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

cmd/gasm/debug_linux_amd64.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"os"
 	"sort"
+	"strings"
 
 	"sourcedock.dev/petrbalvin/gasm-devkit/debug"
 	"sourcedock.dev/petrbalvin/gasm-devkit/verify"
@@ -32,6 +33,7 @@ REPL commands:
 	target := fs.Bool("target", false, "") // hidden: debuggee subprocess mode
 	funcName := fs.String("func", "", "function to debug")
 	argsFile := fs.String("args", "", "file containing the ABI0 argument block")
+	bufSpec := fs.String("buf", "", "buffer specification: name:size:pattern[,name:size:pattern...] where pattern is zero, ones, seq, or hex")
 	fs.Parse(args)
 
 	// --- Debuggee mode (internal, spawned by the debugger) ---
@@ -76,13 +78,80 @@ REPL commands:
 	}
 	sort.Slice(labels, func(i, j int) bool { return labels[i].Offset < labels[j].Offset })
 
-	// Launch the debuggee with a zeroed argument block.
-	argBlock := make([]byte, fl.Args)
-	sess, err := debug.Launch("", path, *funcName, argBlock)
+	// Launch the debuggee with the argument block.
+	var argBlock []byte
+	var bufAddrs []uint64
+	var sess *debug.Session
+	if *bufSpec != "" {
+		// Parse the function signature to determine argument layout.
+		src, err := readSource(path)
 		if err != nil {
 			fmt.Fprintf(os.Stderr, "gasm debug: %v\n", err)
 			return 1
 		}
+		sig, ok := verify.ExtractFuncSig(src, *funcName)
+		if !ok {
+			fmt.Fprintf(os.Stderr, "gasm debug: no // func signature found for %s\n", *funcName)
+			return 1
+		}
+		layout := verify.ArgLayout(sig)
+
+		// Parse the buffer spec to get buffer names.
+		bufNames := parseBufNames(*bufSpec)
+
+		// Allocate buffers in the debuggee.
+		argBlock = make([]byte, fl.Args)
+		sess, bufAddrs, err = debug.LaunchWithBuffers("", path, *funcName, argBlock, *bufSpec)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "gasm debug: %v\n", err)
+			return 1
+		}
+
+		// Construct the argument block with buffer pointers at the correct positions.
+		bufIdx := 0
+		for _, arg := range layout {
+			if !arg.IsPtr {
+				continue
+			}
+			// Find the buffer that matches this argument.
+			for i, name := range bufNames {
+				if i < len(bufAddrs) && (name == arg.Name || strings.HasPrefix(arg.Name, name)) {
+					addr := bufAddrs[i]
+					off := arg.Offset
+					if off+8 <= len(argBlock) {
+						argBlock[off] = byte(addr)
+						argBlock[off+1] = byte(addr >> 8)
+						argBlock[off+2] = byte(addr >> 16)
+						argBlock[off+3] = byte(addr >> 24)
+						argBlock[off+4] = byte(addr >> 32)
+						argBlock[off+5] = byte(addr >> 40)
+						argBlock[off+6] = byte(addr >> 48)
+						argBlock[off+7] = byte(addr >> 56)
+					}
+					// For slices, also set the length and capacity.
+					if strings.HasPrefix(arg.Typ, "[]") && off+24 <= len(argBlock) {
+						// Find the buffer size from the spec.
+						size := parseBufSize(*bufSpec, name)
+						// Length at offset+8, capacity at offset+16.
+						for j := 0; j < 8; j++ {
+							argBlock[off+8+j] = byte(size >> (j * 8))
+							argBlock[off+16+j] = byte(size >> (j * 8))
+						}
+					}
+					bufIdx++
+					break
+				}
+			}
+		}
+		_ = bufIdx
+	} else {
+		argBlock = make([]byte, fl.Args)
+		sess, err = debug.Launch("", path, *funcName, argBlock)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "gasm debug: %v\n", err)
+			return 1
+		}
+	}
 	defer sess.Kill()
 
 	bm := debug.NewBreakpoints(sess)
@@ -96,3 +165,29 @@ REPL commands:
 	debug.REPL(sess, bm, sess.CodeBase(), fl.Offset, fl.Size, fl.Args, labels, srcLines)
 	return 0
 }
+
+// parseBufNames extracts buffer names from a buffer specification.
+// Format: name:size:pattern[,name:size:pattern...]
+func parseBufNames(spec string) []string {
+	var names []string
+	for _, part := range strings.Split(spec, ",") {
+		fields := strings.SplitN(part, ":", 3)
+		if len(fields) >= 1 && fields[0] != "" {
+			names = append(names, fields[0])
+		}
+	}
+	return names
+}
+
+// parseBufSize extracts the size of a named buffer from a buffer specification.
+func parseBufSize(spec, name string) int {
+	for _, part := range strings.Split(spec, ",") {
+		fields := strings.SplitN(part, ":", 3)
+		if len(fields) >= 2 && fields[0] == name {
+			var size int
+			fmt.Sscanf(fields[1], "%d", &size)
+			return size
+		}
+	}
+	return 0
+}

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)]
+			}
+		}
+	}
+}

verify/sigparse.go

@@ -0,0 +1,136 @@
+// Copyright (c) 2026 Petr Balvín <opensource@petrbalvin.org> (https://petrbalvin.org)
+// SPDX-License-Identifier: BSD-3-Clause
+
+package verify
+
+import (
+	"strings"
+)
+
+// FuncSig is a parsed // func signature from the assembly source.
+type FuncSig struct {
+	Name    string
+	Params  []Param
+	Results []Param
+}
+
+// Param is a single function parameter.
+type Param struct {
+	Name string
+	Typ  string // "[]byte", "[]int32", "int", "*[32]uint16", etc.
+}
+
+// IsPointer reports whether the parameter type is a pointer or slice
+// (i.e., it needs a buffer address in the argument block).
+func (p Param) IsPointer() bool {
+	return strings.HasPrefix(p.Typ, "[]") || strings.HasPrefix(p.Typ, "*")
+}
+
+// ParseFuncSig extracts the function signature from a "// func ..." comment.
+func ParseFuncSig(comment string) (FuncSig, bool) {
+	m := funcSigRe.FindStringSubmatch(strings.TrimSpace(comment))
+	if m == nil {
+		return FuncSig{}, false
+	}
+	sig := FuncSig{Name: m[1]}
+	sig.Params = parseParamsExported(m[2])
+	// Results may be "(a int, b int)" or "int" or "(int, error)".
+	res := strings.TrimSpace(m[3])
+	res = strings.TrimPrefix(res, "(")
+	res = strings.TrimSuffix(res, ")")
+	if res != "" {
+		sig.Results = parseParamsExported(res)
+	}
+	return sig, true
+}
+
+// parseParamsExported splits "a []byte, b []int32" into typed parameters.
+func parseParamsExported(s string) []Param {
+	s = strings.TrimSpace(s)
+	if s == "" {
+		return nil
+	}
+	var out []Param
+	for _, field := range strings.Split(s, ",") {
+		field = strings.TrimSpace(field)
+		if field == "" {
+			continue
+		}
+		parts := strings.Fields(field)
+		if len(parts) == 1 {
+			// Unnamed: "int" or "[]byte".
+			out = append(out, Param{Typ: parts[0]})
+		} else {
+			// Named: "a []byte".
+			out = append(out, Param{Name: parts[0], Typ: parts[1]})
+		}
+	}
+	return out
+}
+
+// ExtractFuncSig extracts the // func signature for the named function
+// from the assembly source.
+func ExtractFuncSig(src, funcName string) (FuncSig, bool) {
+	lines := strings.Split(src, "\n")
+	for i, line := range lines {
+		if sig, ok := ParseFuncSig(line); ok && sig.Name == funcName {
+			// Verify the next non-comment line is the TEXT directive.
+			for j := i + 1; j < len(lines); j++ {
+				trimmed := strings.TrimSpace(lines[j])
+				if trimmed == "" || strings.HasPrefix(trimmed, "//") {
+					continue
+				}
+				if strings.HasPrefix(trimmed, "TEXT") {
+					return sig, true
+				}
+				break
+			}
+		}
+	}
+	return FuncSig{}, false
+}
+
+// ArgLayout computes the ABI0 argument layout for a function signature.
+// Returns the offset of each parameter in the argument block.
+func ArgLayout(sig FuncSig) []ArgOffset {
+	var offsets []ArgOffset
+	off := 0
+	for _, p := range sig.Params {
+		size := paramSize(p.Typ)
+		offsets = append(offsets, ArgOffset{Name: p.Name, Typ: p.Typ, Offset: off, Size: size, IsPtr: p.IsPointer()})
+		off += size
+	}
+	return offsets
+}
+
+// ArgOffset describes one parameter's position in the argument block.
+type ArgOffset struct {
+	Name   string
+	Typ    string
+	Offset int
+	Size   int
+	IsPtr  bool
+}
+
+// paramSize returns the size in bytes of a parameter type in the ABI0 layout.
+func paramSize(typ string) int {
+	switch {
+	case strings.HasPrefix(typ, "[]"):
+		// Slice: pointer + length + capacity = 24 bytes.
+		return 24
+	case strings.HasPrefix(typ, "*"):
+		// Pointer: 8 bytes.
+		return 8
+	case typ == "int", typ == "int64", typ == "uint64", typ == "uintptr":
+		return 8
+	case typ == "int32", typ == "uint32", typ == "float32":
+		return 4
+	case typ == "int16", typ == "uint16":
+		return 2
+	case typ == "int8", typ == "uint8", typ == "byte", typ == "bool":
+		return 1
+	default:
+		// Default to 8 bytes for unknown types.
+		return 8
+	}
+}