From 43502cd2e28a523cde4dd148b30cc2266ac3be42 Mon Sep 17 00:00:00 2001
From: dyer-oai <dyer@openai.com>
Date: Sun, 7 Jun 2026 09:23:07 -0700
Subject: [PATCH] guard invalid file info stat in disk usage

Signed-off-by: dyer-oai <dyer@openai.com>
---
 fs/du_unix.go      | 23 ++++++++++++++++++---
 fs/du_unix_test.go | 50 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 70 insertions(+), 3 deletions(-)

diff --git a/fs/du_unix.go b/fs/du_unix.go
index fbd4a00..d960b23 100644
--- a/fs/du_unix.go
+++ b/fs/du_unix.go
@@ -20,6 +20,7 @@ package fs
 
 import (
 	"context"
+	"fmt"
 	"os"
 	"path/filepath"
 	"syscall"
@@ -47,6 +48,16 @@ func newInode(stat *syscall.Stat_t) inode {
 	}
 }
 
+func fileInfoStat(path string, fi os.FileInfo) (*syscall.Stat_t, error) {
+	sys := fi.Sys()
+	stat, ok := sys.(*syscall.Stat_t)
+	if !ok || stat == nil {
+		return nil, fmt.Errorf("failed to get stat for %q: expected *syscall.Stat_t, got %T", path, sys)
+	}
+
+	return stat, nil
+}
+
 func diskUsage(ctx context.Context, roots ...string) (Usage, error) {
 	var (
 		size   int64
@@ -65,7 +76,10 @@ func diskUsage(ctx context.Context, roots ...string) (Usage, error) {
 			default:
 			}
 
-			stat := fi.Sys().(*syscall.Stat_t)
+			stat, err := fileInfoStat(path, fi)
+			if err != nil {
+				return err
+			}
 			inoKey := newInode(stat)
 			if _, ok := inodes[inoKey]; !ok {
 				inodes[inoKey] = struct{}{}
@@ -90,13 +104,16 @@ func diffUsage(ctx context.Context, a, b string) (Usage, error) {
 		inodes = map[inode]struct{}{} // expensive!
 	)
 
-	if err := Changes(ctx, a, b, func(kind ChangeKind, _ string, fi os.FileInfo, err error) error {
+	if err := Changes(ctx, a, b, func(kind ChangeKind, path string, fi os.FileInfo, err error) error {
 		if err != nil {
 			return err
 		}
 
 		if kind == ChangeKindAdd || kind == ChangeKindModify {
-			stat := fi.Sys().(*syscall.Stat_t)
+			stat, err := fileInfoStat(path, fi)
+			if err != nil {
+				return err
+			}
 			inoKey := newInode(stat)
 			if _, ok := inodes[inoKey]; !ok {
 				inodes[inoKey] = struct{}{}
diff --git a/fs/du_unix_test.go b/fs/du_unix_test.go
index 7986734..989ffed 100644
--- a/fs/du_unix_test.go
+++ b/fs/du_unix_test.go
@@ -26,6 +26,7 @@ import (
 	"strings"
 	"syscall"
 	"testing"
+	"time"
 )
 
 func getBsize(root string) (int64, error) {
@@ -91,3 +92,52 @@ func duCheck(root string) (usage int64, err error) {
 
 	return blocks * blocksUnitSize, nil
 }
+
+func TestFileInfoStat(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name    string
+		sys     any
+		wantErr bool
+	}{
+		{
+			name:    "valid stat",
+			sys:     &syscall.Stat_t{},
+			wantErr: false,
+		},
+		{
+			name:    "typed nil stat",
+			sys:     (*syscall.Stat_t)(nil),
+			wantErr: true,
+		},
+		{
+			name:    "unexpected stat type",
+			sys:     struct{}{},
+			wantErr: true,
+		},
+	}
+
+	for _, tc := range tests {
+		tc := tc
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := fileInfoStat("/tmp/file", fakeFileInfo{sys: tc.sys})
+			if (err != nil) != tc.wantErr {
+				t.Fatalf("fileInfoStat() error = %v, wantErr %v", err, tc.wantErr)
+			}
+		})
+	}
+}
+
+type fakeFileInfo struct {
+	sys any
+}
+
+func (fi fakeFileInfo) Name() string       { return "file" }
+func (fi fakeFileInfo) Size() int64        { return 0 }
+func (fi fakeFileInfo) Mode() os.FileMode  { return 0 }
+func (fi fakeFileInfo) ModTime() time.Time { return time.Time{} }
+func (fi fakeFileInfo) IsDir() bool        { return false }
+func (fi fakeFileInfo) Sys() any           { return fi.sys }