[LLD] [COFF] Add options for disabling auto import and runtime pseudo relocs

Allow disabling either the full auto import feature, or just
forbidding the cases that require runtime fixups.

As long as all auto imported variables are referenced from separate
.refptr$<name> sections, we can alias them on top of the IAT entries
and don't actually need any runtime fixups via pseudo relocations.
LLVM generates references to variables in .refptr stubs, if it
isn't known that the variable for sure is defined in the same object
module. Runtime pseudo relocs are needed if the addresses of auto
imported variables are used in constant initializers though.

Fixing up runtime pseudo relocations requires the use of
VirtualProtect (which is disallowed in WinStore/UWP apps) or
VirtualProtectFromApp. To allow any risk of ambiguity, allow
rejecting cases that would require this at the linker stage.

This adds support for the --disable-runtime-pseudo-reloc and
--disable-auto-import options in the MinGW driver (matching GNU ld.bfd)
with corresponding lld private options in the COFF driver.

Differential Revision: https://reviews.llvm.org/D78923

lld/COFF/Config.h

@@ -235,6 +235,8 @@ struct Configuration {
   bool swaprunNet = false;
   bool thinLTOEmitImportsFiles;
   bool thinLTOIndexOnly;
+  bool autoImport = false;
+  bool pseudoRelocs = false;
 };
 
 extern Configuration *config;

lld/COFF/Driver.cpp

@@ -1580,6 +1580,10 @@ void LinkerDriver::link(ArrayRef<const char *> argsArr) {
   config->debugDwarf = debug == DebugKind::Dwarf;
   config->debugGHashes = debug == DebugKind::GHash;
   config->debugSymtab = debug == DebugKind::Symtab;
+  config->autoImport =
+      args.hasFlag(OPT_auto_import, OPT_auto_import_no, config->mingw);
+  config->pseudoRelocs = args.hasFlag(
+      OPT_runtime_pseudo_reloc, OPT_runtime_pseudo_reloc_no, config->mingw);
 
   // Don't warn about long section names, such as .debug_info, for mingw or when
   // -debug:dwarf is requested.
@@ -1841,9 +1845,11 @@ void LinkerDriver::link(ArrayRef<const char *> argsArr) {
   // Needed for MSVC 2017 15.5 CRT.
   symtab->addAbsolute(mangle("__enclave_config"), 0);
 
-  if (config->mingw) {
+  if (config->pseudoRelocs) {
     symtab->addAbsolute(mangle("__RUNTIME_PSEUDO_RELOC_LIST__"), 0);
     symtab->addAbsolute(mangle("__RUNTIME_PSEUDO_RELOC_LIST_END__"), 0);
+  }
+  if (config->mingw) {
     symtab->addAbsolute(mangle("__CTOR_LIST__"), 0);
     symtab->addAbsolute(mangle("__DTOR_LIST__"), 0);
   }
@@ -1901,7 +1907,8 @@ void LinkerDriver::link(ArrayRef<const char *> argsArr) {
     while (run());
   }
 
-  if (config->mingw) {
+  if (config->autoImport) {
+    // MinGW specific.
     // Load any further object files that might be needed for doing automatic
     // imports.
     //

lld/COFF/Options.td

@@ -16,6 +16,13 @@ multiclass B<string name, string help_on, string help_off> {
   def _no : F<name#":no">, HelpText<help_off>;
 }
 
+// Same as B<> above, but without help texts, for private undocumented
+// options.
+multiclass B_priv<string name> {
+  def "" : F<name>;
+  def _no : F<name#":no">;
+}
+
 def align   : P<"align", "Section alignment">;
 def aligncomm : P<"aligncomm", "Set common symbol alignment">;
 def alternatename : P<"alternatename", "Define weak alias">;
@@ -184,6 +191,8 @@ def help : F<"help">;
 def help_q : Flag<["/??", "-??", "/?", "-?"], "">, Alias<help>;
 
 // LLD extensions
+defm auto_import : B_priv<"auto-import">;
+defm runtime_pseudo_reloc : B_priv<"runtime-pseudo-reloc">;
 def end_lib : F<"end-lib">,
   HelpText<"Ends group of objects treated as if they were in a library">;
 def exclude_all_symbols : F<"exclude-all-symbols">;

lld/COFF/SymbolTable.cpp

@@ -438,7 +438,7 @@ void SymbolTable::resolveRemainingUndefines() {
     if (name.contains("_PchSym_"))
       continue;
 
-    if (config->mingw && handleMinGWAutomaticImport(sym, name))
+    if (config->autoImport && handleMinGWAutomaticImport(sym, name))
       continue;
 
     // Remaining undefined symbols are not fatal if /force is specified.

lld/COFF/Writer.cpp

@@ -969,11 +969,11 @@ void Writer::createMiscChunks() {
   if (config->guardCF != GuardCFLevel::Off)
     createGuardCFTables();
 
-  if (config->mingw) {
+  if (config->autoImport)
     createRuntimePseudoRelocs();
 
+  if (config->mingw)
     insertCtorDtorSymbols();
-  }
 }
 
 // Create .idata section for the DLL-imported symbol table.
@@ -1722,6 +1722,15 @@ void Writer::createRuntimePseudoRelocs() {
     sc->getRuntimePseudoRelocs(rels);
   }
 
+  if (!config->pseudoRelocs) {
+    // Not writing any pseudo relocs; if some were needed, error out and
+    // indicate what required them.
+    for (const RuntimePseudoReloc &rpr : rels)
+      error("automatic dllimport of " + rpr.sym->getName() + " in " +
+            toString(rpr.target->file) + " requires pseudo relocations");
+    return;
+  }
+
   if (!rels.empty())
     log("Writing " + Twine(rels.size()) + " runtime pseudo relocations");
   PseudoRelocTableChunk *table = make<PseudoRelocTableChunk>(rels);

lld/MinGW/Driver.cpp

@@ -295,6 +295,16 @@ bool mingw::link(ArrayRef<const char *> argsArr, bool canExitEarly,
   else
     add("-opt:noref");
 
+  if (args.hasFlag(OPT_enable_auto_import, OPT_disable_auto_import, true))
+    add("-auto-import");
+  else
+    add("-auto-import:no");
+  if (args.hasFlag(OPT_enable_runtime_pseudo_reloc,
+                   OPT_disable_runtime_pseudo_reloc, true))
+    add("-runtime-pseudo-reloc");
+  else
+    add("-runtime-pseudo-reloc:no");
+
   if (auto *a = args.getLastArg(OPT_icf)) {
     StringRef s = a->getValue();
     if (s == "all")

lld/MinGW/Options.td

@@ -20,7 +20,15 @@ def L: JoinedOrSeparate<["-"], "L">, MetaVarName<"<dir>">,
   HelpText<"Add a directory to the library search path">;
 def Bdynamic: F<"Bdynamic">, HelpText<"Link against shared libraries">;
 def Bstatic: F<"Bstatic">, HelpText<"Do not link against shared libraries">;
+def disable_auto_import: F<"disable-auto-import">,
+    HelpText<"Don't automatically import data symbols from other DLLs without dllimport">;
+def disable_runtime_pseudo_reloc: F<"disable-runtime-pseudo-reloc">,
+    HelpText<"Don't do automatic imports that require runtime fixups">;
 def dynamicbase: F<"dynamicbase">, HelpText<"Enable ASLR">;
+def enable_auto_import: F<"enable-auto-import">,
+    HelpText<"Automatically import data symbols from other DLLs where needed">;
+def enable_runtime_pseudo_reloc: F<"enable-runtime-pseudo-reloc">,
+    HelpText<"Allow automatic imports that require runtime fixups">;
 defm entry: Eq<"entry", "Name of entry point symbol">, MetaVarName<"<entry>">;
 def exclude_all_symbols: F<"exclude-all-symbols">,
     HelpText<"Don't automatically export any symbols">;
@@ -94,7 +102,6 @@ def: Joined<["-"], "O">;
 def: F<"build-id">;
 def: F<"disable-auto-image-base">;
 def: F<"enable-auto-image-base">;
-def: F<"enable-auto-import">, HelpText<"Ignored; listed for libtool compatibility">;
 def: F<"end-group">;
 def: Flag<["--"], "full-shutdown">;
 def: F<"high-entropy-va">;

lld/test/COFF/autoimport-refptr.s

@@ -4,13 +4,20 @@
 # RUN: llvm-mc -triple=x86_64-windows-gnu %t-lib.s -filetype=obj -o %t-lib.obj
 # RUN: lld-link -out:%t-lib.dll -dll -entry:DllMainCRTStartup %t-lib.obj -lldmingw -implib:%t-lib.lib
 
-# RUN: llvm-mc -triple=x86_64-windows-gnu %s -filetype=obj -o %t.obj
+# RUN: llvm-mc -triple=x86_64-windows-gnu %s -defsym listptrs=1 -filetype=obj -o %t.obj
 # RUN: lld-link -lldmingw -out:%t.exe -entry:main %t.obj %t-lib.lib -verbose
 
 # RUN: llvm-readobj --coff-imports %t.exe | FileCheck -check-prefix=IMPORTS %s
 # RUN: llvm-objdump -d %t.exe | FileCheck --check-prefix=DISASM %s
 # RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=CONTENTS %s
 
+## Check that we can autoimport these variables with pseudo relocs disabled.
+# RUN: llvm-mc -triple=x86_64-windows-gnu %s -defsym listptrs=0 -filetype=obj -o %t.noptrs.obj
+# RUN: lld-link -lldmingw -runtime-pseudo-reloc:no -out:%t.exe -entry:main %t.noptrs.obj %t-lib.lib
+
+## Check that we can't autoimport them with autoimport disabled.
+# RUN: not lld-link -lldmingw -auto-import:no -out:%t.exe -entry:main %t.noptrs.obj %t-lib.lib 2>&1 | FileCheck --check-prefix=NO-AUTOIMPORT %s
+
 # IMPORTS: Import {
 # IMPORTS-NEXT: Name: autoimport-refptr.s.tmp-lib.dll
 # IMPORTS-NEXT: ImportLookupTableRVA: 0x2050
@@ -36,6 +43,8 @@
 # CONTENTS:  140003000 08200040 01000000 08200040 01000000
 # CONTENTS:  140003010 2a000000
 
+# NO-AUTOIMPORT: error: undefined symbol: variable
+
     .global main
     .global localvar
     .text
@@ -47,9 +56,11 @@ main:
     ret
 
     .data
+.if listptrs==1
 relocs:
     .quad __RUNTIME_PSEUDO_RELOC_LIST__
     .quad __RUNTIME_PSEUDO_RELOC_LIST_END__
+.endif
 localvar:
     .int 42
 

lld/test/COFF/autoimport-x86.s

@@ -4,9 +4,12 @@
 # RUN: llvm-mc -triple=x86_64-windows-gnu %t-lib.s -filetype=obj -o %t-lib.obj
 # RUN: lld-link -out:%t-lib.dll -dll -entry:DllMainCRTStartup %t-lib.obj -lldmingw -implib:%t-lib.lib
 
-# RUN: llvm-mc -triple=x86_64-windows-gnu %s -filetype=obj -o %t.obj
+# RUN: llvm-mc -triple=x86_64-windows-gnu -defsym listptrs=1 %s -filetype=obj -o %t.obj
 # RUN: lld-link -lldmingw -debug:symtab -out:%t.exe -entry:main %t.obj %t-lib.lib -verbose
 
+# RUN: llvm-mc -triple=x86_64-windows-gnu -defsym listptrs=0 %s -filetype=obj -o %t.noptrs.obj
+# RUN: not lld-link -lldmingw -runtime-pseudo-reloc:no -debug:symtab -out:%t.exe -entry:main %t.noptrs.obj %t-lib.lib 2>&1 | FileCheck --check-prefix=DISABLED %s
+
 # RUN: llvm-readobj --coff-imports %t.exe | FileCheck -check-prefix=IMPORTS %s
 # RUN: llvm-objdump -d %t.exe | FileCheck --check-prefix=DISASM %s
 # RUN: llvm-objdump -s %t.exe | FileCheck --check-prefix=CONTENTS %s
@@ -46,6 +49,8 @@
 # the symbol table.
 # SYMBOLS-NOT: variable
 
+# DISABLED: error: automatic dllimport of variable in {{.*}}/autoimport-x86.s.tmp.noptrs.obj requires pseudo relocations
+
     .global main
     .text
 main:
@@ -54,6 +59,8 @@ main:
     .data
 ptr:
     .quad variable
+.if listptrs==1
 relocs:
     .quad __RUNTIME_PSEUDO_RELOC_LIST__
     .quad __RUNTIME_PSEUDO_RELOC_LIST_END__
+.endif

lld/test/MinGW/driver.test

@@ -223,3 +223,21 @@ UNKNOWN_ARG: error: unknown argument: --foo
 
 RUN: not ld.lld -m i386pep 2>&1 | FileCheck -check-prefix NO_INPUT_FILES %s
 NO_INPUT_FILES: error: no input files
+
+RUN: ld.lld -### -m i386pep foo.o | FileCheck -check-prefix ENABLE_AUTO_IMPORT %s
+RUN: ld.lld -### -m i386pep foo.o --disable-auto-import --enable-auto-import | FileCheck -check-prefix ENABLE_AUTO_IMPORT %s
+RUN: ld.lld -### -m i386pep foo.o -enable-auto-import | FileCheck -check-prefix ENABLE_AUTO_IMPORT %s
+ENABLE_AUTO_IMPORT: -auto-import{{ }}
+
+RUN: ld.lld -### -m i386pep foo.o --disable-auto-import | FileCheck -check-prefix DISABLE_AUTO_IMPORT %s
+RUN: ld.lld -### -m i386pep foo.o -disable-auto-import | FileCheck -check-prefix DISABLE_AUTO_IMPORT %s
+DISABLE_AUTO_IMPORT: -auto-import:no
+
+RUN: ld.lld -### -m i386pep foo.o | FileCheck -check-prefix ENABLE_RUNTIME_PSEUDO_RELOC %s
+RUN: ld.lld -### -m i386pep foo.o --disable-runtime-pseudo-reloc --enable-runtime-pseudo-reloc | FileCheck -check-prefix ENABLE_RUNTIME_PSEUDO_RELOC %s
+RUN: ld.lld -### -m i386pep foo.o -enable-runtime-pseudo-reloc | FileCheck -check-prefix ENABLE_RUNTIME_PSEUDO_RELOC %s
+ENABLE_RUNTIME_PSEUDO_RELOC: -runtime-pseudo-reloc{{ }}
+
+RUN: ld.lld -### -m i386pep foo.o --disable-runtime-pseudo-reloc | FileCheck -check-prefix DISABLE_RUNTIME_PSEUDO_RELOC %s
+RUN: ld.lld -### -m i386pep foo.o -disable-runtime-pseudo-reloc | FileCheck -check-prefix DISABLE_RUNTIME_PSEUDO_RELOC %s
+DISABLE_RUNTIME_PSEUDO_RELOC: -runtime-pseudo-reloc:no