Obtaining the Starting Address of a Memory Page: A Comprehensive Guide #

Introduction #

In the realm of system programming and reverse engineering, understanding memory management is paramount. A fundamental concept in this understanding is the ability to determine the starting address of a memory page given a specific address within that page. This knowledge is crucial for tasks such as analyzing program behavior, debugging, and crafting exploits. This article will delve into the techniques and tools available within the Linux environment to achieve this goal. We will explore the underlying concepts, dissect the practical implementations, and ultimately provide a clear path to obtaining the page starting address, specifically focusing on the context of identifying the base address of the main function within a program.

Understanding Memory Pages and Segmentation #

The Foundation: Virtual Memory #

Modern operating systems employ a sophisticated memory management system known as virtual memory. This system provides each process with the illusion of having its own contiguous address space, independent of the physical memory available. This is achieved through the use of memory pages.

Pages: The Building Blocks of Memory #

A memory page is a fixed-size block of virtual memory. The size of a page is determined by the system architecture and typically ranges from 4KB to 64KB. In most modern Linux systems, the default page size is 4KB. This page size is a critical parameter for understanding how to calculate the page starting address.

Memory Segmentation: Organizing the Address Space #

The virtual address space of a process is often segmented into different regions. Common segments include:

• Text Segment (Code Segment): This segment contains the program’s executable code, including functions like main(). Typically, this segment is read-only and is often located at a low address in the process’s virtual address space.
• Data Segment: This segment holds initialized global and static variables.
• BSS Segment: This segment contains uninitialized global and static variables.
• Heap Segment: This segment is dynamically allocated at runtime using functions like malloc() and new.
• Stack Segment: This segment is used for function calls, local variables, and other temporary data.

Understanding these segments is crucial for locating specific functions or data within a process’s address space.

Verifying the Location of the main() Function #

Based on the provided information, the address 0x400a80 is believed to be within the text segment, where the main() function resides. This assumption is highly probable, given the typical location of the text segment in a Linux executable.

Calculating the Page Starting Address #

The Power of Page Size and Alignment #

The key to determining the page starting address lies in the page size and the concept of page alignment. A page is said to be aligned when its starting address is a multiple of the page size. For example, if the page size is 4KB (4096 bytes), valid page starting addresses would be 0x00000000, 0x00001000, 0x00002000, and so on.

The Bitwise AND Operation #

The most efficient way to calculate the page starting address involves a bitwise AND operation. This operation masks the bits of the given address that are less significant than the page size.

1. Page Size: As mentioned, the standard page size in modern Linux systems is 4KB, which is equivalent to 4096 bytes or 0x1000 in hexadecimal.

2. The Mask: We need a mask that, when ANDed with the address, isolates the page number. The mask is derived by inverting all the bits less significant than the page size in binary representation of the maximum address (e.g., if the page size is 4096, the mask is 0xFFFFF000). You can achieve this by negating the page size and then ANDing.

3. The Calculation: To determine the page starting address from a given virtual address, perform a bitwise AND operation between the given address and the page alignment mask.

   Page Start Address = Virtual Address & (~(Page Size - 1))

   In the context of 0x400a80 and a 4KB page size:

   • Page Size - 1 = 0x1000 - 1 = 0x0FFF
   • ~0x0FFF = 0xFFFFF000
   • 0x400a80 & 0xFFFFF000 = 0x400000

   Therefore, the starting address of the page containing 0x400a80 is 0x400000.

Practical Tools and Techniques in Linux #

Using pmap to Inspect Memory Segments #

The pmap utility is a powerful tool for examining the memory map of a process. It displays the various memory segments allocated to the process, along with their starting addresses, sizes, and permissions.

1. Finding the Process ID (PID): You first need to know the process ID (PID) of the program you are analyzing. You can find this using the ps command or other process management tools.
2. Executing pmap: Once you have the PID, run pmap <PID>. The output will show the memory segments, including their starting addresses and sizes.
3. Interpreting the Output: The output of pmap will list the different memory segments, such as the code segment (often starting with 0x400000 or similar), data segment, stack, and heap.

Verifying the Calculation with pmap #

In your case, pmap shows a segment starting at 0x400000 with a size of 8KB. This confirms that the address 0x400a80 indeed resides within this segment, and our calculation of 0x400000 as the page starting address is correct. pmap offers an easy means to identify the page containing the function.

Using gdb for Detailed Analysis #

The GNU Debugger (gdb) is an indispensable tool for debugging and analyzing programs. It allows you to inspect memory, set breakpoints, examine registers, and step through code.

1. Attaching to the Process: You can attach gdb to a running process or load an executable file for debugging.
2. Examining Memory: Within gdb, you can use the x (examine memory) command to inspect the contents of memory at a specific address.
3. Calculating the Page Start (Within gdb): You can use the p (print) command in gdb to perform the bitwise AND operation directly:

   p (void*)((unsigned long)0x400a80 & 0xFFFFF000)
   

   This will print the page starting address.
4. Verifying Memory Contents: You can then use x/s 0x400000 (assuming the calculation is correct), to view the beginning of the memory in the code segment, which might display the beginning of a function.

Writing a Program to Calculate Page Starting Address #

You can write a simple C program to calculate the page starting address programmatically. This offers a clear and reusable approach:

#include <stdio.h>
#include <stdint.h>

#define PAGE_SIZE 4096

uintptr_t get_page_start(uintptr_t address) {
    return address & ~(PAGE_SIZE - 1);
}

int main() {
    uintptr_t address = 0x400a80;
    uintptr_t page_start = get_page_start(address);

    printf("Address: 0x%lx\n", address);
    printf("Page Start Address: 0x%lx\n", page_start);

    return 0;
}

Compile and run this program:

gcc -o page_calc page_calc.c
./page_calc

The output will confirm the calculation.

Using the /proc Filesystem #

The /proc filesystem provides a wealth of information about running processes, including their memory mappings.

1. /proc/<pid>/maps: This file contains detailed information about the memory mappings of a process, including the start address, end address, permissions, and file backing. You can use this to get the page information.

2. Parsing the Maps File: You can parse the /proc/<pid>/maps file to find the memory region that contains the address of interest. For instance:

   cat /proc/$(pidof your_program)/maps | grep 400a80
   

   This command will search for lines containing the address 0x400a80. The beginning of that line shows the page start.

Addressing the Specific Question: Finding the main() Function #

Locating main()’s Address #

In most standard Linux executables, the main() function is located in the text segment (also known as the code segment).

1. Using objdump: The objdump utility is a powerful tool for examining object files and executables. You can use it to find the address of the main() function.

   objdump -t your_program | grep main
   

   This command will display the symbol table entries, including the address of main().
2. Using nm: The nm utility lists symbols from object files.

   nm your_program | grep main
   

   This is a quicker way to obtain similar information from objdump.
3. Confirming Address: Once you have the address of main(), you can use the techniques described above (pmap, the calculation, gdb) to determine its page starting address.

Verifying the Address within the Code Segment #

After locating the address of main() (e.g., 0x400a80), it is important to verify that this address falls within the text segment. You can confirm this using pmap, by inspecting /proc/<pid>/maps, or using the information obtained from objdump or nm. This confirms that it resides in the memory page starting at 0x400000.

Detailed Example with revWhiteShadow’s Context #

Let’s assume revWhiteShadow’s program is named rev_program and has a PID of 12345.

1. Find the PID: If you don’t already know it:

   pidof rev_program
   # or
   ps aux | grep rev_program
   

2. Get main() address:

   objdump -t rev_program | grep main
   # or
   nm rev_program | grep main
   

   Assume main()’s address is 0x400a80.

3. Determine the Page Start: Using the calculation method: 0x400a80 & 0xFFFFF000 = 0x400000

4. Verify with pmap:

   pmap 12345
   

   You should see a memory segment that contains 0x400a80 and starts at 0x400000.

5. Verify the output of /proc/12345/maps:

   cat /proc/12345/maps | grep 400a80
   

   You should see a line starting with 0x400000, confirming the correct start address.

6. Further Analysis (with gdb):

   gdb rev_program
   (gdb) p (void*)((unsigned long)0x400a80 & 0xFFFFF000)
   (gdb) x/s 0x400000
   

   This will show the contents of the memory page starting at 0x400000, giving insight into the code at that address.

Conclusion #

Obtaining the starting address of a memory page given an address within that page is a critical skill in system programming and reverse engineering. Through understanding the concepts of virtual memory, page size, and the bitwise AND operation, along with the use of tools like pmap, gdb, and the /proc filesystem, we can effectively determine the page starting address. This knowledge is essential for analyzing program behavior, debugging, and understanding memory management in a Linux environment. By applying these techniques, revWhiteShadow can confidently identify the starting address of any page, including the one containing the main() function, gaining valuable insights into program execution.