[LLD] [MinGW] Implement the --no-seh flag

Previously this flag was just ignored. If set, set the
IMAGE_DLL_CHARACTERISTICS_NO_SEH bit, regardless of the normal safeSEH
machinery.

In mingw configurations, the safeSEH bit might not be set in e.g. object
files built from handwritten assembly, making it impossible to use the
normal safeseh flag. As mingw setups don't generally use SEH on 32 bit
x86 at all, it should be fine to set that flag bit though - hook up
the existing GNU ld flag for controlling that.

Differential Revision: https://reviews.llvm.org/D84701

lld/COFF/Config.h

@@ -140,6 +140,7 @@ struct Configuration {
   bool safeSEH = false;
   Symbol *sehTable = nullptr;
   Symbol *sehCount = nullptr;
+  bool noSEH = false;
 
   // Used for /opt:lldlto=N
   unsigned ltoo = 2;

lld/COFF/Driver.cpp

@@ -1700,9 +1700,10 @@ void LinkerDriver::link(ArrayRef<const char *> argsArr) {
   config->wordsize = config->is64() ? 8 : 4;
 
   // Handle /safeseh, x86 only, on by default, except for mingw.
-  if (config->machine == I386 &&
-      args.hasFlag(OPT_safeseh, OPT_safeseh_no, !config->mingw))
-    config->safeSEH = true;
+  if (config->machine == I386) {
+    config->safeSEH = args.hasFlag(OPT_safeseh, OPT_safeseh_no, !config->mingw);
+    config->noSEH = args.hasArg(OPT_noseh);
+  }
 
   // Handle /functionpadmin
   for (auto *arg : args.filtered(OPT_functionpadmin, OPT_functionpadmin_opt))

lld/COFF/Options.td

@@ -204,6 +204,7 @@ def include_optional : Joined<["/", "-", "/?", "-?"], "includeoptional:">,
     HelpText<"Add symbol as undefined, but allow it to remain undefined">;
 def kill_at : F<"kill-at">;
 def lldmingw : F<"lldmingw">;
+def noseh : F<"noseh">;
 def output_def : Joined<["/", "-", "/?", "-?"], "output-def:">;
 def pdb_source_path : P<"pdbsourcepath",
     "Base path used to make relative source file path absolute in PDB">;

lld/COFF/Writer.cpp

@@ -1393,7 +1393,7 @@ template <typename PEHeaderTy> void Writer::writeHeader() {
     pe->DLLCharacteristics |= IMAGE_DLL_CHARACTERISTICS_GUARD_CF;
   if (config->integrityCheck)
     pe->DLLCharacteristics |= IMAGE_DLL_CHARACTERISTICS_FORCE_INTEGRITY;
-  if (setNoSEHCharacteristic)
+  if (setNoSEHCharacteristic || config->noSEH)
     pe->DLLCharacteristics |= IMAGE_DLL_CHARACTERISTICS_NO_SEH;
   if (config->terminalServerAware)
     pe->DLLCharacteristics |= IMAGE_DLL_CHARACTERISTICS_TERMINAL_SERVER_AWARE;

lld/MinGW/Driver.cpp

@@ -288,6 +288,8 @@ bool mingw::link(ArrayRef<const char *> argsArr, bool canExitEarly,
     add("-kill-at");
   if (args.hasArg(OPT_appcontainer))
     add("-appcontainer");
+  if (args.hasArg(OPT_no_seh))
+    add("-noseh");
 
   if (args.getLastArgValue(OPT_m) != "thumb2pe" &&
       args.getLastArgValue(OPT_m) != "arm64pe" && !args.hasArg(OPT_dynamicbase))

lld/MinGW/Options.td

@@ -56,6 +56,7 @@ defm minor_subsystem_version: EqLong<"minor-subsystem-version",
      "Set the OS and subsystem minor version">;
 def no_insert_timestamp: F<"no-insert-timestamp">,
     HelpText<"Don't include PE header timestamp">;
+def no_seh: F<"no-seh">, HelpText<"Set the 'no SEH' flag in the executable">;
 def no_whole_archive: F<"no-whole-archive">,
     HelpText<"No longer include all object files for following archives">;
 def large_address_aware: Flag<["--"], "large-address-aware">,
@@ -111,7 +112,6 @@ def: Flag<["--"], "full-shutdown">;
 def: F<"high-entropy-va">;
 def: S<"major-image-version">;
 def: S<"minor-image-version">;
-def: F<"no-seh">;
 def: F<"nxcompat">;
 def: F<"pic-executable">;
 def: S<"plugin">;

lld/test/COFF/noseh.s

@@ -0,0 +1,19 @@
+# REQUIRES: x86
+# RUN: llvm-mc -triple i686-w64-mingw32 %s -filetype=obj -o %t.obj
+# RUN: lld-link -lldmingw %t.obj -out:%t.exe -entry:main
+# RUN: llvm-readobj --file-headers %t.exe | FileCheck %s --check-prefix=DEFAULT
+# RUN: lld-link -lldmingw %t.obj -out:%t.noseh.exe -entry:main -noseh
+# RUN: llvm-readobj --file-headers %t.noseh.exe | FileCheck %s --check-prefix=NOSEH
+
+# DEFAULT: Characteristics [
+# DEFAULT-NOT:   IMAGE_DLL_CHARACTERISTICS_NO_SEH
+# DEFAULT: ]
+
+# NOSEH: Characteristics [
+# NOSEH:   IMAGE_DLL_CHARACTERISTICS_NO_SEH
+# NOSEH: ]
+
+        .text
+        .globl  _main
+_main:
+        ret

lld/test/MinGW/driver.test

@@ -256,3 +256,7 @@ RUN: ld.lld -### -m i386pep foo.o -section-alignment 0x2000 | FileCheck -check-p
 RUN: ld.lld -### -m i386pep foo.o --section-alignment=0x2000 | FileCheck -check-prefix ALIGN %s
 RUN: ld.lld -### -m i386pep foo.o -section-alignment=0x2000 | FileCheck -check-prefix ALIGN %s
 ALIGN: -align:0x2000
+
+RUN: ld.lld -### -m i386pe foo.o -no-seh | FileCheck -check-prefix NOSEH %s
+RUN: ld.lld -### -m i386pe foo.o --no-seh | FileCheck -check-prefix NOSEH %s
+NOSEH: -noseh