Consider reading when the data segment is separated

Author: p-x9

Sources/MachOObjCSection/Extension/FullDyldCache+.swift

@@ -0,0 +1,42 @@
+//
+//  FullDyldCache+.swift
+//  MachOObjCSection
+//
+//  Created by p-x9 on 2025/09/28
+//  
+//
+
+import Foundation
+import MachOKit
+#if compiler(>=6.0) || (compiler(>=5.10) && hasFeature(AccessLevelOnImport))
+internal import FileIO
+#else
+@_implementationOnly import FileIO
+#endif
+
+extension FullDyldCache {
+    internal typealias File = ConcatenatedMemoryMappedFile
+
+    var fileHandle: File {
+        let mainCache = try! DyldCache(url: url)
+
+        let subCacheSuffixes = mainCache.subCaches?.map {
+            $0.fileSuffix
+        } ?? []
+        var urls = [url]
+        urls += subCacheSuffixes.map {
+            URL(fileURLWithPath: url.path + $0)
+        }
+
+        return try! .open(
+            urls: urls,
+            isWritable: false
+        )
+    }
+}
+
+extension FullDyldCache {
+    func fileSegment(forOffset offset: UInt64) -> File.FileSegment? {
+        try? fileHandle._file(for: numericCast(offset))
+    }
+}

Sources/MachOObjCSection/MachOFile+ObjectiveC.swift

@@ -35,9 +35,10 @@ extension MachOFile.ObjectiveC {
             return nil
         }
 
-        return try! machO.fileHandle.read(
-            offset: numericCast(__objc_imageinfo.offset + machO.headerStartOffset)
-        )
+        guard let fileSlice = machO._fileSliceForSection(section: __objc_imageinfo) else {
+            return nil
+        }
+        return try? fileSlice.read(offset: 0)
     }
 }
 
@@ -272,8 +273,11 @@ extension MachOFile.ObjectiveC {
         in machO: MachOFile,
         isCatlist2: Bool = false
     ) -> [Categgory]? {
-        let data = try! machO.fileHandle.readData(
-            offset: numericCast(section.offset + machO.headerStartOffset),
+        guard let fileSlice = machO._fileSliceForSection(section: section) else {
+            return nil
+        }
+        let data = try! fileSlice.readData(
+            offset: 0,
             length: section.size
         )
 
@@ -311,8 +315,11 @@ extension MachOFile.ObjectiveC {
         from section: any SectionProtocol,
         in machO: MachOFile
     ) -> [Class]? {
-        let data = try! machO.fileHandle.readData(
-            offset: numericCast(section.offset + machO.headerStartOffset),
+        guard let fileSlice = machO._fileSliceForSection(section: section) else {
+            return nil
+        }
+        let data = try! fileSlice.readData(
+            offset: 0,
             length: section.size
         )
 
@@ -346,8 +353,11 @@ extension MachOFile.ObjectiveC {
         from section: any SectionProtocol,
         in machO: MachOFile
     ) -> [Protocol]? {
-        let data = try! machO.fileHandle.readData(
-            offset: numericCast(section.offset + machO.headerStartOffset),
+        guard let fileSlice = machO._fileSliceForSection(section: section) else {
+            return nil
+        }
+        let data = try! fileSlice.readData(
+            offset: 0,
             length: section.size
         )
 
@@ -373,3 +383,45 @@ extension MachOFile.ObjectiveC {
             }
     }
 }
+
+extension MachOFile {
+    fileprivate func _fileSliceForSection(
+        section: any SectionProtocol
+    ) -> File.FileSlice? {
+        let text: (any SegmentCommandProtocol)? = loadCommands.text64 ?? loadCommands.text
+        guard let text else { return nil }
+
+        let maxFileOffsetToCheck = text.fileOffset + section.address - text.virtualMemoryAddress
+        let isWithinFileRange: Bool = fileHandle.size >= maxFileOffsetToCheck
+
+        // 1) text.vmaddr < linkedit.vmaddr
+        // 2) fileoff_diff <= vmaddr_diff
+        // 3) If both exist in the same file
+        //    text.fileoff < linkedit.fileoff <= text.fileoff + vmaddr_diff
+        // 4) if fileHandle.size < text.fileoff + vmaddr_diff
+        //    both exist in the same file
+
+        // The linkedit data in iOS is stored together in a separate, independent cache.
+        // (.0x.linkeditdata)
+        if isLoadedFromDyldCache && !isWithinFileRange {
+            guard let fullCache = self.fullCache,
+                  let fileOffset = fullCache.fileOffset(
+                    of: numericCast(section.address)
+                  ),
+                  let segment = fullCache.fileSegment(
+                    forOffset: fileOffset
+                  ) else {
+                return nil
+            }
+            return try? segment._file.fileSlice(
+                offset: numericCast(fileOffset) - segment.offset,
+                length: section.size
+            )
+        } else {
+            return try? fileHandle.fileSlice(
+                offset: headerStartOffset + section.offset,
+                length: section.size
+            )
+        }
+    }
+}