Syscall/Sysenter on Llvm

Syscall/sysenter on LLVM

Posting an answer here since exa has put up a bounty.

I realized this was somewhat a silly question to ask after Ross Ridge's comments, and some playing around with clang.

Let's assume we have the following program, which uses inline assembly to directly call write().

#include <stdio.h>
int main(void)
{
    char *buf = "test\n";
    ssize_t n;
    asm volatile (
        "movl $0x00000002, %%edi\n"  /* first argument == stderr */
        "movl $0x00000006, %%edx\n"  /* third argument == number of bytes */
        "movl $1, %%eax\n"  /* syscall number == write on amd64 linux */
        "syscall\n"
        : "=A"(n)         /* %rax: return value */
        : "S"(buf));      /* %rsi: second argument == address of data to write */
    return n;
}

We can compile this with either gcc or clang and get roughly the same result.

$ gcc -o syscall.gcc syscall.c
$ clang -o syscall.clang syscall.c
$ ./syscall.gcc
test
$ ./syscall.clang
test

If we wish to see the exact LLVM instructions which would be used to emit this code, we can simply use the -emit-llvm flag. As you can see, there is a call i64 asm sideeffect line which has the full inline assembly string.

$ clang -S -emit-llvm syscall.c
$ cat syscall.ll
; ModuleID = 'syscall.c'
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"

@.str = private unnamed_addr constant [6 x i8] c"test\0A\00", align 1

; Function Attrs: nounwind uwtable
define i32 @main() #0 {
  %1 = alloca i32, align 4
  %buf = alloca i8*, align 8
  %n = alloca i64, align 8
  store i32 0, i32* %1
  store i8* getelementptr inbounds ([6 x i8]* @.str, i32 0, i32 0), i8** %buf, align 8
  %2 = load i8** %buf, align 8
  %3 = call i64 asm sideeffect "movl $$0x00000002, %edi\0Amovl $$0x00000006, %edx\0Amovl $$1, %eax\0Asyscall\0A", "=A,{si},~{dirflag},~{fpsr},~{flags}"(i8* %2) #1, !srcloc !1
  store i64 %3, i64* %n, align 8
  %4 = load i64* %n, align 8
  %5 = trunc i64 %4 to i32
  ret i32 %5
}

attributes #0 = { nounwind uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "stack-protector-buffer-size"="8" "unsafe-fp-math"="false" "use-soft-float"="false" }
attributes #1 = { nounwind }

!llvm.ident = !{!0}

!0 = metadata !{metadata !"Ubuntu clang version 3.5-1ubuntu1 (trunk) (based on LLVM 3.5)"}
!1 = metadata !{i32 134, i32 197, i32 259, i32 312}

OS-independent LLVM IR system calls

Absolutely not. LLVM is a compiler backend; it does not concern itself with system calls. System calls are usually employed inside the platform's C library, which implements them with a mixture of low-level C and target-specific assembly. System calls are both OS and target (CPU) dependent.

As for more materials on studying this stuff - you have my sympathy. It's not a well documented area, because 99.9% of programmers never need to operate at this level. I suggest you start picking up some basic assembly programming and go from there.

Can LLVM-based languages be used in OS development?

LLVM is an abstract machine. As such, it does not directly allow you to access certain hardware registers. However, you can still use inline assembly (through the call asm LLVM bitcode mnemonic) or program the few functions that need access to fixed hardware registers in assembly and call them from your LLVM code.

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