From 67eca8a21251c5b30bb8fb744ce8ecb464f3c87f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Pawe=C5=82=20Gronowski?= <pawel.gronowski@docker.com>
Date: Tue, 24 Mar 2026 01:57:02 +0100
Subject: [PATCH] ext4: return error instead of panicking on special file
 inodes
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OpenFile and openFileViaInode panic with a nil pointer dereference when
called on character device, block device, FIFO, or socket inodes. These
inode types store device major/minor numbers instead of an extent tree,
so inode.extents is nil and the call to inode.extents.blocks() crashes.

Add a nil guard that returns a descriptive error before accessing the
extent tree.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
---
 filesystem/ext4/ext4.go              |  6 ++++++
 filesystem/ext4/specialfile_test.go  | 31 ++++++++++++++++++++++++++++
 filesystem/ext4/testdata/buildimg.sh |  2 ++
 3 files changed, 39 insertions(+)
 create mode 100644 filesystem/ext4/specialfile_test.go

diff --git a/filesystem/ext4/ext4.go b/filesystem/ext4/ext4.go
index 1224560..1831880 100644
--- a/filesystem/ext4/ext4.go
+++ b/filesystem/ext4/ext4.go
@@ -1272,6 +1272,9 @@ func (fs *FileSystem) OpenFile(p string, flag int) (filesystem.File, error) {
 		}
 		return fs.OpenFile(linkTarget, flag)
 	}
+	if inode.extents == nil {
+		return nil, fmt.Errorf("cannot open special file %s (inode %d): no extent tree", p, inodeNumber)
+	}
 	offset := int64(0)
 	if flag&os.O_APPEND == os.O_APPEND {
 		offset = int64(inode.size)
@@ -1306,6 +1309,9 @@ func (fs *FileSystem) openFileViaInode(inodeNumber uint32, flag int) (filesystem
 	if inode.fileType == fileTypeSymbolicLink {
 		return nil, fmt.Errorf("cannot open file via inode: inode %d is a symbolic link", inodeNumber)
 	}
+	if inode.extents == nil {
+		return nil, fmt.Errorf("cannot open special file (inode %d): no extent tree", inodeNumber)
+	}
 	offset := int64(0)
 	if flag&os.O_APPEND == os.O_APPEND {
 		offset = int64(inode.size)
diff --git a/filesystem/ext4/specialfile_test.go b/filesystem/ext4/specialfile_test.go
new file mode 100644
index 0000000..00c5299
--- /dev/null
+++ b/filesystem/ext4/specialfile_test.go
@@ -0,0 +1,31 @@
+package ext4
+
+import (
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/diskfs/go-diskfs/backend/file"
+)
+
+func TestOpenFileSpecialFileReturnsError(t *testing.T) {
+	f, err := os.Open(imgFile)
+	if err != nil {
+		t.Fatalf("Error opening test image: %v", err)
+	}
+	defer f.Close()
+
+	b := file.New(f, true)
+	fs, err := Read(b, 100*MB, 0, 512)
+	if err != nil {
+		t.Fatalf("Error reading filesystem: %v", err)
+	}
+
+	_, err = fs.OpenFile("/chardev", os.O_RDONLY)
+	if err == nil {
+		t.Fatal("OpenFile on character device should return an error, not succeed")
+	}
+	if !strings.Contains(err.Error(), "cannot open special file") {
+		t.Errorf("expected 'cannot open special file' error, got: %v", err)
+	}
+}
diff --git a/filesystem/ext4/testdata/buildimg.sh b/filesystem/ext4/testdata/buildimg.sh
index a399b3f..ddb272c 100755
--- a/filesystem/ext4/testdata/buildimg.sh
+++ b/filesystem/ext4/testdata/buildimg.sh
@@ -31,6 +31,8 @@ ln -s nonexistent deadlink
 ln -s /some/really/long/path/that/does/not/exist/and/does/not/fit/in/symlink deadlonglink # the target here is >60 chars and so will not fit within the inode
 # hardlink
 ln random.dat hardlink.dat
+# character device for special file tests
+mknod chardev c 1 3
 cd /data
 umount /mnt