[BOLT] Overwrite .eh_frame_hdr in-place (#116730)

If the new EH frame header can fit into the original .eh_frame_hdr
section, overwrite it in-place and pad with zeroes.

Author: maksfb

bolt/lib/Rewrite/RewriteInstance.cpp

@@ -5791,42 +5791,64 @@ void RewriteInstance::writeEHFrameHeader() {
   LLVM_DEBUG(dbgs() << "BOLT: writing a new " << getEHFrameHdrSectionName()
                     << '\n');
 
-  NextAvailableAddress =
-      appendPadding(Out->os(), NextAvailableAddress, EHFrameHdrAlign);
+  // Try to overwrite the original .eh_frame_hdr if the size permits.
+  uint64_t EHFrameHdrOutputAddress = 0;
+  uint64_t EHFrameHdrFileOffset = 0;
+  std::vector<char> NewEHFrameHdr;
+  BinarySection *OldEHFrameHdrSection = getSection(getEHFrameHdrSectionName());
+  if (OldEHFrameHdrSection) {
+    NewEHFrameHdr = CFIRdWrt->generateEHFrameHeader(
+        RelocatedEHFrame, NewEHFrame, OldEHFrameHdrSection->getAddress());
+    if (NewEHFrameHdr.size() <= OldEHFrameHdrSection->getSize()) {
+      BC->outs() << "BOLT-INFO: rewriting " << getEHFrameHdrSectionName()
+                 << " in-place\n";
+      EHFrameHdrOutputAddress = OldEHFrameHdrSection->getAddress();
+      EHFrameHdrFileOffset = OldEHFrameHdrSection->getInputFileOffset();
+    } else {
+      OldEHFrameHdrSection->setOutputName(getOrgSecPrefix() +
+                                          getEHFrameHdrSectionName());
+      OldEHFrameHdrSection = nullptr;
+    }
+  }
 
-  const uint64_t EHFrameHdrOutputAddress = NextAvailableAddress;
-  const uint64_t EHFrameHdrFileOffset =
-      getFileOffsetForAddress(NextAvailableAddress);
+  // If there was not enough space, allocate more memory for .eh_frame_hdr.
+  if (!OldEHFrameHdrSection) {
+    NextAvailableAddress =
+        appendPadding(Out->os(), NextAvailableAddress, EHFrameHdrAlign);
 
-  std::vector<char> NewEHFrameHdr = CFIRdWrt->generateEHFrameHeader(
-      RelocatedEHFrame, NewEHFrame, EHFrameHdrOutputAddress);
+    EHFrameHdrOutputAddress = NextAvailableAddress;
+    EHFrameHdrFileOffset = getFileOffsetForAddress(NextAvailableAddress);
+
+    NewEHFrameHdr = CFIRdWrt->generateEHFrameHeader(
+        RelocatedEHFrame, NewEHFrame, EHFrameHdrOutputAddress);
+
+    NextAvailableAddress += NewEHFrameHdr.size();
+    if (!BC->BOLTReserved.empty() &&
+        (NextAvailableAddress > BC->BOLTReserved.end())) {
+      BC->errs() << "BOLT-ERROR: unable to fit " << getEHFrameHdrSectionName()
+                 << " into reserved space\n";
+      exit(1);
+    }
+
+    // Create a new entry in the section header table.
+    const unsigned Flags = BinarySection::getFlags(/*IsReadOnly=*/true,
+                                                   /*IsText=*/false,
+                                                   /*IsAllocatable=*/true);
+    BinarySection &EHFrameHdrSec = BC->registerOrUpdateSection(
+        getNewSecPrefix() + getEHFrameHdrSectionName(), ELF::SHT_PROGBITS,
+        Flags, nullptr, NewEHFrameHdr.size(), /*Alignment=*/1);
+    EHFrameHdrSec.setOutputFileOffset(EHFrameHdrFileOffset);
+    EHFrameHdrSec.setOutputAddress(EHFrameHdrOutputAddress);
+    EHFrameHdrSec.setOutputName(getEHFrameHdrSectionName());
+  }
 
   Out->os().seek(EHFrameHdrFileOffset);
   Out->os().write(NewEHFrameHdr.data(), NewEHFrameHdr.size());
 
-  const unsigned Flags = BinarySection::getFlags(/*IsReadOnly=*/true,
-                                                 /*IsText=*/false,
-                                                 /*IsAllocatable=*/true);
-  BinarySection *OldEHFrameHdrSection = getSection(getEHFrameHdrSectionName());
+  // Pad the contents if overwriting in-place.
   if (OldEHFrameHdrSection)
-    OldEHFrameHdrSection->setOutputName(getOrgSecPrefix() +
-                                        getEHFrameHdrSectionName());
-
-  BinarySection &EHFrameHdrSec = BC->registerOrUpdateSection(
-      getNewSecPrefix() + getEHFrameHdrSectionName(), ELF::SHT_PROGBITS, Flags,
-      nullptr, NewEHFrameHdr.size(), /*Alignment=*/1);
-  EHFrameHdrSec.setOutputFileOffset(EHFrameHdrFileOffset);
-  EHFrameHdrSec.setOutputAddress(EHFrameHdrOutputAddress);
-  EHFrameHdrSec.setOutputName(getEHFrameHdrSectionName());
-
-  NextAvailableAddress += EHFrameHdrSec.getOutputSize();
-
-  if (!BC->BOLTReserved.empty() &&
-      (NextAvailableAddress > BC->BOLTReserved.end())) {
-    BC->errs() << "BOLT-ERROR: unable to fit " << getEHFrameHdrSectionName()
-               << " into reserved space\n";
-    exit(1);
-  }
+    Out->os().write_zeros(OldEHFrameHdrSection->getSize() -
+                          NewEHFrameHdr.size());
 
   // Merge new .eh_frame with the relocated original so that gdb can locate all
   // FDEs.

bolt/test/eh-frame-hdr.test

@@ -0,0 +1,12 @@
+# Check that llvm-bolt overwrites .eh_frame_hdr in-place.
+
+REQUIRES: system-linux
+
+RUN: %clang %cflags %p/Inputs/hello.c -o %t -Wl,-q
+RUN: llvm-bolt %t -o %t.bolt --use-old-text \
+RUN:   | FileCheck %s --check-prefix=CHECK-BOLT
+RUN: llvm-readelf -WS %t.bolt | FileCheck %s
+
+CHECK-BOLT: rewriting .eh_frame_hdr in-place
+
+CHECK-NOT: .bolt.org.eh_frame_hdr