[COFF] Don't error out on undefined references to __enclave_config

[mstorsjo]

Summary

This is required for linking the CRT from MSVC 2017 15.5.

While this is unrelated to guardcf.test in general, it's the same kind of issue (avoiding errors due to undefined symbols), so I added the symbol to that test instead of creating a completely new one.

[ruiu]

What does this symbol do in MSVC 2017?

[mstorsjo]

In D41089#951580, @ruiu wrote:

What does this symbol do in MSVC 2017?

I'm not quite sure - I just ran into it after updating to 15.5 (which is the latest update to 2017, released last week). I think the symbol is related to Intel SGX things; googling it landed me here: https://www.reddit.com/r/cpp/comments/6zjub9/msvc_now_has_partial_twophase_name_lookup/dn13dxz/

[ruiu]

LGTM

Got it. We can figure this out later.

lld/trunk/COFF/Driver.cpp

void LinkerDriver::link(ArrayRef<const char *> ArgsArr) {
   // Define CFG symbols anyway so that we can link MSVC 2015 CRT.
   Symtab->addAbsolute(mangle("__guard_fids_count"), 0);
   Symtab->addAbsolute(mangle("__guard_fids_table"), 0);
   Symtab->addAbsolute(mangle("__guard_flags"), 0x100);
   Symtab->addAbsolute(mangle("__guard_iat_count"), 0);
   Symtab->addAbsolute(mangle("__guard_iat_table"), 0);
   Symtab->addAbsolute(mangle("__guard_longjmp_count"), 0);
   Symtab->addAbsolute(mangle("__guard_longjmp_table"), 0);
+  // Needed for MSVC 2017 15.5 CRT.
+  Symtab->addAbsolute(mangle("__enclave_config"), 0);
 
   // This code may add new undefined symbols to the link, which may enqueue more
   // symbol resolution tasks, so we need to continue executing tasks until we
   // converge.
   do {
     // Windows specific -- if entry point is not found,
     // search for its mangled names.
     if (Config->Entry)

lld/trunk/test/COFF/guardcf.test

- Name:            __guard_longjmp_count
     ComplexType:     IMAGE_SYM_DTYPE_NULL
     StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
   - Name:            __guard_longjmp_table
     Value:           0
     SectionNumber:   0
     SimpleType:      IMAGE_SYM_TYPE_NULL
     ComplexType:     IMAGE_SYM_DTYPE_NULL
     StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
+  - Name:            __enclave_config
+    Value:           0
+    SectionNumber:   0
+    SimpleType:      IMAGE_SYM_TYPE_NULL
+    ComplexType:     IMAGE_SYM_DTYPE_NULL
+    StorageClass:    IMAGE_SYM_CLASS_EXTERNAL
 ...