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

debug/ptrace_linux_amd64.go

@@ -33,9 +33,17 @@ type Session struct {
 // JIT code, calls PTRACE_TRACEME and raises SIGSTOP; Launch waits for
 // that initial stop and returns a ready Session.
 func Launch(gasmBin, asmPath, funcName string, args []byte) (*Session, error) {
+	sess, _, err := LaunchWithBuffers(gasmBin, asmPath, funcName, args, "")
+	return sess, err
+}
+
+// LaunchWithBuffers is like Launch but also allocates buffers in the debuggee
+// based on the buffer specification.  Returns the Session and the buffer
+// addresses (in the order they appear in the spec).
+func LaunchWithBuffers(gasmBin, asmPath, funcName string, args []byte, bufSpec string) (*Session, []uint64, error) {
 	self, err := os.Executable()
 	if err != nil {
-		return nil, fmt.Errorf("debug: cannot find gasm binary: %w", err)
+		return nil, nil, fmt.Errorf("debug: cannot find gasm binary: %w", err)
 	}
 	if gasmBin != "" {
 		self = gasmBin
@@ -44,12 +52,20 @@ func Launch(gasmBin, asmPath, funcName string, args []byte) (*Session, error) {
 	// Write the arg block to a temp file (the child reads it).
 	tmpDir, err := os.MkdirTemp("", "gasm-debug-*")
 	if err != nil {
-		return nil, fmt.Errorf("debug: tempdir: %w", err)
+		return nil, nil, fmt.Errorf("debug: tempdir: %w", err)
 	}
 	argsFile := filepath.Join(tmpDir, "args.bin")
 	if err := os.WriteFile(argsFile, args, 0o644); err != nil {
 		os.RemoveAll(tmpDir)
-		return nil, fmt.Errorf("debug: write args: %w", err)
+		return nil, nil, fmt.Errorf("debug: write args: %w", err)
+	}
+
+	// Write the buffer spec if present.
+	if bufSpec != "" {
+		if err := os.WriteFile(filepath.Join(tmpDir, "bufspec"), []byte(bufSpec), 0o644); err != nil {
+			os.RemoveAll(tmpDir)
+			return nil, nil, fmt.Errorf("debug: write bufspec: %w", err)
+		}
 	}
 
 	cmd := exec.Command(self, "debug", "--target", "--func", funcName, "--args", argsFile, asmPath)
@@ -60,7 +76,7 @@ func Launch(gasmBin, asmPath, funcName string, args []byte) (*Session, error) {
 
 	if err := cmd.Start(); err != nil {
 		os.RemoveAll(tmpDir)
-		return nil, fmt.Errorf("debug: start debuggee: %w", err)
+		return nil, nil, fmt.Errorf("debug: start debuggee: %w", err)
 	}
 
 	s := &Session{pid: cmd.Process.Pid, cmd: cmd}
@@ -79,8 +95,28 @@ func Launch(gasmBin, asmPath, funcName string, args []byte) (*Session, error) {
 	if _, err := syscall.Wait4(s.pid, &ws, syscall.WUNTRACED, nil); err != nil {
 		cmd.Process.Kill()
 		os.RemoveAll(tmpDir)
-		return nil, fmt.Errorf("debug: wait for debuggee: %w", err)
+		return nil, nil, fmt.Errorf("debug: wait for stop: %w", err)
 	}
+
+	// Wait for the debuggee to reach the function entry point.
+	entryFile := filepath.Join(tmpDir, "entry")
+	for i := 0; i < 500; i++ {
+		if _, err := os.Stat(entryFile); err == nil {
+			break
+		}
+		time.Sleep(5 * time.Millisecond)
+	}
+
+	// Continue the debuggee to the entry point.
+	if err := s.Continue(); err != nil {
+		return nil, nil, fmt.Errorf("debug: continue to entry: %w", err)
+	}
+
+	// Wait for the entry stop.
+	if _, err := syscall.Wait4(s.pid, &ws, syscall.WUNTRACED, nil); err != nil {
+		return nil, nil, fmt.Errorf("debug: wait for entry: %w", err)
+	}
+
 	s.stopped = true
 
 	// Read the code base from /proc/pid/maps (find the RWX mapping).
@@ -93,7 +129,21 @@ func Launch(gasmBin, asmPath, funcName string, args []byte) (*Session, error) {
 		}
 	}
 
-	return s, nil
+	// Read buffer addresses if buffers were allocated.
+	var bufAddrs []uint64
+	if bufSpec != "" {
+		addrFile := filepath.Join(tmpDir, "bufaddrs")
+		if data, err := os.ReadFile(addrFile); err == nil {
+			for _, line := range strings.Split(strings.TrimSpace(string(data)), "\n") {
+				var addr uint64
+				if _, err := fmt.Sscanf(line, "%d", &addr); err == nil {
+					bufAddrs = append(bufAddrs, addr)
+				}
+			}
+		}
+	}
+
+	return s, bufAddrs, nil
 }
 
 // wait waits for the debuggee to stop and returns the wait status.

debug/repl.go

@@ -34,21 +34,7 @@ type SourceLine struct {
 func REPL(s *Session, bm *Breakpoints, codeBase uint64, funcOffset, funcSize, argsSize int, labels []Label, lines []SourceLine) {
 	entryAddr := codeBase + uint64(funcOffset)
 
-	// Run to the function entry.
-	bp, err := bm.Set(entryAddr, "(entry)")
-	if err != nil {
-		fmt.Printf("warning: cannot set entry breakpoint: %v\n", err)
-	} else {
-		if err := s.Continue(); err != nil {
-			fmt.Printf("warning: continue to entry: %v\n", err)
-		}
-		regs, _ := s.GetRegs()
-		bm.HandleTrap(&regs)
-		// Remove the temporary entry breakpoint.
-		bm.Clear(entryAddr)
-		_ = bp
-	}
-
+	// The debuggee is already stopped at the function entry point.
 	fmt.Printf("stopped at function entry: %#x (%d bytes)\n", entryAddr, funcSize)
 	fmt.Println("commands: break <label|addr> | step [n] | continue | disas [n] | regs | where | x <addr> [len] | w <addr> <val...> | labels | quit")
 

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)]
+			}
+		}
+	}
+}