I am wondering what load-time relocation actually means on a system with virtual memory support.I was thinking that in a system with virtual memory every executable will have addresses starting from zero and at run-time the addresses will be translated into physical addresses using page tables.Therefore the executable can be loaded anywhere in memory without the need of any relocation. However this article on shared libraries mentions that linker specifies an address in the executable where the executable is to be loaded (Entry-point address).
http://eli.thegreenplace.net/2011/08/25/load-time-relocation-of-shared-libraries/
Also there are many articles on dynamic linking which talk about absolute addresses.
Is my understanding wrong ?
Load-time relocation and virtual memory support are two different concepts. Almost all CPUs and OSes these days have virtual memory support. The only really important point to understand about virtual memory is this: forget physical addresses. That is now a hardware and OS responsibility and, unless you are writing a paging system, you can forget about physical addresses. All addresses that a program uses are virtual addresses. This is a huge advantage and immensely simplifies the programming model. On 32-bit systems, this simply means that each process gets its own 4 GiB memory space, ranging from 0x00000000 to 0xffffffff.
An .exe represents a process. A linker produces .exe from .obj files. While both are binary files, .obj files are not executable because they do not contain the addresses of all the variables and functions. It is the job of the linker to provide these addresses, which it determines by placing these .obj files end-to-end and then computing the exact addresses of all the symbols (functions and variables). Thus, the .exe that is created has every address of functions and variables "hard-coded" into it. But there is still one critical information needed before the .exe can be created. The linker has to have insider knowledge about where in memory the .exe will be loaded. Will it be at address 0x00000000, or at 0xffff0000, or somewhere else? For example, in Windows all .exes are always loaded at an absolute starting address of 0x00400000. This is called the base address. When the linker generates the final addresses of symbols (functions and variables), it computes those from this address onward.
Now, .exes rarely need to be loaded at any other address. But the same is not true for .dlls. .ddls are the same as .exes (both are formatted in the portable executable (PE) file format, which describes the memory layout, for example, where text goes, where data goes, and how to find which one). .dlls have a preferred address, too. This simply means that the linker uses this value when it computes the addresses for symbols inside the .dll. If the .dll is loaded at this address, then we are all set.
But if the .dll cannot be loaded at this address (say it was 0x10000000) because some other .dll had already been loaded at this address, then the loader will find some other space in memory and load the .dll there. However, the global addresses of functions and symbols in the .dll are now incorrect. Thus, the loader has to do a relocation (also called "fixup"), in which it adjusts the addresses of all global symbols and functions to reflect their actual addresses.
In order to do this adjustment, the loader needs to be able to find all such symbols in the .dll. The PE file has a .reloc section that contains the internal offset of all such symbols.
Of course, there are other details, for example, regarding how indirection can be used when the compiler generated the code so that, instead of making direct calls, the calls are indirect and variables are accessed via known memory locations in the header of the .exe.
Finally, the gist is this: You need relocation (of some sort) to adjust addresses in the call and jump as well as variable access instructions when the code does not load at the position (within the 4 GiB address space) it was expected to load. When the OS loads a .exe, it has to pick a suitable place in this 4 GiB address space where it will copy the code and data chunks from this .exe on disk.
Related
I was reading about how programs get loaded into memory. I wanted to understand how different sections (like text,data,rodata etc.) of a PE/ELF file are mapped at different places in the virtual address space of a process. I particularly wanted to understand how does the linker know where will a particular section (say for example rodata) will be mapped onto the address space, so that it can resolve correct addresses for all the symbols before creating the executable.
Do operating systems (eg. Windows) have a fixed range of virtual addresses for a process where they load/map a particular section? If not, then how will the linker resolve the correct addresses for the symbols in different sections?
It doesn't. Linker can only propose the executable image be loaded at certain VA, usually 0x00400000 for PE or 0x10000000 for DLL. Virtual adresses of sections (.text, .rodata, .data etc) are aligned by the section alignment (usually 0x00001000) and their proposed VA are therefore 0x00401000, 0x00402000 etc. Linker then fixes adresses of symbols to those assumed VAs.
The default ImageBase address (taken from linker script or linker arguments) is not required by OS loader, but I don't see a reason to change it, its a good habit to see nice rounded addresses during debugging.
In rare cases the Windows loader may find out that part of the proposed address space is occupied, so it will load the image at a different VA and fix position-dependent (absolute) references to their new VA.
Special PE section relocs contains addresses of references to program symbols which need relocation at load-time.
I'm learning about the layout of executable binaries. My end goal is to analyze a specific executable for things that could be refactored (in its source) to reduce the compiled output size.
I've been using https://www.embeddedrelated.com/showarticle/900.php and https://www.geeksforgeeks.org/memory-layout-of-c-program/ as references for this initial learning.
From what I've learned, a linker script specifies the addresses where sections of compiled binaries are placed. E.g.
> ld --verbose | grep text
PROVIDE (__executable_start = SEGMENT_START("text-segment", 0x400000)); . = SEGMENT_START("text-segment", 0x400000) + SIZEOF_HEADERS;
*(.rela.text .rela.text.* .rela.gnu.linkonce.t.*)
I think this means that the text segments of compiled binaries starts at memory address 0x400000 - true?
What does that value, 0x400000, represent? I'm probably not understanding something properly, but surely that 0x400000 does not represent a physical memory location, does it? E.g. if I were to run two instances of my compiled a.out executable in parallel, they couldn't both simultaneously occupy the space at 0x400000, right?
0x4000000 is not a physical address in the sense how your memory chips see it. This is a virtual address as it's seen from CPU's point of view.
Loader of your program will map a few pages of physical memory to VA 0x400000 and copy the contents of text-segment to it. And yes, another instance of your program could occupy the same physical and virtual block of memory for the text-segment, because text (code) is readable and executable but not writeable. Other segments (data, bss, stack, heap) may have identical VA but each will be mapped to their private protected physical block of memory.
What is 0x400000
I think this means that the text segments of compiled binaries starts at memory address 0x400000 - true?
No, this is well explained in the official documentation at: https://sourceware.org/binutils/docs/ld/Builtin-Functions.html
SEGMENT_START(segment, default)
Return the base address of the named segment. If an explicit value has already been given for this segment (with a command-line ‘-T’ option) then that value will be returned otherwise the value will be default. At present, the ‘-T’ command-line option can only be used to set the base address for the “text”, “data”, and “bss” sections, but you can use SEGMENT_START with any segment name.
Therefore, SEGMENT_START is not setting the address, but rather it is returning it, and 0x4000000 in your case is just the default if that value was not deterministically set by some CLI mechanism mentioned in the documentation (e.g. -Ttext=0x200 as mentioned in man ld)
Physical vs virtual
As you've said, doing things in physical addresses is very uncommon in userland, and would at the very least always require sudo as it would break process separation. Here is an example of userland doing physical address stuff for example: How to access physical addresses from user space in Linux?
Therefore, when the kernel loads an ELF binary with the exec syscalls, all addresses are interpreted as virtual addresses.
Note however that this is just a matter of convention. For example, when I give my Linux kernel ELF binary for QEMU to load into memory to start simulation, or when a bootloader does that in a real system, the ELF addresses would then be treated as physical addresses since there is no page table available at that point.
I just learned about different memory segments like: Text, Data, Stack and Heap. My question is:
1- Where the boundaries between these sections are defined? Is it in Compiler or OS?
2- How the compiler or OS know which addresses belong to each section? Should we define it anywhere?
This answer is from the point of view of a more special-purpose embedded system rather than a more general-purpose computing platform running an OS such as Linux.
Where the boundaries between these sections are defined? Is it in Compiler or OS?
Neither the compiler nor the OS do this. It's the linker that determines where the memory sections are located. The compiler generates object files from the source code. The linker uses the linker script file to locate the object files in memory. The linker script (or linker directive) file is a file that is a part of the project and identifies the type, size and address of the various memory types such as ROM and RAM. The linker program uses the information from the linker script file to know where each memory starts. Then the linker locates each type of memory from an object file into an appropriate memory section. For example, code goes in the .text section which is usually located in ROM. Variables go in the .data or .bss section which are located in RAM. The stack and heap also go in RAM. As the linker fills one section it learns the size of that section and can then know where to start the next section. For example, the .bss section may start where the .data section ended.
The size of the stack and heap may be specified in the linker script file or as project options in the IDE.
IDEs for embedded systems typically provide a generic linker script file automatically when you create a project. The generic linker file is suitable for many projects so you may never have to customize it. But as you customize your target hardware and application further you may find that you also need to customize the linker script file. For example, if you add an external ROM or RAM to the board then you'll need to add information about that memory to the linker script so that the linker knows how to locate stuff there.
The linker can generate a map file which describes how each section was located in memory. The map file may not be generated by default and you may need to turn on a build option if you want to review it.
How the compiler or OS know which addresses belong to each section?
Well I don't believe the compiler or OS actually know this information, at least not in the sense that you could query them for the information. The compiler has finished its job before the memory sections are located by the linker so the compiler doesn't know the information. The OS, well how do I explain this? An embedded application may not even use an OS. The OS is just some code that provides services for an application. The OS doesn't know and doesn't care where the boundaries of memory sections are. All that information is already baked into the executable code by the time the OS is running.
Should we define it anywhere?
Look at the linker script (or linker directive) file and read the linker manual. The linker script is input to the linker and provides the rough outlines of memory. The linker locates everything in memory and determines the extent of each section.
For your Query :-
Where the boundaries between these sections are defined? Is it in Compiler or OS?
Answer is OS.
There is no universally common addressing scheme for the layout of the .text segment (executable code), .data segment (variables) and other program segments. However, the layout of the program itself is well-formed according to the system (OS) that will execute the program.
How the compiler or OS know which addresses belong to each section? Should we define it anywhere?
I divided your this question into 3 questions :-
About the text (code) and data sections and their limitation?
Text and Data are prepared by the compiler. The requirement for the compiler is to make sure that they are accessible and pack them in the lower portion of address space. The accessible address space will be limited by the hardware, e.g. if the instruction pointer register is 32-bit, then text address space would be 4 GiB.
About Heap Section and limit? Is it the total available RAM memory?
After text and data, the area above that is the heap. With virtual memory, the heap can practically grow up close to the max address space.
Do the stack and the heap have a static size limit?
The final segment in the process address space is the stack. The stack takes the end segment of the address space and it starts from the end and grows down.
Because the heap grows up and the stack grows down, they basically limit each other. Also, because both type of segments are writeable, it wasn't always a violation for one of them to cross the boundary, so you could have buffer or stack overflow. Now there are mechanism to stop them from happening.
There is a set limit for heap (stack) for each process to start with. This limit can be changed at runtime (using brk()/sbrk()). Basically what happens is when the process needs more heap space and it has run out of allocated space, the standard library will issue the call to the OS. The OS will allocate a page, which usually will be manage by user library for the program to use. I.e. if the program wants 1 KiB, the OS will give additional 4 KiB and the library will give 1 KiB to the program and have 3 KiB left for use when the program ask for more next time.
Most of the time the layout will be Text, Data, Heap (grows up), unallocated space and finally Stack (grows down). They all share the same address space.
The sections are defined by a format which is loosely tied to the OS. For example on Linux you have ELF and on Mac OS you have Mach-O.
You do not define the sections explicitly as a programmer, in 99.9% of cases. The compiler knows what to put where.
Virtual memory along with logical memory helps to make sure programs do not corrupt each others data.
Program relocation does an almost similar thing of making sure that multiple programs does not corrupt each other.Relocation modifies object program so that it can be loaded at a new, alternate address.
How are virtual memory, logical memory and program relocation related ? Are they similar ?
If they are same/similar, then why do we need program relocation ?
Relocatable programs, or said another way position-independent code, is traditionally used in two circumstances:
systems without virtual memory (or too basic virtual memory, e.g. classic MacOS), for any code
for dynamic libraries, even on systems with virtual memory, given that a dynamic library could find itself lodaded on an address that is not its preferred one if other code is already at that space in the address space of the host program.
However, today even main executable programs on systems with virtual memory tend to be position-independent (e.g. the PIE* build flag on Mac OS X) so that they can be loaded at a randomized address to protect against exploits, e.g. those using ROP**.
* Position Independent Executable
** Return-Oriented Programming
Virtual memory does not prevent programs from interfering with out other. It is logical memory that does so. Unfortunately, it is common for the two concepts to be conflated to "virtual memory."
There are two types of relocation and it is not clear which you are referring to. However, they are connected. On the other hand, the concept is not really related to virtual memory.
The first concept of relocatable code. This is critical for shared libraries that usually have to be mapped to different addresses.
Relocatable code uses offsets rather than absolute addresses. When a program results in an instruction sequence something like:
JMP SOMELABEL
. . .
SOMELABEL:
The computer or assembler encodes this as
JUMP the-number-of-bytes-to-SOMELABEL
rather than
JUMP to-the-address-of-somelabel.
By using offsets the code works the same way no matter where the JMP instruction is located.
The second type of relocation uses the first. In the past relocation was mostly used for libraries. Now, some OS's will load program segments at different places in memory. That is intended for security. It is designed to keep malicious cracks that depend upon the application being loaded at a specific address.
Both of these concepts work with or without virtual memory.
Note that generally the program is not modified to relocated it. I generally, because an executable file will usually have some addresses that need to be fixed up at run time.
I am writing a simple user-space ELF loader under Linux (why? for 'fun'). My loader at the moment is quite simple and is designed to load only statically-linked ELF files containing position-independent code.
Normally, when a program is loaded by the kernel's ELF loader, it is loaded into its own address space. As such, the data segment and code segment can be loaded at the correct virtual address as specified in the ELF segments.
In my case, however, I am requesting addresses from the kernel via mmap, and may or may not get the addresses requested in the ELF segments. This is not a problem for the code segment since it is position independent. However, if the data segment is not loaded at the expected address, code will not be able to properly reference anything stored in the data segment.
Indeed, my loader appears to work fine with a simple assembly executable that does not contain any data. But as soon as I add a data segment and reference it, the executable fails to run correctly or SEGFAULTs.
How, if possible, can I fixup any references to the data segment to point to the correct place? Is there a relocation section stored in the (static) ELF file for this purpose?
If you modify the absolute addresses available in the .got section, (global offset table) your program should work. Make sure to modify the absolute address calculation to cater for the new distance between .text and .data, I'm afraid you need to figure out where this information comes from, for your architecture.
See this: Global Offset Table (Processor-Specific)
Good luck.
I don't see any way you can do that, unless you emulate the kernel-provided virtual address space completely, and run the code inside that virtual space. When you mmap the data section from the file, you are intrinsically relocating it to an unknown address of the virtual address space of your ELF interpreter, and your code will not be able to reference to it in any way.
Glad to be proven wrong. There's something very cool to learn here.