[RFC] [LLD] [COFF] Support options for enabling/disabling autoimport and pseudo relocs in .drectve sections

[mstorsjo]

Summary

This is the other bit I split out from D78923 as mentioned there. However since I wrote this, I'm less convinced that it's a good idea.

Firstly, it makes the lld private option (used on the lld-link interface - the one on the mingw driver interface is public) essentially public, if projects would be to start embedding it in .drectve sections.

Secondly, placing such an option in an object file is tricky, because it's essentially impossible to override it by linker options, as the linker options are read first and the object file is parsed later, overriding it.

Thirdly, after remembering more of the details about this, it turned out that the full pseudo reloc mechanism should be usable even in restricted UWP mode, as long as VirtualProtect is remapped to VirtualProtectFromApp (which already is done in https://sourceforge.net/p/mingw-w64/mingw-w64/ci/8f078a256769c84a021f5512ba0400f1eb8d0c05/), and as long as all places needing fixup is in data sections. If the executable code is generated with LLVM, it should avoid placing any such references within code sections, but always use .refptr stubs - so then this shouldn't really be an issue.

In any case, posting this for discussion as it was mentioned in D78923 - but I'm less convinced about its necessity now. Or what does @jacek think?

[jacek]

With the solution for UWP, we don't have a use case .drectve change, so dropping that part sounds reasonable.

[rnk]

I guess I would prefer not to add .drectve support for this unless there is a use case for it. It is a somewhat fragile interface.

[mstorsjo]

Yeah, let's shelve this one for now.

lld/COFF/Driver.cpp

case OPT_incl:
       addUndefined(arg->getValue());
       break;
     case OPT_merge:
       parseMerge(arg->getValue());
       break;
     case OPT_nodefaultlib:
       config->noDefaultLibs.insert(doFindLib(arg->getValue()).lower());
       break;
+    case OPT_auto_import:
+      config->autoImport = true;
+      break;
+    case OPT_auto_import_no:
+      config->autoImport = false;
+      break;
+    case OPT_runtime_pseudo_reloc:
+      config->pseudoRelocs = true;
+      break;
+    case OPT_runtime_pseudo_reloc_no:
+      config->pseudoRelocs = false;
+      break;
     case OPT_section:
       parseSection(arg->getValue());
       break;
     case OPT_subsystem:
       parseSubsystem(arg->getValue(), &config->subsystem,
                      &config->majorOSVersion, &config->minorOSVersion);
       break;
     // Only add flags here that link.exe accepts in

lld/test/COFF/autoimport-drectve.s

+# REQUIRES: x86
+
+# RUN: echo -e ".global variable\n.global DllMainCRTStartup\n.text\nDllMainCRTStartup:\nret\n.data\nvariable:\n.long 42" > %t-lib.s
+# 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: not lld-link -lldmingw -out:%t.exe -entry:main %t.obj %t-lib.lib 2>&1 | FileCheck %s
+
+## Check that we can disable runtime pseudo relocs via a .drectve in a
+## linked object file:
+##
+# CHECK: error: automatic dllimport of variable in {{.*}}/autoimport-drectve.s.tmp.obj requires pseudo relocations
+
+    .global main
+    .text
+main:
+    movl variable(%rip), %eax
+    ret
+    .data
+ptr:
+    .quad variable
+
+    .section .drectve
+    .ascii "-runtime-pseudo-reloc:no"