[lld] [LLD][COFF] Add support for custom DOS stub (PR #122561)

[via llvm-commits]

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@@ -1668,21 +1680,31 @@ template <typename PEHeaderTy> void Writer::writeHeader() {
   // When run under Windows, the loader looks at AddressOfNewExeHeader and uses
   // the PE header instead.
   Configuration *config = &ctx.config;
+
   uint8_t *buf = buffer->getBufferStart();
   auto *dos = reinterpret_cast<dos_header *>(buf);
-  buf += sizeof(dos_header);
-  dos->Magic[0] = 'M';
-  dos->Magic[1] = 'Z';
-  dos->UsedBytesInTheLastPage = dosStubSize % 512;
-  dos->FileSizeInPages = divideCeil(dosStubSize, 512);
-  dos->HeaderSizeInParagraphs = sizeof(dos_header) / 16;
-
-  dos->AddressOfRelocationTable = sizeof(dos_header);
-  dos->AddressOfNewExeHeader = dosStubSize;
 
   // Write DOS program.
-  memcpy(buf, dosProgram, sizeof(dosProgram));
-  buf += sizeof(dosProgram);
+  if (config->stub.size()) {
+    memcpy(buf, config->stub.data(), config->stub.size());
+    // MS link.exe accepts an invalid `e_lfanew` and updates it automatically.
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kkent030315 wrote:

Yes, `dos_header::AddressOfNewExeHeader` is `IMAGE_DOS_HEADER::e_lfanew` defined in [COFF.h](https://github.com/llvm/llvm-project/blob/051612c0180e4e5a9ba750a994a91d2c1b05b00c/llvm/include/llvm/Object/COFF.h#L57). I'll mention that in the comment.

https://github.com/llvm/llvm-project/pull/122561