Fix ISO GRUB modification to search entire ISO and use robust kernel matching

Two fixes: (1) Search entire ISO for grub.cfg content instead of only at
2048-byte sector boundaries — catches the copy inside the EFI boot
partition image that UEFI firmware actually reads. (2) Match kernel lines
by GRUB 'linux' command prefix instead of requiring '/boot/linux' path.
Added detailed logging of original and modified grub.cfg content.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/image/isomod.go

@@ -3,7 +3,6 @@ package image
 import (
 	"bytes"
 	"fmt"
-	"io"
 	"log"
 	"os"
 	"strings"
@@ -17,10 +16,12 @@ func ModifyISOForAutoInstall(isoPath, answerURL string) error {
 		return err
 	}
 	if origConfig == nil {
-		log.Printf("image: no grub.cfg found in ISO, skipping auto-install modification")
-		return nil
+		return fmt.Errorf("no grub.cfg found in ISO filesystem")
 	}
 
+	log.Printf("image: found grub.cfg (%d bytes), first 200 chars: %s",
+		len(origConfig), truncate(string(origConfig), 200))
+
 	newConfig := rewriteGrubConfig(string(origConfig), answerURL)
 
 	if len(newConfig) > len(origConfig) {
@@ -33,22 +34,27 @@ func ModifyISOForAutoInstall(isoPath, answerURL string) error {
 		padded[i] = '\n'
 	}
 
-	offset, err := findContentInISO(isoPath, origConfig)
+	offsets, err := findAllOccurrences(isoPath, origConfig)
 	if err != nil {
 		return fmt.Errorf("locate grub.cfg in ISO: %w", err)
 	}
 
+	log.Printf("image: found grub.cfg at %d location(s) in ISO: %v", len(offsets), offsets)
+
 	f, err := os.OpenFile(isoPath, os.O_WRONLY, 0)
 	if err != nil {
 		return fmt.Errorf("open ISO for writing: %w", err)
 	}
 	defer f.Close()
 
-	if _, err := f.WriteAt(padded, offset); err != nil {
-		return fmt.Errorf("write modified grub.cfg: %w", err)
+	for _, offset := range offsets {
+		if _, err := f.WriteAt(padded, offset); err != nil {
+			return fmt.Errorf("write at offset %d: %w", offset, err)
+		}
 	}
 
-	log.Printf("image: modified grub.cfg at offset %d for auto-install", offset)
+	log.Printf("image: modified grub.cfg at %d location(s) for auto-install", len(offsets))
+	log.Printf("image: new grub.cfg:\n%s", newConfig)
 	return nil
 }
 
@@ -75,7 +81,7 @@ func readGrubCfgFromISO(isoPath string) ([]byte, error) {
 	}
 
 	reader := grubFile.Reader()
-	data, err := io.ReadAll(reader)
+	data, err := readAll(reader)
 	if err != nil {
 		return nil, fmt.Errorf("read grub.cfg: %w", err)
 	}
@@ -103,6 +109,7 @@ func rewriteGrubConfig(original, answerURL string) string {
 	lines := strings.Split(original, "\n")
 	var result []string
 	depth := 0
+	firstMenuModified := false
 
 	for _, line := range lines {
 		trimmed := strings.TrimSpace(line)
@@ -120,10 +127,14 @@ func rewriteGrubConfig(original, answerURL string) string {
 			depth++
 		}
 		if trimmed == "}" && depth > 0 {
+			if depth == 1 {
+				firstMenuModified = true
+			}
 			depth--
 		}
 
-		if depth == 1 && strings.Contains(trimmed, "linux ") && strings.Contains(trimmed, "/boot/linux") {
+		if depth > 0 && !firstMenuModified &&
+			(strings.HasPrefix(trimmed, "linux ") || strings.HasPrefix(trimmed, "linux\t")) {
 			if !strings.Contains(line, "proxmox-start-auto-installer") {
 				line = strings.TrimRight(line, " \t") + " proxmox-start-auto-installer"
 			}
@@ -138,32 +149,48 @@ func rewriteGrubConfig(original, answerURL string) string {
 	return strings.Join(result, "\n")
 }
 
-func findContentInISO(isoPath string, content []byte) (int64, error) {
-	f, err := os.Open(isoPath)
+// findAllOccurrences searches the entire ISO file for all locations where
+// the grub.cfg content appears — this catches both the ISO9660 filesystem
+// copy and any copy inside an embedded EFI boot partition image.
+func findAllOccurrences(isoPath string, content []byte) ([]int64, error) {
+	data, err := os.ReadFile(isoPath)
 	if err != nil {
-		return 0, err
-	}
-	defer f.Close()
-
-	prefix := content
-	if len(prefix) > 256 {
-		prefix = prefix[:256]
+		return nil, fmt.Errorf("read ISO: %w", err)
 	}
 
-	buf := make([]byte, 2048)
-	stat, _ := f.Stat()
+	needle := content
+	if len(needle) > 256 {
+		needle = needle[:256]
+	}
 
-	for offset := int64(0); offset < stat.Size(); offset += 2048 {
-		n, err := f.ReadAt(buf, offset)
-		if n == 0 {
+	var offsets []int64
+	start := 0
+	for {
+		idx := bytes.Index(data[start:], needle)
+		if idx == -1 {
 			break
 		}
-		if err != nil && err != io.EOF {
-			return 0, err
-		}
-		if bytes.HasPrefix(buf[:n], prefix) {
-			return offset, nil
-		}
+		offsets = append(offsets, int64(start+idx))
+		start += idx + 1
 	}
-	return 0, fmt.Errorf("grub.cfg content not found in ISO raw data")
+
+	if len(offsets) == 0 {
+		return nil, fmt.Errorf("grub.cfg content not found in ISO raw data (searched %d bytes with %d-byte needle)",
+			len(data), len(needle))
+	}
+
+	return offsets, nil
+}
+
+func readAll(r interface{ Read([]byte) (int, error) }) ([]byte, error) {
+	var buf bytes.Buffer
+	_, err := buf.ReadFrom(r)
+	return buf.Bytes(), err
+}
+
+func truncate(s string, n int) string {
+	if len(s) <= n {
+		return s
+	}
+	return s[:n] + "..."
 }