[LLD] [COFF] Support ENTRY and SUBSYSTEM in .drectve sections

[dmajor]

Summary

Adds support for "/ENTRY" and "/SUBSYSTEM" linker options in .drectve sections. Some Mozilla binaries were using these directives and MSVC link.exe appears to allow them. No attempt is made to reconcile these with the options on the command line.

[ruiu]

Can you add a test?

COFF/Driver.cpp

11

Are you really sure if you need mangle? IIRC, there are some command line options that behave differently in terms of name mangling when they are given via .drctve section, so I wonder if that is not the case for /entry.

[mstorsjo]

COFF/Driver.cpp

11

Also wrt mangle, it is possible that it also is done differently between gcc or clang in gnu mode, and msvc and clang in msvc mode. But if this is something that only is emitted e.g. with msvc specific pragmas that aren't enabled by default in gnu mode, it might not matter. (It does differ e.g. for mangling of symbol names in dllexport, which is supported in both modes.)

[dmajor]

COFF/Driver.cpp

11

Sorry for the slow response. I finally got around to testing an I386 build (which is the only arch where this matters, according to the body of mangle) and I get an undefined symbol if I remove the call to mangle. So it seems that it's necessary.

11

As far as I can tell (based on the comment above PragmaCommentHandler::HandlePragma) this is an msvc specific thing, so I wouldn't expect it to be enabled in gnu mode.

[ruiu]

I think I can approve if you add a test.

COFF/Driver.cpp

11

Cool! Thank you very much for investigating.

[dmajor]

While writing a test I found a problem: the code that infers /entry should not run if we've previously defined one in a drectve. (In Mozilla's build, it worked by accident because the drectve was redundantly specifying the same function that got inferred.) Updated the patch.

[ruiu]

LGTM

COFF/Driver.cpp

 //===- Driver.cpp ---------------------------------------------------------===//
 //
 //                             The LLVM Linker
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 
 #include "Driver.h"
 #include "Config.h"
 #include "InputFiles.h"
 #include "MinGW.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
 #include "Writer.h"
 #include "lld/Common/Driver.h"
 #include "lld/Common/ErrorHandler.h"
 #include "lld/Common/Memory.h"
case OPT_aligncomm:
       break;
     case OPT_alternatename:
       parseAlternateName(Arg->getValue());
       break;
     case OPT_defaultlib:
       if (Optional<StringRef> Path = findLib(Arg->getValue()))
         enqueuePath(*Path, false);
       break;
+    case OPT_entry:
+      Config->Entry = addUndefined(mangle(Arg->getValue()));
+      break;
     case OPT_export: {
       Export E = parseExport(Arg->getValue());
       if (Config->Machine == I386 && Config->MinGW) {
         if (!isDecorated(E.Name))
           E.Name = Saver.save("_" + E.Name);
         if (!E.ExtName.empty() && !isDecorated(E.ExtName))
           E.ExtName = Saver.save("_" + E.ExtName);
       }
case OPT_merge:
       parseMerge(Arg->getValue());
       break;
     case OPT_nodefaultlib:
       Config->NoDefaultLibs.insert(doFindLib(Arg->getValue()));
       break;
     case OPT_section:
       parseSection(Arg->getValue());
       break;
+    case OPT_subsystem:
+      parseSubsystem(Arg->getValue(), &Config->Subsystem,
+                     &Config->MajorOSVersion, &Config->MinorOSVersion);
+      break;
     case OPT_editandcontinue:
     case OPT_fastfail:
     case OPT_guardsym:
     case OPT_natvis:
     case OPT_throwingnew:
       break;
     default:
       error(Arg->getSpelling() + " is not allowed in .drectve");
void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   // Handle /highentropyva
   Config->HighEntropyVA =
       Config->is64() &&
       Args.hasFlag(OPT_highentropyva, OPT_highentropyva_no, true);
 
   // Handle /entry and /dll
   if (auto *Arg = Args.getLastArg(OPT_entry)) {
     Config->Entry = addUndefined(mangle(Arg->getValue()));
-  } else if (Args.hasArg(OPT_dll) && !Config->NoEntry) {
+  } else if (!Config->Entry && !Config->NoEntry) {
+    if (Args.hasArg(OPT_dll)) {
       StringRef S = (Config->Machine == I386) ? "__DllMainCRTStartup@12"
                                               : "_DllMainCRTStartup";
       Config->Entry = addUndefined(S);
-  } else if (!Config->NoEntry) {
+    } else {
       // Windows specific -- If entry point name is not given, we need to
       // infer that from user-defined entry name.
       StringRef S = findDefaultEntry();
       if (S.empty())
         fatal("entry point must be defined");
       Config->Entry = addUndefined(S);
       log("Entry name inferred: " + S);
     }
+  }
 
   // Handle /export
   for (auto *Arg : Args.filtered(OPT_export)) {
     Export E = parseExport(Arg->getValue());
     if (Config->Machine == I386) {
       if (!isDecorated(E.Name))
         E.Name = Saver.save("_" + E.Name);
       if (!E.ExtName.empty() && !isDecorated(E.ExtName))

test/COFF/entry-drectve.test

+# RUN: yaml2obj < %s > %t.obj
+# RUN: lld-link /subsystem:console /out:%t.exe %t.obj
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_I386
+  Characteristics: []
+sections:
+  - Name:            .text
+    Characteristics: [ IMAGE_SCN_CNT_CODE, IMAGE_SCN_MEM_EXECUTE, IMAGE_SCN_MEM_READ ]
+    Alignment:       4
+    SectionData:     B800000000506800000000680000000050E80000000050E800000000
+  - Name:            .drectve
+    Characteristics: [ IMAGE_SCN_LNK_INFO, IMAGE_SCN_LNK_REMOVE ]
+    Alignment:       1
+    SectionData:     2f656e7472793a437573746f6d456e74727900 # /entry:CustomEntry
+symbols:
+  - Name:            _CustomEntry
+    Value:           0
+    SectionNumber:   1
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+...

test/COFF/subsystem-drectve.test

+# RUN: yaml2obj < %s > %t.obj
+# RUN: lld-link /dll /noentry /out:%t.dll %t.obj
+# RUN: llvm-readobj -file-headers %t.dll | FileCheck %s
+
+# CHECK: MajorOperatingSystemVersion: 42
+# CHECK: MinorOperatingSystemVersion: 43
+# CHECK: MajorSubsystemVersion: 42
+# CHECK: MinorSubsystemVersion: 43
+# CHECK: Subsystem: IMAGE_SUBSYSTEM_WINDOWS_CUI
+
+--- !COFF
+header:
+  Machine:         IMAGE_FILE_MACHINE_I386
+  Characteristics: []
+sections:
+  - Name:            .drectve
+    Characteristics: [ IMAGE_SCN_LNK_INFO, IMAGE_SCN_LNK_REMOVE ]
+    Alignment:       1
+    SectionData:     2f73756273797374656d3a636f6e736f6c652c34322e343300 # /subsystem:console,42.43
+symbols:
+...