[lld] r326920 - Write a hash of the executable into the PE timestamp fields.

[Zachary Turner via llvm-commits]

Author: zturner
Date: Wed Mar  7 10:13:41 2018
New Revision: 326920

URL: http://llvm.org/viewvc/llvm-project?rev=326920&view=rev
Log:
Write a hash of the executable into the PE timestamp fields.

Windows tools treats the timestamp fields as sort of a build id,
using it to archive executables on a symbol server, as well as
for matching executables to PDBs.  We were writing 0 for these
fields, which would cause symbol servers to break as they are
indexed in the symbol server based on this value.

Although the field is called timestamp, it can really be any
value that is unique per build, so to support reproducible builds
we use a hash of the executable here.

Differential Revision: https://reviews.llvm.org/D43978

Modified:
    lld/trunk/COFF/Writer.cpp

Modified: lld/trunk/COFF/Writer.cpp
URL: http://llvm.org/viewvc/llvm-project/lld/trunk/COFF/Writer.cpp?rev=326920&r1=326919&r2=326920&view=diff
==============================================================================
--- lld/trunk/COFF/Writer.cpp (original)
+++ lld/trunk/COFF/Writer.cpp Wed Mar  7 10:13:41 2018
@@ -28,6 +28,7 @@
 #include "llvm/Support/Parallel.h"
 #include "llvm/Support/Path.h"
 #include "llvm/Support/RandomNumberGenerator.h"
+#include "llvm/Support/xxhash.h"
 #include <algorithm>
 #include <cstdio>
 #include <map>
@@ -71,11 +72,18 @@ public:
       uint64_t Offs = OS->getFileOff() + (Record->getRVA() - OS->getRVA());
       D->PointerToRawData = Offs;
 
+      TimeDateStamps.push_back(&D->TimeDateStamp);
       ++D;
     }
   }
 
+  void setTimeDateStamp(uint32_t TimeDateStamp) {
+    for (support::ulittle32_t *TDS : TimeDateStamps)
+      *TDS = TimeDateStamp;
+  }
+
 private:
+  mutable std::vector<support::ulittle32_t *> TimeDateStamps;
   const std::vector<Chunk *> &Records;
 };
 
@@ -157,7 +165,7 @@ private:
   RVATableChunk *GuardFidsTable = nullptr;
   RVATableChunk *SEHTable = nullptr;
 
-  Chunk *DebugDirectory = nullptr;
+  DebugDirectoryChunk *DebugDirectory = nullptr;
   std::vector<Chunk *> DebugRecords;
   CVDebugRecordChunk *BuildId = nullptr;
   Optional<codeview::DebugInfo> PreviousBuildId;
@@ -1026,22 +1034,50 @@ void Writer::writeSections() {
 }
 
 void Writer::writeBuildId() {
-  // If we're not writing a build id (e.g. because /debug is not specified),
-  // then just return;
-  if (!Config->Debug)
-    return;
+  // There are two important parts to the build ID.
+  // 1) If building with debug info, the COFF debug directory contains a
+  //    timestamp as well as a Guid and Age of the PDB.
+  // 2) In all cases, the PE COFF file header also contains a timestamp.
+  // For reproducibility, instead of a timestamp we want to use a hash of the
+  // binary, however when building with debug info the hash needs to take into
+  // account the debug info, since it's possible to add blank lines to a file
+  // which causes the debug info to change but not the generated code.
+  //
+  // To handle this, we first set the Guid and Age in the debug directory (but
+  // only if we're doing a debug build).  Then, we hash the binary (thus causing
+  // the hash to change if only the debug info changes, since the Age will be
+  // different).  Finally, we write that hash into the debug directory (if
+  // present) as well as the COFF file header (always).
+  if (Config->Debug) {
+    assert(BuildId && "BuildId is not set!");
+    if (PreviousBuildId.hasValue()) {
+      *BuildId->BuildId = *PreviousBuildId;
+      BuildId->BuildId->PDB70.Age = BuildId->BuildId->PDB70.Age + 1;
+    } else {
+      BuildId->BuildId->Signature.CVSignature = OMF::Signature::PDB70;
+      BuildId->BuildId->PDB70.Age = 1;
+      llvm::getRandomBytes(BuildId->BuildId->PDB70.Signature, 16);
+    }
+  }
 
-  assert(BuildId && "BuildId is not set!");
+  // At this point the only fields in the COFF file which remain unset are the
+  // "timestamp" in the COFF file header, and the ones in the coff debug
+  // directory.  Now we can hash the file and write that hash to the various
+  // timestamp fields in the file.
+  StringRef OutputFileData(
+      reinterpret_cast<const char *>(Buffer->getBufferStart()),
+      Buffer->getBufferSize());
 
-  if (PreviousBuildId.hasValue()) {
-    *BuildId->BuildId = *PreviousBuildId;
-    BuildId->BuildId->PDB70.Age = BuildId->BuildId->PDB70.Age + 1;
-    return;
-  }
+  uint32_t Hash = static_cast<uint32_t>(xxHash64(OutputFileData));
+
+  if (DebugDirectory)
+    DebugDirectory->setTimeDateStamp(Hash);
 
-  BuildId->BuildId->Signature.CVSignature = OMF::Signature::PDB70;
-  BuildId->BuildId->PDB70.Age = 1;
-  llvm::getRandomBytes(BuildId->BuildId->PDB70.Signature, 16);
+  uint8_t *Buf = Buffer->getBufferStart();
+  Buf += DOSStubSize + sizeof(PEMagic);
+  object::coff_file_header *CoffHeader =
+      reinterpret_cast<coff_file_header *>(Buf);
+  CoffHeader->TimeDateStamp = Hash;
 }
 
 // Sort .pdata section contents according to PE/COFF spec 5.5.