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by glitchc2 hours ago |
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upvoteby kees992 hours ago|
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Yes, something like this:

  static inline long read(int fd, void *buf, long count) {
    register long rax asm("rax") = __NR_read;
    register long rdi asm("rdi") = (long)fd;
    register long rsi asm("rsi") = (long)buf;
    register long rdx asm("rdx") = count;

    asm volatile(
        "syscall"
        : "+r"(rax)
        : "r"(rdi), "r"(rsi), "r"(rdx)
        : "rcx", "r11", "memory"
    );
    return rax;
  }
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upvoteby asveikau1 hours ago|
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The linux kernel also has its own headers (IIRC it was something like <asm/unistd.h> and/or <sys/syscall.h>, but might depend on architecture and version) where it will provide the stub asm statements.

In my memory the syscall ABI has changed a few times (i386 had int $0x80, then sysenter, then abstracting it in the vdso, then amd64 has 'syscall'), so it may be easier to let the kernel header provide the mechanism.

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upvoteby yjftsjthsd-h29 minutes ago|
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The Linux syscall ABI is (famously) stable; on x86* you can still call into 0x80 just fine. There might be new interfaces made available, but existing programs shouldn't break. Although, in C there could be value in abstracting over the per-platform differences like int/syscall. I've only ever done raw syscalls from assembly where portability was rather moot:)
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upvoteby Chu4eeno2 hours ago|
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It's quite easy to get it to do syscalls directly, I even got Fable to do a simple standalone TLS implementation (in C). Not really useful since it hardcoded a set of supported algorithms etc., but it's fun to see how "simple" you can force it to go.
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