[LLD][COFF] Ignore /kernel flag

[pzheng]

Summary

There exists some description of the flag from Microsoft, but not sure if
there's more to it. We ignore the flag for now until we find out more about it.

https://docs.microsoft.com/en-us/cpp/build/reference/kernel-create-kernel-mode-binary?view=msvc-170

[pzheng]

minor change

[hans]

Do you have a use case for this, or have you checked that it works?

I'm not familiar with this flag, but I suspect there could be more to it (for example, do any PE headers change).

If we're not sure that this is everything that's required, I think it's better to leave the flag as unsupported.

[thieta]

I was just also about to comment that it seems like it's just ignoring the incremental flag and that it has to be more to it?

[pzheng]

Thanks for the comments, @hans and @thieta! I haven't done any verification of anything built with this flag yet. So, I am not very confident whether there's more to it than what's documented by Microsoft. I guess, for now, we'll just ignore the flag. At some point, I'll probably get a chance to do more testing on this and maybe we can revisit it then.

[pzheng]

Ignore the flag

[thieta]

Lgtm. Wait with merge until Hans checks it as well.

[pzheng]

Thanks, @thieta! @hans, please let me know if there is anything I need to address.

[hans]

lgtm

[pzheng]

Thanks!

lld/COFF/Driver.cpp

void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   config->ltoPGOWarnMismatch = args.hasFlag(OPT_lto_pgo_warn_mismatch,
                                             OPT_lto_pgo_warn_mismatch_no, true);
   config->allowBind = args.hasFlag(OPT_allowbind, OPT_allowbind_no, true);
   config->allowIsolation =
       args.hasFlag(OPT_allowisolation, OPT_allowisolation_no, true);
   config->incremental =
       args.hasFlag(OPT_incremental, OPT_incremental_no,
                    !config->doGC && config->doICF == ICFLevel::None &&
-                       !args.hasArg(OPT_order) && !args.hasArg(OPT_profile));
+                       !args.hasArg(OPT_order) && !args.hasArg(OPT_profile) &&
+                       !args.hasArg(OPT_kernel));
   config->integrityCheck =
       args.hasFlag(OPT_integritycheck, OPT_integritycheck_no, false);
   config->cetCompat = args.hasFlag(OPT_cetcompat, OPT_cetcompat_no, false);
   config->nxCompat = args.hasFlag(OPT_nxcompat, OPT_nxcompat_no, true);
   for (auto *arg : args.filtered(OPT_swaprun))
     parseSwaprun(arg->getValue());
   config->terminalServerAware =
       !config->dll && args.hasFlag(OPT_tsaware, OPT_tsaware_no, true);
void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   config->mapFile = getMapFile(args, OPT_map, OPT_map_file);
 
   if (config->lldmapFile != "" && config->lldmapFile == config->mapFile) {
     warn("/lldmap and /map have the same output file '" + config->mapFile +
          "'.\n>>> ignoring /lldmap");
     config->lldmapFile.clear();
   }
 
+  if (config->incremental && args.hasArg(OPT_kernel)) {
+    warn("ignoring '/incremental' due to '/kernel' specification");
+    config->incremental = false;
+  }
+
   if (config->incremental && args.hasArg(OPT_profile)) {
     warn("ignoring '/incremental' due to '/profile' specification");
     config->incremental = false;
   }
 
   if (config->incremental && args.hasArg(OPT_order)) {
     warn("ignoring '/incremental' due to '/order' specification");
     config->incremental = false;

lld/COFF/Options.td

 def dll : F<"dll">, HelpText<"Create a DLL">;
 def driver : F<"driver">, HelpText<"Generate a Windows NT Kernel Mode Driver">;
 def driver_wdm : F<"driver:wdm">,
     HelpText<"Set IMAGE_FILE_UP_SYSTEM_ONLY bit in PE header">;
 def driver_uponly : F<"driver:uponly">,
     HelpText<"Set IMAGE_DLL_CHARACTERISTICS_WDM_DRIVER bit in PE header">;
 def driver_wdm_uponly : F<"driver:wdm,uponly">;
 def driver_uponly_wdm : F<"driver:uponly,wdm">;
+def kernel : F<"kernel">;
 def nodefaultlib_all : F<"nodefaultlib">,
     HelpText<"Remove all default libraries">;
 def noentry : F<"noentry">,
     HelpText<"Don't add reference to DllMainCRTStartup; only valid with /dll">;
 def profile : F<"profile">;
 def repro : F<"Brepro">,
     HelpText<"Use a hash of the executable as the PE header timestamp">;
 def reproduce : Joined<["/", "-", "/?", "-?"], "reproduce:">,