Category Archives: Computers

Memory is an abstraction

I was learning about the different types of RAM recently (e.g. SRAM and DRAM) and it occurred to me that “computer memory” is just an abstraction. This is obvious once you think about it but I think as a programmer, it’s very easy to not realize this. The idea of memory as a linear array of bits is an abstraction created and implemented by an electrical device.

Most programmers when they think of memory are thinking of virtual memory, which is a completely different abstraction. While it’s also a linear array of bits, the abstraction is created by the operating system and lives at a higher level.

One level below, the abstraction the operating system itself uses โ€” “physical memory” โ€” is the one I’m talking about, created by a a set of electrical devices connected to the CPU with wires (the memory bus).

I’m projecting without any basis, but I presume the reason so few programmers think of memory as an abstraction is because the abstraction is so strong. Nearly all of the time, it “just works” โ€” you write bits and read them out later.1 The abstraction can leak slightly during programming disciplines that require awareness of low level details like the memory hierarchy and cache coherence (i.e. lockfree) but this is a leak of the abstraction of the memory hierarchy. The core abstraction of physical memory stays in tact โ€” for example, programmers never need to be aware of the internal refresh mechanism of DRAM.2

Of course, you can go infinitely far with this โ€” it’s turtles all the way down.

Syscall ABI compatibility: Linux vs Windows/macOS

The Linux kernel has an interesting difference compared to the Windows and macOS kernels: it offers syscall ABI compatibility.

This means that applications that program directly against the raw syscall interface are more or less guaranteed to always keep working, even with arbitrarily newer kernel versions. “Programming against the raw syscall interface” means including assembly code in your app that triggers syscalls:

  • setting the appropriate syscall number in the syscall register
  • setting arguments in the defined argument registers
  • executing a syscall instruction
  • reading the syscall return value register

Here are the ABIs for some common architectures.

Syscall Number RegisterSyscall ArgumentsSyscall Return Value
x86EAXEBX, ECX, EDX, ESI, EDI, EBPEAX
x86_64RAXRDI, RSI, RDX, R10, R8, R9RAX
Armv7R7R0-R6R0
AArch64X8X0-X5X0
Manticore is my go-to source to quickly look these up: https://github.com/trailofbits/manticore

Once you’ve done this, now you’re relying on the kernel to not change any part of this. If the kernel changes any of these registers, or changes the syscall number mapping, your app will not longer trigger the desired syscall correctly and will break.

Aside from writing raw assembly in your app, there’s a more innocuous way of accidentally “programming directly against the syscall interface”: statically linking to libc. When you statically link to a library, that library’s code is directly included in your binary. libc is generally the system component responsible for implementing the assembly to trigger syscalls, and by statically linking to it, you effectively inline those assembly instructions directly into your application.

So why does Linux offer this and Windows and macOS don’t?

In general, compatibility is cumbersome. As a developer, if you can avoid having to maintain compatibility, it’s better. You have more freedom to change, improve, and refactor in the future. So by default it’s preferable to not maintain compatibility โ€” including for kernel development.

Windows and macOS are able to not offer compatibility because they control the libc for their platforms and the rules for using it. And one of their rules is “you are not allowed to statically link libc”. For the exact reason that this would encourage apps that depend directly on the syscall ABI, hindering the kernel developers’ ability to freely change the kernel’s implementation.

If all app developers are forced to dynamically link against libc, then as long as kernel developers also update libc with the corresponding changes to the syscall ABI, everything works. Old apps run on a new kernel will dynamically link against the new libc, which properly implements the new ABI. Compatibility is of course still maintained at the app/libc level โ€” just not at the libc/kernel level.

Linux doesn’t control the libc in the same way Windows and macOS do because in the Linux world, there is a distinct separation between kernel and userspace that isn’t present in commercial operating systems. Strictly speaking Linux is just the kernel, and you’re free to run whatever userspace on top. Most people run GNU userspace components (glibc), but alternatives are not unheard of (musl libc, also bionic libc on Android).

So because Linux kernel developers can’t 100% control the libc that resides on the other end of the syscall interface, they bite the bullet and retain ABI compatibility. This technically allows you to statically link with more confidence than on other OSs. That said, there are other reasons why you shouldn’t statically link libc, even on Linux.


Links:

https://news.ycombinator.com/item?id=21908824
https://www.kernel.org/doc/Documentation/ABI/README

This directory documents the interfaces that the developer has
defined to be stable.  Userspace programs are free to use these
interfaces with no restrictions, and backward compatibility for
them will be guaranteed for at least 2 years.  Most interfaces
(like syscalls) are expected to never change and always be
available.

kernel docs

What:		The kernel syscall interface
Description:
	This interface matches much of the POSIX interface and is based
	on it and other Unix based interfaces.  It will only be added to
	over time, and not have things removed from it.

	Note that this interface is different for every architecture
	that Linux supports.  Please see the architecture-specific
	documentation for details on the syscall numbers that are to be
	mapped to each syscall.

apple developer docs

Q:  I'm trying to link my binary statically, but it's failing to link because it can't find crt0.o. Why?
A: Before discussing this issue, it's important to be clear about terminology:

A static library is a library of code that can be linked into a binary that will, eventually, be dynamically linked to the system libraries and frameworks.
A statically linked binary is one that does not import system libraries and frameworks dynamically, but instead makes direct system calls into the kernel.
Apple fully supports static libraries; if you want to create one, just start with the appropriate Xcode project or target template.

Apple does not support statically linked binaries on Mac OS X. A statically linked binary assumes binary compatibility at the kernel system call interface, and we do not make any guarantees on that front. Rather, we strive to ensure binary compatibility in each dynamically linked system library and framework.

If your project absolutely must create a statically linked binary, you can get the Csu (C startup) module from Darwin and try building crt0.o for yourself. Obviously, we won't support such an endeavor.

stackoverflow

  • Solaris also stopped supporting static linking against libc.