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187 lines
8 KiB
Markdown
187 lines
8 KiB
Markdown
# Under the hood: System Calls
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The term "function" is used many places in this book and needs no
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additional explanation here. The term "system call" has also been used
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in many places, often with a comment that making a system call through
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the C runtime is actually just calling an ordinary function acting as
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a wrapper. An explanation of this has been promised...
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## Wrappers
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"Wrapper" is a term used to describe a function which hides the details
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of something else, often another function or functions. Hiding details
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is a form of abstraction and can be a good thing. Broadly speaking,
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an API (Application Programmer's Interface) is itself another example
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of wrappers in common use.
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## C runtime as a wrapper
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Many C runtime functions are just wrappers for system calls. For example
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if you call `open()` from the C runtime, the function will perform a few
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bookkeeping operations and then make the actual system call.
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## What IS a system call?
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The short answer is a system call is a sort-of function call that is
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serviced by the operating system itself, within its own private region
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of memory and with access to internal features and data structures.
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Our programs run in "userland". The technical name for userland on the
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ARM64 processor is EL0 (Exception Level 0).
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We can operate within the kernel's space only through carefully
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controlled mechanisms - such as system calls. The technical name for
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where the kernel (or system) generally operates is called EL1.
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There are two higher Exception Levels (EL2 and EL3) which are beyond
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the scope of this book.
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## Mechanism of making a system call
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First, like any function call, parameters need to be set up. The first
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parameter goes in the first register, etc.
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Second, a number associated with the specific system call we wish to
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make is loaded in a specific register (`w8`).
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Finally, a special instruction `svc` causes a trap which elevates us out
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of userland into kernel space. Said differently, `svc` causes a
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transition from EL0 to EL1. There, various checks are done and the
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actual code for the system call is run.
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A description of returning from a system call is beyond the scope of
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this book. Hint: just as there's a special instruction that escalates
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from EL0 to EL1, there is a special instruction that does the reverse.
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## What is the number associated with a particular system call?
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Hard question.
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In a perfect world, each Linux distribution would use the same set of
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system call numbers. But no.
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[This](https://marcin.juszkiewicz.com.pl/download/tables/syscalls.html)
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is the most comprehensive list of system call numbers we have seen. It
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shows system call numbers for many architectures and distributions.
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## Example: calling `getpid()`
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The system call `getpid()` fetches the running process's process ID.
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Every executing entity has one.
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We present four different versions of the same program:
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1. Written in C++
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2. Written in C
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3. Written using C runtime from assembly language
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4. Calling the system call directly from assembly language
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### Written in C++
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```c++
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#include <iostream> // 1
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#include <unistd.h> // 2
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// 3
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using std::cout; // 4
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using std::endl; // 5
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// 6
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int main() { // 7
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cout << "Greetings from: " << getpid() << endl; // 8
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return 0; // 9
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} // 10
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```
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### Written in C
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```c
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#include <stdio.h> // 1
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#include <unistd.h> // 2
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// 3
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int main() { // 4
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printf("Greetings from: %d\n", getpid()); // 5
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return 0; // 6
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} // 7
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```
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### Written in assembly language using C runtime
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```text
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.global main // 1
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.text // 2
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.align 2 // 3
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// 4
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main: stp x29, x30, [sp, -16]! // 5
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bl getpid // 6
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mov w1, w0 // 7
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ldr x0, =fmt // 8
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bl printf // 9
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ldp x29, x30, [sp], 16 // 10
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mov w0, wzr // 11
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ret // 12
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// 13
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.data // 14
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fmt: .asciz "Greetings from: %d\n" // 15
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// 16
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.end // 17
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```
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### And finally: calling the system call directly
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```text
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.global main // 1
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.text // 2
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.align 2 // 3
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// 4
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main: stp x29, x30, [sp, -16]! // 5
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mov x8, 172 // getpid on ARM64 // 6
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svc 0 // trap to EL1 // 7
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mov w1, w0 // 8
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ldr x0, =fmt // 9
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bl printf // 10
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ldp x29, x30, [sp], 16 // 11
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mov w0, wzr // 12
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ret // 13
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// 14
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.data // 15
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fmt: .asciz "Greetings from: %d\n" // 16
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// 17
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.end // 18
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```
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We chose `getpid()` because it doesn't require any parameters. Using
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the C runtime, we simply `bl` to it. Calling the system call directly
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is different in that we must first load `x8` with the number that
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corresponds to `getpid()` for the AARCH64 architecture.
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Consulting [this](https://marcin.juszkiewicz.com.pl/download/tables/syscalls.html)
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awesome website, we find that the number we want is 172.
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The constant specific to the system call we want is loaded into `x8`.
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Recall that `x0` through `x7` are scratch registers.
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Then on line 7, the `svc` with the argument 0 initiates the escalation
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from EL0 to EL1 where the kernel implements our desired functionality
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and returns to us.
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### Review
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System calls are functions implemented inside the operating system.
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To get there, at some point perhaps behind a wrapper function found in
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the CRT (C Run Time library), a distro specific system call number is
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placed in `x8` with other scratch registers getting the system call's
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documented parameters and the `svc` instruction is executed with
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argument 0.
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### Preference
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We suggest using the CRT wrapper functions where possible because:
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* They are easier to code
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* They are portable between distributions of the OS
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