Understanding Dynamic Library loading in Linux

Question

I am trying to understand Dynamic Library loading in Linux from here [1] and want to clarify the concept. Concretely, when a dynamic library is loaded in a process in a Linux environment, it is loaded at any point in the address space. Now, a library has a code segment, and a data segment. The code segment's address is not defined pre-linking so it is 0x0000000 while for data segment, some number is defined to be an address.

But here is the trick, this address of data segment is not actually the true address. Actually, at whatever position code segment is loaded, data segment's pre-defined address is added to it.

Am I correct here?

One more thing from the referenced article. What does this statement mean?

However, we have the constraint that the shared library must still have a unqiue data instance in each process. While it would be possible to put the library data anywhere we want at runtime, this would require leaving behind relocations to patch the code and inform it where to actually find the data — destroying the always read-only property of the code and thus sharability.

[1] http://www.technovelty.org/linux/plt-and-got-the-key-to-code-sharing-and-dynamic-libraries.html

Answer 1

Actually, at whatever position code segment is loaded, data segment's pre-defined address is added to it.

Yes. The "VirtAddr" of the data segment will be added to base address.

What does this statement mean?

It means that when library accesses its own static data, we should not use relocations in the library code. Otherwise linker may need to patch the binary code, which leads to unsharing some parts of library codes between processes (if process1 loads library lib1 at 0x40000000, and process2 loads lib1 at 0x50000000, their data relocations will be different).

So, different solution is used in real life. Both library code and data are loaded together, and the offset between code and data is fixed for all cases. There is the "solution" after text you cited: http://www.technovelty.org/linux/plt-and-got-the-key-to-code-sharing-and-dynamic-libraries.html

As you can see from the above headers, the solution is that the read-write data section is always put at a known offset from the code section of the library. This way, via the magic of virtual-memory, every process sees its own data section but can share the unmodified code. All that is needed to access data is some simple maths; address of thing I want = my current address + known fixed offset.