[llvm-xray] Support for PIE

[phosek]

Summary

When the instrumented binary is linked as PIE, we need to apply the
relative relocations to sleds. This is handled by the dynamic linker
at runtime, but when processing the file we have to do it ourselves.

[dberris]

LGTM -- Thanks @phosek!

[MaskRay]

llvm/lib/XRay/InstrumentationMap.cpp

90

Section.relocations() applies on either SHT_REL or SHT_RELA. I think the code can be applied on just SHT_RELA.

if (Section.isBSS() || Section.isVirtual()) is not necessary.

[MaskRay]

llvm/lib/XRay/InstrumentationMap.cpp

97

The code only works on x86_64 and ppc64le. So the relative relocations can just be enumerated here.

[joerg]

llvm/lib/XRay/InstrumentationMap.cpp

97

Please explicitly check for the relocation types you care about. They are not the only relocations you can find in a PIE binary, even with static linking.

[dberris]

LGTM still, thanks @phosek!

[MaskRay]

llvm/lib/XRay/InstrumentationMap.cpp

105

It can be simplified with
if (auto AddendOrErr = object::ELFRelocationRef(Reloc).getAddend())

llvm/lib/XRay/InstrumentationMap.cpp

 //===- InstrumentationMap.cpp - XRay Instrumentation Map ------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // Implementation of the InstrumentationMap type for XRay sleds.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/XRay/InstrumentationMap.h"
+#include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/ADT/Triple.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Object/Binary.h"
+#include "llvm/Object/ELFObjectFile.h"
 #include "llvm/Object/ObjectFile.h"
 #include "llvm/Support/DataExtractor.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/YAMLTraits.h"
 #include <algorithm>
 #include <cstddef>
 #include <cstdint>
if (I == Sections.end())
     return make_error<StringError>(
         "Failed to find XRay instrumentation map.",
         std::make_error_code(std::errc::executable_format_error));
 
   if (I->getContents(Contents))
     return errorCodeToError(
         std::make_error_code(std::errc::executable_format_error));
 
+  using RelocMap = DenseMap<uint64_t, uint64_t>;
+  RelocMap Relocs;
+
+  if (ObjFile.getBinary()->isELF()) {
+    for (const object::SectionRef &Section : Sections) {
+      // Skip sections that will never have any relocations.
+      if (Section.isBSS() || Section.isVirtual())
+        continue;
+
+      for (const object::RelocationRef &Reloc : Section.relocations()) {
+        // We assume the R_*_RELATIVE relocations are being used so we
+        // compute the addend here and later use it as the value. This
+        // logic will need to be extended if other relocation types are
+        // ever being used.
+        auto AddendOrErr = object::ELFRelocationRef(Reloc).getAddend();
+        if (AddendOrErr)
+          Relocs.insert({Reloc.getOffset(), *AddendOrErr});
+      }
+    }
+  }
+
   // Copy the instrumentation map data into the Sleds data structure.
   auto C = Contents.bytes_begin();
   static constexpr size_t ELF64SledEntrySize = 32;
 
   if ((C - Contents.bytes_end()) % ELF64SledEntrySize != 0)
     return make_error<StringError>(
         Twine("Instrumentation map entries not evenly divisible by size of "
               "an XRay sled entry in ELF64."),
         std::make_error_code(std::errc::executable_format_error));
 
+  auto RelocIfNeeded = [&](DataExtractor &Extractor, uint32_t *Offset) {
+    uint64_t Address = I->getAddress() + C - Contents.bytes_begin() + *Offset;
+    uint64_t Value = Extractor.getU64(Offset);
+    if (!Value) {
+      RelocMap::const_iterator R = Relocs.find(Address);
+      if (R != Relocs.end())
+        return R->second;
+    }
+    return Value;
+  };
+
   int32_t FuncId = 1;
   uint64_t CurFn = 0;
   for (; C != Contents.bytes_end(); C += ELF64SledEntrySize) {
     DataExtractor Extractor(
         StringRef(reinterpret_cast<const char *>(C), ELF64SledEntrySize), true,
         8);
     Sleds.push_back({});
     auto &Entry = Sleds.back();
     uint32_t OffsetPtr = 0;
-    Entry.Address = Extractor.getU64(&OffsetPtr);
-    Entry.Function = Extractor.getU64(&OffsetPtr);
+    Entry.Address = RelocIfNeeded(Extractor, &OffsetPtr);
+    Entry.Function = RelocIfNeeded(Extractor, &OffsetPtr);
     auto Kind = Extractor.getU8(&OffsetPtr);
     static constexpr SledEntry::FunctionKinds Kinds[] = {
         SledEntry::FunctionKinds::ENTRY, SledEntry::FunctionKinds::EXIT,
         SledEntry::FunctionKinds::TAIL,
         SledEntry::FunctionKinds::LOG_ARGS_ENTER,
         SledEntry::FunctionKinds::CUSTOM_EVENT};
     if (Kind >= sizeof(Kinds))
       return errorCodeToError(

llvm/test/tools/llvm-xray/X86/extract-instrmap-pie.ll

+; This test makes sure we can extract the instrumentation map from an
+; XRay-instrumented PIE file.
+;
+; RUN: llvm-xray extract %S/Inputs/elf64-pie.bin -s | FileCheck %s
+
+; CHECK:      ---
+; CHECK-NEXT: - { id: 1, address: 0x00000000000299C0, function: 0x00000000000299C0, kind: function-enter, always-instrument: true, function-name: {{.*foo.*}} }
+; CHECK-NEXT: - { id: 1, address: 0x00000000000299D0, function: 0x00000000000299C0, kind: function-exit, always-instrument: true, function-name: {{.*foo.*}} }
+; CHECK-NEXT: - { id: 2, address: 0x00000000000299E0, function: 0x00000000000299E0, kind: function-enter, always-instrument: true, function-name: {{.*bar.*}} }
+; CHECK-NEXT: - { id: 2, address: 0x00000000000299F6, function: 0x00000000000299E0, kind: function-exit, always-instrument: true, function-name: {{.*bar.*}} }
+; CHECK-NEXT: ...