linux 不同进程 maps,linux的进程地址空间映射图解

转自：http://duartes.org/gustavo/blog/post/anatomy-of-a-program-in-memory

整理备忘

Anatomy of a Program in Memory

Memory management is the heart of operating systems; it is crucial

for both programming and system administration. In the next few posts

I’ll cover memory with an eye towards practical aspects, but without

shying away from internals. While the concepts are generic, examples are

mostly from Linux and Windows on 32-bit x86. This first post describes

how programs are laid out in memory.

Each process in a multi-tasking OS runs in its own memory sandbox. This sandbox is the virtual address space, which in 32-bit mode is always a 4GB block of memory addresses. These virtual addresses are mapped to physical memory by page tables,

which are maintained by the operating system kernel and consulted by

the processor. Each process has its own set of page tables, but there is

a catch. Once virtual addresses are enabled, they apply to all software running in the machine, including the kernel itself. Thus a portion of the virtual address space must be reserved to the kernel:

This does not mean the kernel uses that much

physical memory, only that it has that portion of address space

available to map whatever physical memory it wishes. Kernel space is

flagged in the page tables as exclusive to privileged code

(ring 2 or lower), hence a page fault is triggered if user-mode

programs try to touch it. In Linux, kernel space is constantly present

and maps the same physical memory in all processes. Kernel code and data

are always addressable, ready to handle interrupts or system calls at

any time. By contrast, the mapping for the user-mode portion of the

address space changes whenever a process switch happens:

Blue regions represent virtual addresses that are mapped to physical

memory, whereas white regions are unmapped. In the example above,

Firefox has used far more of its virtual address space due to its

legendary memory hunger. The distinct bands in the address space

correspond to memory segments like the heap, stack, and so on. Keep in mind these segments are simply a range of memory addresses and have nothing to do with Intel-style segments. Anyway, here is the standard segment layout in a Linux process:

When computing was happy and safe and cuddly, the starting virtual addresses for the segments shown above were exactly the same

for nearly every process in a machine. This made it easy to exploit

security vulnerabilities remotely. An exploit often needs to reference

absolute memory locations: an address on the stack, the address for a

library function, etc. Remote attackers must choose this location

blindly, counting on the fact that address spaces are all the same. When

they are, people get pwned. Thus address space randomization has become

popular. Linux randomizes the , , and

by adding offsets to their starting addresses. Unfortunately the 32-bit

address space is pretty tight, leaving little room for randomization

and .

The topmost segment in the process address space is the stack, which

stores local variables and function parameters in most programming

languages. Calling a method or function pushes a new stack frame

onto the stack. The stack frame is destroyed when the function returns.

This simple design, possible because the data obeys strict

order, means that no complex data structure is needed to track stack

contents – a simple pointer to the top of the stack will do. Pushing and

popping are thus very fast and deterministic. Also, the constant reuse

of stack regions tends to keep active stack memory in the cpu caches, speeding up access. Each thread in a process gets its own stack.

It is possible to exhaust the area mapping the stack by pushing more

data than it can fit. This triggers a page fault that is handled in

Linux by , which in turn calls to check whether it’s appropriate to grow the stack. If the stack size is below RLIMIT_STACK

(usually 8MB), then normally the stack grows and the program continues

merrily, unaware of what just happened. This is the normal mechanism

whereby stack size adjusts to demand. However, if the maximum stack size

has been reached, we have a stack overflow and the

program receives a Segmentation Fault. While the mapped stack area

expands to meet demand, it does not shrink back when the stack gets

smaller. Like the federal budget, it only expands.

Dynamic stack growth is the

in which access to an unmapped memory region, shown in white above,

might be valid. Any other access to unmapped memory triggers a page

fault that results in a Segmentation Fault. Some mapped areas are

read-only, hence write attempts to these areas also lead to segfaults.

Below the stack, we have the memory mapping segment. Here the kernel

maps contents of files directly to memory. Any application can ask for

such a mapping via the Linux system call () or /

in Windows. Memory mapping is a convenient and high-performance way to

do file I/O, so it is used for loading dynamic libraries. It is also

possible to create an anonymous memory mapping that

does not correspond to any files, being used instead for program data.

In Linux, if you request a large block of memory via , the C library will create such an anonymous mapping instead of using heap memory. ‘Large’ means larger than MMAP_THRESHOLD bytes, 128 kB by default and adjustable via mallopt().

Speaking of the heap, it comes next in our plunge into address space.

The heap provides runtime memory allocation, like the stack, meant for

data that must outlive the function doing the allocation, unlike the

stack. Most languages provide heap management to programs. Satisfying

memory requests is thus a joint affair between the language runtime and

the kernel. In C, the interface to heap allocation is and friends, whereas in a garbage-collected language like C# the interface is the new keyword.

If there is enough space in the heap to satisfy a memory request, it

can be handled by the language runtime without kernel involvement.

Otherwise the heap is enlarged via the system call () to make room for the requested block. Heap management is ,

requiring sophisticated algorithms that strive for speed and efficient

memory usage in the face of our programs’ chaotic allocation patterns.

The time needed to service a heap request can vary substantially.

Real-time systems have to deal with this problem. Heaps also become fragmented, shown below:

Finally, we get to the lowest segments of memory: BSS, data, and

program text. Both BSS and data store contents for static (global)

variables in C. The difference is that BSS stores the contents of uninitialized

static variables, whose values are not set by the programmer in source

code. The BSS memory area is anonymous: it does not map any file. If you

say static int cntActiveUsers, the contents of cntActiveUsers live in the BSS.

The data segment, on the other hand, holds the contents for static variables initialized in source code. This memory area is not anonymous. It maps the part of the program’s binary image that contains the initial static values given in source code. So if you say static int cntWorkerBees = 10, the contents of cntWorkerBees live in the data segment and start out as 10. Even though the data segment maps a file, it is a private memory mapping,

which means that updates to memory are not reflected in the underlying

file. This must be the case, otherwise assignments to global variables

would change your on-disk binary image. Inconceivable!

The data example in the diagram is trickier because it uses a pointer. In that case, the contents of pointer gonzo

– a 4-byte memory address – live in the data segment. The actual string

it points to does not, however. The string lives in the text

segment, which is read-only and stores all of your code in addition to

tidbits like string literals. The text segment also maps your binary

file in memory, but writes to this area earn your program a Segmentation

Fault. This helps prevent pointer bugs, though not as effectively as

avoiding C in the first place. Here’s a diagram showing these segments

and our example variables:

You can examine the memory areas in a Linux process by reading the file /proc/pid_of_process/maps.

Keep in mind that a segment may contain many areas. For example, each

memory mapped file normally has its own area in the mmap segment, and

dynamic libraries have extra areas similar to BSS and data. The next

post will clarify what ‘area’ really means. Also, sometimes people say

“data segment” meaning all of data + bss + heap.

You can examine binary images using the and

commands to display symbols, their addresses, segments, and so on.

Finally, the virtual address layout described above is the “flexible”

layout in Linux, which has been the default for a few years. It assumes

that we have a value for RLIMIT_STACK. When that’s not the case, Linux reverts back to the “classic” layout shown below:

That’s it for virtual address space layout. The next post discusses

how the kernel keeps track of these memory areas. Coming up we’ll look

at memory mapping, how file reading and writing ties into all this and

what memory usage figures mean.