feat(debug): add named buffer allocation with pattern filling

debug/target_linux_amd64.go

@@ -6,9 +6,12 @@
 package debug
 
 import (
+	"encoding/hex"
 	"fmt"
 	"os"
 	"runtime"
+	"strconv"
+	"strings"
 	"syscall"
 	"unsafe"
 
@@ -85,6 +88,15 @@ func RunTarget(asmPath, funcName, argsFile, tmpDir string) error {
 		args = padded
 	}
 
+	// Read buffer specification if present.
+	bufSpecFile := tmpDir + "/bufspec"
+	if bufSpec, err := os.ReadFile(bufSpecFile); err == nil && len(bufSpec) > 0 {
+		args, err = setupBuffers(string(bufSpec), args, fl.Args, tmpDir)
+		if err != nil {
+			return fmt.Errorf("debug target: setup buffers: %w", err)
+		}
+	}
+
 	// Lock this goroutine to the current OS thread so the parent's
 	// ptrace (attached to this thread) controls the JIT execution.
 	runtime.LockOSThread()
@@ -100,6 +112,11 @@ func RunTarget(asmPath, funcName, argsFile, tmpDir string) error {
 
 	// --- Execution resumes here after the parent continues us ---
 
+	// Stop at the function entry point so the debugger can set breakpoints.
+	// The parent will continue us when ready.
+	os.WriteFile(tmpDir+"/entry", []byte("ok"), 0o644)
+	syscall.Kill(syscall.Getpid(), syscall.SIGSTOP)
+
 	// Prepare the ABI0 stack and call the function.
 	fnAddr := codeBase + uintptr(fl.Offset)
 	stackArgs := make([]byte, fl.Args)
@@ -129,3 +146,82 @@ func mapRWX(code []byte) ([]byte, error) {
 	copy(mem, code)
 	return mem, nil
 }
+
+// setupBuffers allocates buffers in the debuggee's memory and updates the
+// argument block with pointers to them.
+// Format: name:size:pattern[,name:size:pattern...]
+// Patterns: zero, ones, seq, or hex (e.g. "deadbeef").
+func setupBuffers(spec string, args []byte, argSize int, tmpDir string) ([]byte, error) {
+	// Parse the buffer spec.
+	type bufSpec struct {
+		name    string
+		size    int
+		pattern string
+	}
+	var specs []bufSpec
+	for _, part := range strings.Split(spec, ",") {
+		fields := strings.SplitN(part, ":", 3)
+		if len(fields) != 3 {
+			continue
+		}
+		size, err := strconv.Atoi(fields[1])
+		if err != nil || size <= 0 {
+			continue
+		}
+		specs = append(specs, bufSpec{name: fields[0], size: size, pattern: fields[2]})
+	}
+
+	if len(specs) == 0 {
+		return args, nil
+	}
+
+	// Allocate buffers and write their addresses to a file for the parent.
+	var bufAddrs []uint64
+	for _, s := range specs {
+		buf, err := syscall.Mmap(-1, 0, s.size,
+			syscall.PROT_READ|syscall.PROT_WRITE,
+			syscall.MAP_PRIVATE|syscall.MAP_ANON)
+		if err != nil {
+			return nil, fmt.Errorf("mmap buffer %s: %w", s.name, err)
+		}
+		fillBuffer(buf, s.pattern)
+		bufAddrs = append(bufAddrs, uint64(uintptr(unsafe.Pointer(&buf[0]))))
+	}
+
+	// Write buffer addresses to a file for the parent to read.
+	addrFile, err := os.Create(tmpDir + "/bufaddrs")
+	if err != nil {
+		return nil, err
+	}
+	for _, addr := range bufAddrs {
+		fmt.Fprintf(addrFile, "%d\n", addr)
+	}
+	addrFile.Close()
+
+	// For now, return the args unchanged. The parent will read bufaddrs
+	// and construct the final argument block with the correct pointers.
+	return args, nil
+}
+
+// fillBuffer fills a buffer with the specified pattern.
+func fillBuffer(buf []byte, pattern string) {
+	switch pattern {
+	case "zero":
+		// Already zeroed by mmap.
+	case "ones":
+		for i := range buf {
+			buf[i] = 0xFF
+		}
+	case "seq":
+		for i := range buf {
+			buf[i] = byte(i)
+		}
+	default:
+		// Try to parse as hex.
+		if data, err := hex.DecodeString(pattern); err == nil && len(data) > 0 {
+			for i := range buf {
+				buf[i] = data[i%len(data)]
+			}
+		}
+	}
+}