[ELF] Allow getErrPlace() to work before Out::bufferStart is set

[arichardson]

Summary

So far it seems like the only test affected by this change is the one I
recently added for R_MIPS_JALR relocations since the other test cases that
use this function early (unknown-relocation-*) do not have a valid input
section for the relocation offset.

[ruiu]

lld/ELF/Target.cpp

109

If Out::bufferStart is a nullptr, does this if statement always succeed?

[arichardson]

lld/ELF/Target.cpp

109

I don't believe so, since isec->data().data() will only be in range for the matching input section

[ruiu]

LGTM

[arichardson]

Fix for UBSAN failure

[merge_guards_bot]

Unit tests: pass. 60867 tests passed, 0 failed and 726 were skipped.

clang-format: pass.

Build artifacts: console-log.txt, CMakeCache.txt, test-results.xml, diff.json

lld/ELF/Target.cpp

 //
 //===----------------------------------------------------------------------===//
 
 #include "Target.h"
 #include "InputFiles.h"
 #include "OutputSections.h"
 #include "SymbolTable.h"
 #include "Symbols.h"
+#include "SyntheticSections.h"
 #include "lld/Common/ErrorHandler.h"
 #include "llvm/Object/ELF.h"
 
 using namespace llvm;
 using namespace llvm::object;
 using namespace llvm::ELF;
 
 namespace lld {
case EM_SPARCV9:
     return getSPARCV9TargetInfo();
   case EM_X86_64:
     return getX86_64TargetInfo();
   }
   llvm_unreachable("unknown target machine");
 }
 
 template <class ELFT> static ErrorPlace getErrPlace(const uint8_t *loc) {
-  if (!Out::bufferStart)
-    return {};
-
+  assert(loc != nullptr);
   for (InputSectionBase *d : inputSections) {
     auto *isec = cast<InputSection>(d);
     if (!isec->getParent())
       continue;
 
-    uint8_t *isecLoc = Out::bufferStart + isec->getParent()->offset + isec->outSecOff;
+    const uint8_t *isecLoc =
+        Out::bufferStart
+            ? (Out::bufferStart + isec->getParent()->offset + isec->outSecOff)
+            : isec->data().data();
+    if (isecLoc == nullptr) {
+      assert(isa<SyntheticSection>(isec) && "No data but not synthetic?");
+      continue;
+    }
     if (isecLoc <= loc && loc < isecLoc + isec->getSize())
       return {isec, isec->template getLocation<ELFT>(loc - isecLoc) + ": "};
   }
   return {};
 }
 
 ErrorPlace getErrorPlace(const uint8_t *loc) {
   switch (config->ekind) {
   case ELF32LEKind:

lld/test/ELF/mips-jalr-non-functions.s

 # RUN: llvm-objdump --no-show-raw-insn --no-leading-addr -d %t.so | FileCheck %s
 
 .set  noreorder
 test:
   .reloc .Ltmp1, R_MIPS_JALR, tls_obj
 .Ltmp1:
   jr  $t9
   nop
-# WARNING-MESSAGE: warning: found R_MIPS_JALR relocation against non-function symbol tls_obj. This is invalid and most likely a compiler bug.
+# WARNING-MESSAGE: warning: {{.+}}.tmp.o:(.text+0x0): found R_MIPS_JALR relocation against non-function symbol tls_obj. This is invalid and most likely a compiler bug.
 
   .reloc .Ltmp2, R_MIPS_JALR, reg_obj
 .Ltmp2:
   jr  $t9
   nop
-# WARNING-MESSAGE: warning: found R_MIPS_JALR relocation against non-function symbol reg_obj. This is invalid and most likely a compiler bug.
+# WARNING-MESSAGE: warning: {{.+}}.tmp.o:(.text+0x8): found R_MIPS_JALR relocation against non-function symbol reg_obj. This is invalid and most likely a compiler bug.
 
   .reloc .Ltmp3, R_MIPS_JALR, untyped
 .Ltmp3:
   jr  $t9
   nop
 
 ## However, we do perform the optimization for untyped symbols:
 untyped: