66 lines
2.1 KiB
Zig
66 lines
2.1 KiB
Zig
const std = @import("std");
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const linux = std.os.linux;
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const clone = linux.CLONE;
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var child_stack: [4096 * 4]u8 align(16) = undefined;
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pub fn main() !void {
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// SIGCHLD: Send signal to parent on exit (required for waitpid)
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const flags = clone.VM | clone.FILES | clone.FS | clone.SIGHAND | linux.SIG.CHLD;
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// Stack grows downwards. Point to the end.
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const stack_top = @intFromPtr(&child_stack) + child_stack.len;
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const msg = "Child: Hello\n";
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const msg_len = msg.len;
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// We use inline assembly to perform the clone syscall and handle the child path completely to
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// avoid the compiler generating code that relies on the parent's stack frame in the child
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// process (where the stack is empty).
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const ret = asm volatile (
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\\ syscall
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\\ test %%rax, %%rax
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\\ jnz 1f
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\\
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\\ # Child Path
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\\ # Write to stdout
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\\ mov $1, %%rdi # fd = 1 (stdout)
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\\ mov %[msg], %%rsi # buffer
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\\ mov %[len], %%rdx # length
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\\ mov $1, %%rax # SYS_write
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\\ syscall
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\\
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\\ # Exit
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\\ mov $0, %%rdi # code = 0
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\\ mov $60, %%rax # SYS_exit
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\\ syscall
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\\
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\\ 1:
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\\ # Parent Path continues
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: [ret] "={rax}" (-> usize),
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: [number] "{rax}" (@intFromEnum(linux.syscalls.X64.clone)),
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[arg1] "{rdi}" (flags),
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[arg2] "{rsi}" (stack_top),
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[arg3] "{rdx}" (0),
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[arg4] "{r10}" (0),
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[arg5] "{r8}" (0),
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[msg] "r" (msg.ptr),
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[len] "r" (msg_len),
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: .{ .rcx = true, .r11 = true, .memory = true });
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// Parent Process
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const child_pid: i64 = @bitCast(ret);
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if (child_pid < 0) {
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std.debug.print(
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"Parent: Clone failed with: {}\n",
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.{@as(linux.E, @enumFromInt(-child_pid))},
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);
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return;
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}
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var status: u32 = 0;
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// wait4 for the child to exit
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_ = linux.syscall4(.wait4, @as(usize, @intCast(child_pid)), @intFromPtr(&status), 0, 0);
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_ = linux.syscall3(.write, 1, @intFromPtr("Parent: Goodbye\n"), 16);
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}
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