual address (RVA)

The RVA it's an important piece in the PE file. It's used to located objects after the file is loaded in memory. When a PE file is loaded in memory it starts at some determined address that we call ImageBase (this address will be the HMODULE) it's located in the OpitionalHeader->ImageBase. To simplify everything let's see in the CFF Explorer (PE Viewer) how they are expressed:

PE File - Section Header / Virtual Address

So to locate the .text section in-memory we have to use the VirtualAddress that is the Relative Virtual Address of the section in-memory, it's relative because the final address to the section depends on the ImageBase address. So to locate any section in-memory we need to add: ImageBase + RVA. If we have the ImageBase 0x400000 and the .text section RVA 0x1000, the final address would be 0x401000, is where the .text section starts in-memory.

So in the header we have both the RVA(VirtualAddress) and the Offset(RawAddress) in disk. If we don't map the PE file on the memory we will use only the Raw Address, if the PE file is loaded we will use only the ImageBase+VirtualAddress.

Data Directories

Data directories are data structure used keep information that the PE files need. For example the imports section have a data structure that contains all the information necessary to the Windows Loader when loading the PE file in memory. So it can load the imports before it starts to execute the code.

Examples of data directories is imports, export and resources. So in the PE file we have a header to located each one of these structures. In the nex image is the Header of the Data Directories. here you have the RVA for each Directory in memory and it's size.

PE File - Data Directories

Importing Functions

In the PE file we have an Directory containing all the information about the imported functions. Which functions from which DLLs, then the Loader can load and locate all the symbols it need to run the module.

When using imported functions from DLLs the compiler automatically compiles and generates the PE file specifying in the import section which DLLs is been used inside the file, so the Windows Loader can properly load the DLL and prepare it to be used by the file in run-time. Note that in my source-code I didn't imported all these functions, but the compiler did. I used the GCC and as you can see it imported lot of functions for internal purposes like security and so on.

PE File - Import Directory

The PE file keep an array of data structures with all DLL's imported. Each data structures have two arrays known as Import Address Table (IAT) and Import Name Table (INT). In the previous image we can see that each of these data structures have the name of the DLL (ModuleName) along with two arrays OFT (OriginalFirstThunk / INT) and FT (FirstThunk / IAT).

The tricky part here is that both arrays has the "same" structure, because the  structure itself is an union that could be any of the values defined in the structure. I recommend you to read the references to get a more comprehensive understanding about it, take time reading and exploring the PE file. Though the tricky part, in general, the FirstThunk field generally points to the IAT array that is overwritten by the Windows Loader with all the API function addresses and OriginalFirstThunk is an array with 2 fields Hint and Name. The Hint field it's the name of the imported function and hint is the ordinal of the function API might be.

PE File - Import Descriptor

Once the Windows Loader loaded the DLLs and overwritten the Import Address Table (IAT) with all addresses that the PE file need to import, all the calls to imported symbols (function API) is redirected to the IAT and finally to the real API address.

In the run-time if the call to the imported symbol is redirected to a JMP instruction, then it's accessing the IAT before reach the API. If the call doesn't passes through any JMP then probably it's going directly to the API.

Malloc CALL (IDA View)

IAT Jumping To Imported Malloc (IDA View)

In future post I will go in more details here, doing a manual DLL hijacking overwriting IAT. Stay tuned. :)

Exporting Functions

Exports is another data directory containing all the information about everything the PE file exports. We refer to this exports as "symbols", for example the API LoadLibraryA is an export symbol of kernel32.dll. This directory is a little tricky, because it have some confusing pointers and rules, I will try to keep it simple and objective, but for a in-depth information please check the references.

When exporting functions or data to others modules all the information must be in the Export Directory, because it's this information the Windows Loader searches for when the other module is importing the symbols in this PE file. Symbols it's a term that includes anything that could be exported. Generally when some module exports symbols, the name of these symbols is the same as was originally coded on the source file. Let's have a look inside the export directory:

PE File - Export Directory

When we are consuming some DLL and we need to import it's function, generally we call the GetProcAddress to give us the address to that function. When we do that, internally the Windows Loader goes into the array Export Name Table (ENT / Field AddressOfNames) gets the index of this function in the array and then access the same index in the array pointed by field AddressOfNameOrdinals, the Loader saves the ordinal in the array at the index before mentioned (ENT). The ordinal is actually the real index used to get the RVA (Relative Virtual Address) of the imported function. In the field AddressOfFunctions has an array of all the exported functions each index of the array is a RVA that points to the function. The tricky part is the field Base that is used with the ordinal, so to find the real index we need to add the field Base+Ordinal resulting in the index for the AddressOfFunctions array. Generally this field is 0x00000001 and all symbol is in order.

I think this part is the most important of all, then I will make it more detailed debugging the GetProcAddress. I think it is interesting to see how the things works. GetProcAddress is a API imported from kernel32.dll subsystem that uses the kernelbase.dll that uses native API ntdll.dll (undocumented).

If you want to try it I will let the code in my github so you can compile the DLL and the code that consumes it.

• Github - PE Portable File
• Look for Dll Consume in README

Remember at this point to be objective, follow the address that matter to you. I used the x64dbg is a very good debugger has a lot of functionality and has a great community developing it. After the LoadLibraryA our dll is is already loaded in memory, you can see it in the tab Memory Map (inside x64dbg). In my case it was loaded in the address 0x6C300000. So this is the address that matter to us. Let's debug it. I breakpoint the GetProcAddress:

Breakpoint - GetProcAddress

After the breakpoint I steped into until I found the begning of the process where the "Windows Loader" begins to search the "NONAME" symbol in the Export Directory in our DLL. I will not make this part too long so I will get right to the point. Debugging you can see that it received the BaseAddress of our DLL, then it got the NT Header, checked if it is a valid PE (OptionalHeader->Magic value), got the Export Directory RVA and Export Directory Size, and now the ntdll.dll is inside our export directory. As you can see in the next image EAX already have the AddressOfNames(0x6C30602C), then it start to compare if the name provided in the GetProcAddress is the same as the exported from the DLL.

EAX=AddressOfNames // ECX=PointerTo_NONAME_Function // EDX=NameToCompare

After the confirmation that is the same function and it's the index 0, because was the first function of the AddressOfNames. In the next image we will see that now it got the value in the index 0 of the field array AddressOfNameOrdinals and with this value it was able to sought the function address in the array field AddressOfFunctions.

Getting the AddressOfNameOrdinals

Getting the NONAME symbol Address

Well, I hope that I could be clear engough explaining all the process to you and introducing you to the Windows Loader, basics of the hierarchy process of subsystem and native API, and how important is the PE format to the Operating System. Any question just let me know, you can email me or call me on twitter.

Some PE files use only the ordinal value of the symbol to export it's symbols. Ordinal it's just an index of the symbol as a mentioned. So when some module try to import some symbol by ordinal in the Import Section will be specified which ordinal the Loader must search in the Export Section of the module imported.

Resource files

The resources files is another data directory in the PE file. Generally have it's own section (.rsrc), but it is not a rule. As I mentioned before the PE file have all ingredients included in the recipe, so anything the PE file needs it can include in itself. Any type of file can be included in the PE file, after all any kind of files are just binary.

The resources are just embed files. There is some ways to get this resources in the run-time, advanced ways and basic ways. Generally we use the Windows API to load the resources. I pretend to introduce to this methods in the future, for now let's see how it works.

PE File - Resource Directory

The resource directory it's a little confusing if you have to read these structures, but using a PE reader it's very easy. Works as a chain of structures, the main structure is the IMAGE_RESOURCE_DIRECTORY that contains some fields. There are only two important fields NumberOfNamedEntries and NumberOfIdEntries, these two fields values has the size of the array of the next structure, IMAGE_RESOURCE_DIRECTORY_ENTRY.

The IMAGE_RESOURCE_DIRECTORY_ENTRY structure has two fields, Name and OffsetToData. Now come the tricky part. If the most significant bit of the field Name is set (differ from 0), then the remaining bits is the offset to the name of the resource, if it's not set then it's a ID for the resource. If the most significant bit of the field OffsetToData is set, then the remain bits is the offset to another IMAGE_RESOURCE_DIRECTORY.  If this field is not set, then the offset points to the resource itself. It's important to remember that this offset is always relative to the beginning of the resource section.

In malware analysis it's a good place to keep additional parts of the malware to drop (droppers) on the machine. Generally malware do this trying to bypass antivirus alert, because lots of antivirus if not all of them do a heuristics analysis searching for malicious behavior. So in many cases the malware (PE file) in the resources are packed, when the main program is executed it unpack the file from resource section and drops it in some folder. To help us we have some tools like CFF Explorer and Resource Hacker.

To make it more clear let's compile a program with another executable inside of it. I will let the link to the source used here. I compiled everything with GCC. Follow the link:

• Github - PE Portable File
• Look for Resource Files in README

Well as you can see I used an icon and another source, follow the steps in the github to compile the first source inversing.c that will be inserted inside our main source blog_resource.c.

After you compiled the inversing.c move the executable to the blog_resource.c folder. In the res.rc you can find the files names to compile, I used "inversing.exe" you can use whataver you want, just change the name in the .rc file. So you first have to compile the res.rc to a object file, for this we use the tool windres from GCC. Read the README that will have all commands to compile, any questions please contact me.

After you have compiled the blog_resources.exe you can execute it to see that the code gets the first two bytes from the inversing.exe file that is {"MZ"} or (big-endian{0x4D, 0x5A}, little-endian{0x5A, 0x4D}), these two bytes represents the IMAGE_DOS_SIGNATURE of all executable files. We can see in the Resource Hacker.

Resource Hacker - inversing.exe inside blog_resources.exe

It's confusing, but you can read more about it on the reference. In future posts I want to bring a more detailed post on how a dropper could work using Windows API and without any API (it's possible too). Anyway in the reference you can read more about the functions I used to find the resource file.

.NET Header

This header is present in PE files compiled in the .NET Framework (obviously). This section is needed for specific information about .NET compilation such as metadata and intermediate language (IL). Differently from directly assembly compiled languages like C/C++, .NET has it's entry point on MSCOREE.DLL which is the DLL that will starts the execution based on the information from the .NET header.

To make a little clear in the below image you can see a little overview on the .NET execution flow:

.NET Framework - Book Eldad Eilam - Reversing: Secrets of Reverse Engineering

The assembly part in this case is from the .NET Framework, the application itself is the Intermediate Language (IL) interpreted by the Common Language Runtime (CLR). In this case the disassembler tool utilized is for the IL.

Conclusion

First of all I do apologizes any mistake that I've been made and I would appreciate that if I did any, please contact me. Any question you may have I would be glad to help if I can. Well, as I already said I tried to keep it simple and objective. I hope this can be useful for anyone. This post is one of the series of introduction topics to my future posts with more technical text.

I linked all references in the bottom. So as I already said, the response to any questions that maybe arises certainly is in the references, anyway feel free to contact me. My post is simple and objective to give a direction about the PE Files, for more in-depth information the references is the way. Thanks! :)

References

• Matt Pietrek - Peering Inside the PE: A Tour of the Win32 Portable Executable File Format
• Matt Pietrek - In-Depth Look into the Win32 Portable Executable File Format
• PE Format Overview (Wikipedia)
• MSDN - PE Format
• Windows Executable Files
• Eldad Eilam - Reversing: Secrets of Reverse Engineering
• Setting ICONS for Windows programs with GCC
• FindResource Reference
• LoadResource Reference
• Resource Types

Solving the first exercise

Hello, there!

In this post we will reverse engineering the exercise-program that I left in my previous post about compiling C programs.  I tried to make well explained and objective. So any question let me know.

Prerequisites:

• Writing and compiling a program in C

In the end of the post linked above there is a code that we are going to use in this post. Compile it and let's begin. Tools in this post:

• strings.exe
• IDA Pro Free
• x64dbg

In my last post I left an exercise (simple one) that the goal was to reach the "Secret Stuff!". In this post I will show two ways to do it. It's very simple program coded in C. I assume that already have compiled the program as the link in the prerequisite.

First I ran it to see what happens:

Fig1 - Running the program

Nothing much, just a simple "Not Valid". Well I will run the strings.exe from SysInternals to see if we can find something interesting. I configured my strings program in the Path environment variable to be easy to execute. Take some time and look closely to anything that could be interesting. There are a lot of strings here, but the 90% is standard, besides this doesn't seem encrypted. Taking a time rolling up and down, I could find some interesting strings:

Fig2 - Strings the program

Well I see that I will have to type some password, but when I first ran it nothing was asked to me. Why this ?. And we have the secret stuff too, so we just began with it since our goal is to reach that part, we deal with password in the way to it if we had too.

We could open in some software like CFF Explorer to dig into the imports and so on, but doesn't seem necessary at this point. So let's open it in the IDA Pro and see if we can find anything interesting. With the program opened in the IDA Pro let's seek for our goal in the tab Strings, click in the tab and search for the "You reach the secret stuff!!", then double click in the line with the string. IDA sent us to the part of the section rdata in the offset that starts the string. We have to click in the offset name aYouReachTheSec and press the X key, then we can choose the references for this offset, since we just have one let's double click it. We were drove to a function that uses this string sub_401517, we can click on it's name and press N to rename it, I renamed to SecretSub. Ok, this would be enough to crack it. I will show two manners to do it, cracking it(Patching) and bypassing it. To bypass we just have to find how to reach the secret stuff without hardcode/patch the binary.

Fig3 - String offset in the section rdata

Bypassing the binary

To bypass the binary first we have to find how the binary reach this SecretSub in normal way, so click in the name of the sub and press X to find the references to this sub. We only find one reference:

Fig4 - Main flow

As we can see we have a little piece of the program that asks for some password using the scanf (C API) to get the password, call sub_401460 and right after uses a cmp eax, 1. We can deduce that we get the password and then sub_401460 will check for the password to see if it is right and return to eax if the password is right or not. Let's see how the binary validates the password double clicking in the sub_401460. We were drove into the sub and I already rename the sub to PasswordCheck making it easy to identify.

Fig5 - Password checking

We can see some loops, some ifs and in the end of the sub we have only one piece of code that will drive us to the good end (mov eax, 1), the other pieces just print "Not Valid" in the screen. The first part is interesting, many hex values stock in variables at the first piece of code, we will look at it after. IDA name the variables for us to make our work more easy, this variables are just offsets in the stack for example [ebp+var_c] without the IDA would be [ebp-C], so IDA put variable names in the offset to make our lives easier, we can even rename the variables clinking in their names and pressing N to make it more easy to read the code.

After looking a little at this code we are able to identify that the first part seems like a counter adding in the variable var_10 to be compared in cmp [ebp+var_10],14h , the size in this variable has to be lower than 14h (decimal 20) if it was larger then we got a "Not valid" message.

We can deduce to this point that the password must be under 20 characters. In the next block it uses the variables var_c (counter) and var_1B (string reference), the var_c is an index reference to the string array for both the argument arg_0 (types password) and the var_1B (hardcoded password). It compares byte per byte from both variables to check if the password matches. So we can take the var_1B till var_11 to get the expected password. If we get the whole and put it together we can convert hexadecimal to unicode characters, but we have to keep in mind that since the value is in the stack it is disposed in little-endian order, thus to convert it we must write them in the reverse order like 3332312D3132332D323331 converting this we get "321-123-231". Now we have the password, but if we run it we can't get in the part to type the password, so we have to dig a little more. Let's back a little and see what it have to does to reach the part that asks for the password.

As we can see in the Fig4 there is a cmp with the arg_0 in the main function and right after a conditional jump JG, since this program was written in C we know that in the main function the first argument by default is the amount of arguments passed to the program to execute. If you ran the program in the CMD the first argument will be always the complete path to the program that is been executed. Therefore we must have more than one argument when execute this program, to do this we just add any text after the program name like ">main.exe newargument" if you execute like this we can bypass this part and reach the password part. Let's try it:

Fig6 - Program bypassed

Voilá! We did it, we bypassed the exercise without patch anything.

Patching the binary

Now let's do a quick and functional patch to get this working without type anything. We want to reach the secret stuff without efforts, execute the program and reach it. How can we do it, now we know the address that calls the SecretSub, we just have to alter that conditional jump JG that was made right after the comparative in the program's argument and JMP directly to the call to the SecretSub. We can do this using the x64dbg. Open the program in the debugger reach the address using the CTRL+G in the address 40153E and press space bar to patch this line. To work properly we can't simple call or jump inside the secret stuff because the program must return to properly exits the execution, then we simply jump directly to the right call using a unconditional jump jmp 0x40158E. Press CTRL+P to open the patches click to patch file and choose a name to the new executable and save it. Now try to run it.

Fig7 - Patched program

Voilá! Now we access the secret stuff without have to type anything.

Well that is it folks! This program was quite simple with no anti-technique at all. In the next posts I will try to bring more difficult exercises. :) Thank you for you time and see you soon! Any questions at all please let me know! :)

Thanks! Best regards!

Writing and compiling a program in C

Hello there!

Today I want to bring a short post where I will show you how to write a program in C and compile it with GCC on Windows. This post will be useful, because we will need to compile the sources that I will bring on future technical posts. My goal is not to teach C programming, there is a lot of content about it on the web. Of course that I will help you along the posts with references about the code sources.

I recommend reading the previous topcis:

• https://verseinversing.blogspot.com.br/2017/06/reverse-engineering-didactic-point-of.html
• https://verseinversing.blogspot.com.br/2017/06/basics-of-assembly.html

The GCC

Since I will not bring the executable here instead just the C code for you to compile on your own environment. In this way you can understand better how C code turns into Assembly code. Like a said in the previous topics, it's important to know programming in reverse engineering even if somebody tells you that isn't. There is no ultimate truth, so I'm open to any comment, tips or criticisms.

Well, let's do it! First we have to download the GCC, I'm using the MinGW to install it. There is a lot of C compilers out there, I pick the MinGW for convenience.

• https://sourceforge.net/projects/mingw/files/

Download and install it at "C:\mingw" to be easy to access. Skip any checks to install components, just finish the installation. We will install the GCC via console on the next step. Once you have installed it, open the CMD or PowerShell goes to the mingw folder.

cd c:\mingw\bin
mingw-get.exe install mingw32-gcc-bin

We will only install the bin component, because we only want to compile our code. After the download process by the MinGW we are good to go, to write and compile our program.

To use the GCC directly, I mean through any folder in console you have to add the path of GCC to the Environment Path property.

GCC compiles our source with some initialization code that is default for every compilation, as I could see so far it starts to write the assembly from given address (0x401460), this is for the standard compilation setting. Let's see it in the next example. I wrote this simple code in C to test our GCC whether it's ok or not:

#include <stdio.h>

int main(){
   
    printf("Hello There. VerseInversing!\n");
   
    return 0;
}

Copy the code below on your favorite editor, save it like "sample.c" on C:/. Now open the console and run the command:

cd C:\
gcc sample.c -o sample

If everything it's ok you should see a sample.exe created on C:\. Just type it on console and execute it.

./sample.exe

Cool uh ? haha

Well, now we have to see how the assembly was generated. I was using the OllyDbg, but since the x64dbg has becoming very popular I'm giving it a try. It's a nice debug tool updated very often, have many features embed in it and many other good things. Download at:

• http://x64dbg.com/

Program loading (Very simple approach)

Before we get our hands dirty, let's see basically how the programming execution works. So first of all I assuming that you read the previous topics. So, to execute a program (binary file) the windows needs to load this file into memory so it takes the file and simply put it into the memory. Then each file type is interpreted in different ways when loaded into memory. The executable file have the header with key information that let the windows load it and execute it properly.

So the executable file always have the PE (Portable executable) header, without it the OS just can't load it. In the PE Header we have vital information that will help our analysis. Is this same information that let the debugger loads it and show us all the information that will help us on the debugging process.

In this PE Header we have many information, like the EntryPoint. EntryPoint like it's name, is the offset where the execution must to begin. This offset belong to the (code, text) section where all the code of our program is in. Again, everything on the file is hex data, but when you put this data on code or text section the OS reads it as instructions to CPU execute. All the rest is hex data too, but it is interpreted in a different way. As I said all depends on how you want to interpret information.

So as I said the executable mainly uses the OS to run, so it will need the libraries on the OS to run it's code. Unless the coder uses the ordinal number to call the API, it's easy to see which api the program uses by looking at the .idata section. IDATA stands for "Import Data" that is the imports the executable do to use on the execution.

On windows each DLL library has specific functionality so you can basically use some program like  CFF Explorer to look at the imports of the PE and google each DLL to see the specifics of each one. In our case it uses the KERNEL32.dll and msvcrt.dll. KERNEL32.dll it's pretty basic with core functions, it's very very common. msvcrt.dll it's the library of C on Windows, so as we are using C it make sense to importing it.

Anyway this is subject to another post, but I just doing a little intro on it. Let's continue our debugging.

Let's debug

After you have downloaded and installed it. Let's open our fresh compiled program. Well, it starts little different from others, not at all.. but a little.
It started at ntdll.dll module, but it's already set the breakpoint on the entry point of the executable (EntryPoint information is in the PE Header).

As I said before the program imports some DLLs to uses it's functionality. This DLLs run with our executable so each one of these DLLs and also our executable is a module running. In this case the ntdll.dll was imported by KERNEL32.dll because the the DLL itself uses the ntdll.dll that does some interface with the kernel.

Let's continue. In the x64dbg goes to the Breakpoints tab and look at the breakpoints:

Shortcuts used in this post:

• F9: Run the program to the end if it doesn't find any breakpoint ahead.
• F8: Step by step or Line by line.
• F7: Step into. If there is any CALL instruction you are going inside the CALL. If you F8 then you jump to the next line and skip the CALL.

Ignore the TLS Callback for now, we will not need it in this topic. To do this you go to Options=>Preferences=>[Tab]Events and uncheck the "TLS Callbacks*". I will bring this in the future posts. You can uncheck it or you can simply ignore it skipping with F9.

As you can see the address 0x4012E0 holds our entry point, so lets play until it stops on that address. Press F9 until you reach the address. This is the entry point on programs compiled with GCC, so our program starts after all the initialization code. My tip is to step through it to familiarize with the debugger.

But to make it shorter the call for our program block is on 0x401280 with a CALL instruction to the address 0x401460 (this is generally where the GCC starts to write our program assembly).

So let's do a little analysis. First it set the stack frame (see Assembly Basics) then it enlarges the stack frame by doing an AND followed by a SUB, then it calls a sample.sub_401970 at 0x401469 it's only check for some value at address 0x407028. If you go to the Memory tab you can see that (this information came from PE Header) the address 0x407000 is of the section .bss (uninitialized data), anyway doesn't do anything crucial and we know that, because we write it.

After this step, it move the address 0x405064 to the stack directly it doesn't uses any register to keep this value, it just MOV to the stack and call the PUTS API from the C library of the Windows. Now, why it uses puts if we write printf ? And why it calling from sample.puts ?

First question, if you see the difference from puts to printf you see why it uses the puts. Reference to the puts:

• http://www.cprogramming.com/fod/puts.html

As you can see, the puts receives a pointer to a string then it put a newline character at the end of the string and that was exactly what we did in our code, but with printf. So the GCC just saw it and make it better.

Second question, if you press F7 you will step into the CALL and will see that inside the call it's just a Table JUMP to the original API. No secrets.

That's it. So we could see what the compiler can do with code, it work on it to make it simpler and "faster" in it's own way. So coding and see how compiler handles our code it's good to a better understating of assembly in practice.

I coded a simple program in C that have many ways to bypass, so the goal is to the reach the ultimate function which prints "You reach the secret stuff!!". I recommend that you do not see what it does and try to understand what it is doing and how to bypass it through x64dbg (or any other debugger). I will do a post explaining how to do it and how to patch it. Consider as an exercise. (The program is buggy, I will show why on the next post)

• https://github.com/bernardopadua/blog-inversing/blob/master/write-compile-c/main.c

Any doubts, comments, tips, criticism just tell me. We are all here to learn with each other. Hope that's can be useful to anyone. I will bring some other analysis here very soon. Thanks! Bye!