fix: brain_tunnel, brain_update and session_log confine caller-supplied paths (#169) (#170)

brain_tunnel accepts only clean wiki/.../*.md paths and reads/rewrites both notes through brainfs; new brainfs.WriteRegular never creates, never writes through a final symlink (Lstat + O_NOFOLLOW|O_NONBLOCK + os.SameFile on the handle, since os.Root follows in-root final symlinks even with O_NOFOLLOW). brain_update resolves like brain_edit (knowledge/ and wiki/ notes only) and reads/rewrites through brainfs. session_log requires a plain-name session id and opens with O_NOFOLLOW|O_NONBLOCK.

Fixes #169. Adversarial review: BLOCKED, BLOCKED, PASS (round 3 by the estate session's reviewer). Follow-ups: #173, #174.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: Mathias <mthbqv@gmail.com>

ingestion/internal/brain/tunnel.go

@@ -4,9 +4,12 @@ import (
 	"bufio"
 	"fmt"
 	"os"
+	"path"
 	"path/filepath"
 	"strings"
 	"time"
+
+	"github.com/mathiasbq/hyperguild/ingestion/internal/brainfs"
 )
 
 // seeAlsoHeader is the markdown heading used to group cross-wing links.
@@ -197,6 +200,12 @@ func fileExists(p string) bool {
 // section headers. Rejects same-wing pairs (a tunnel is by definition
 // cross-wing) and missing notes.
 func WriteTunnel(brainDir, sourcePath, targetPath string) error {
+	if err := checkNotePath(sourcePath); err != nil {
+		return fmt.Errorf("source: %w", err)
+	}
+	if err := checkNotePath(targetPath); err != nil {
+		return fmt.Errorf("target: %w", err)
+	}
 	srcWing, err := wingOf(sourcePath)
 	if err != nil {
 		return fmt.Errorf("source: %w", err)
@@ -209,20 +218,50 @@ func WriteTunnel(brainDir, sourcePath, targetPath string) error {
 		return fmt.Errorf("tunnel must cross wings; got both in %q", srcWing)
 	}
 
-	srcFull := filepath.Join(brainDir, filepath.FromSlash(sourcePath))
-	tgtFull := filepath.Join(brainDir, filepath.FromSlash(targetPath))
-	if _, err := os.Stat(srcFull); err != nil {
+	// Both notes are read and rewritten through the brain root, regular
+	// files only: brain_tunnel passes caller-supplied paths, and the brain is
+	// a Syncthing folder that can hold symlinks.
+	bfs, err := brainfs.Open(brainDir)
+	if err != nil {
+		return fmt.Errorf("open brain: %w", err)
+	}
+	defer func() { _ = bfs.Close() }()
+	srcContent, err := bfs.ReadRegular(sourcePath)
+	if err != nil {
 		return fmt.Errorf("source note: %w", err)
 	}
-	if _, err := os.Stat(tgtFull); err != nil {
+	tgtContent, err := bfs.ReadRegular(targetPath)
+	if err != nil {
 		return fmt.Errorf("target note: %w", err)
 	}
 
-	if err := appendSeeAlso(srcFull, wikilinkOf(targetPath)); err != nil {
+	if out, changed, err := withSeeAlso(srcContent, wikilinkOf(targetPath)); err != nil {
 		return fmt.Errorf("update source: %w", err)
+	} else if changed {
+		if err := bfs.WriteRegular(sourcePath, out); err != nil {
+			return fmt.Errorf("update source: %w", err)
+		}
 	}
-	if err := appendSeeAlso(tgtFull, wikilinkOf(sourcePath)); err != nil {
+	if out, changed, err := withSeeAlso(tgtContent, wikilinkOf(sourcePath)); err != nil {
 		return fmt.Errorf("update target: %w", err)
+	} else if changed {
+		if err := bfs.WriteRegular(targetPath, out); err != nil {
+			return fmt.Errorf("update target: %w", err)
+		}
+	}
+	return nil
+}
+
+// checkNotePath accepts only a clean, local wiki/<wing>/<hall>/.../<slug>.md
+// path: no "." or ".." segments, no empty segments, no leading slash.
+func checkNotePath(p string) error {
+	if p == "" || path.Clean(p) != p || !filepath.IsLocal(p) || !strings.HasSuffix(p, ".md") {
+		return fmt.Errorf("not a clean wiki note path: %q", p)
+	}
+	for _, part := range strings.Split(p, "/") {
+		if part == "" || part == "." || part == ".." {
+			return fmt.Errorf("not a clean wiki note path: %q", p)
+		}
 	}
 	return nil
 }
@@ -248,16 +287,13 @@ func wingOf(relPath string) (string, error) {
 	return parts[1], nil
 }
 
-// appendSeeAlso inserts `- [[link]]` under the file's See also section,
-// creating the section if absent. No-op when the link is already present.
-func appendSeeAlso(filePath, link string) error {
-	content, err := os.ReadFile(filePath)
-	if err != nil {
-		return err
-	}
+// withSeeAlso returns content with `- [[link]]` under its See also section,
+// creating the section if absent. changed is false when the link is already
+// present.
+func withSeeAlso(content []byte, link string) (out []byte, changed bool, err error) {
 	wikilink := "[[" + link + "]]"
 	if strings.Contains(string(content), wikilink) {
-		return nil
+		return content, false, nil
 	}
 
 	bullet := "- " + wikilink
@@ -266,8 +302,7 @@ func appendSeeAlso(filePath, link string) error {
 		// No section yet — append a fresh one. Always emit a trailing
 		// newline so subsequent appends don't merge into the previous line.
 		trimmed := strings.TrimRight(string(content), "\n")
-		out := trimmed + "\n\n" + seeAlsoHeader + "\n\n" + bullet + "\n"
-		return os.WriteFile(filePath, []byte(out), 0o644)
+		return []byte(trimmed + "\n\n" + seeAlsoHeader + "\n\n" + bullet + "\n"), true, nil
 	}
 
 	// Section exists — splice the bullet in just before the next `## `
@@ -293,12 +328,11 @@ func appendSeeAlso(filePath, link string) error {
 		b.WriteByte('\n')
 	}
 	if err := scanner.Err(); err != nil {
-		return err
+		return nil, false, err
 	}
 	if !inserted {
 		// section was the last thing in the file — just append bullet
-		out := strings.TrimRight(b.String(), "\n") + "\n" + bullet + "\n"
-		return os.WriteFile(filePath, []byte(out), 0o644)
+		return []byte(strings.TrimRight(b.String(), "\n") + "\n" + bullet + "\n"), true, nil
 	}
-	return os.WriteFile(filePath, []byte(b.String()), 0o644)
+	return []byte(b.String()), true, nil
 }