[lld] [ELF] Add a dummySym member to Ctx (PR #150796)

[Jessica Clarke via llvm-commits]

https://github.com/jrtc27 created https://github.com/llvm/llvm-project/pull/150796

This ensures subsequent calls to elf::postScanRelocations with a new Ctx
will correctly use an instance with the right internalFile (with the old
one presumably deleted, even). It also avoids having to create a new
instance in elf::getErrorPlace, and will allow more uses of such a dummy
symbol in future commits.


>From 84153f35f3d28bee40c55450fa7c89c5b68abdfc Mon Sep 17 00:00:00 2001
From: Jessica Clarke <jrtc27 at jrtc27.com>
Date: Sat, 26 Jul 2025 21:32:53 +0100
Subject: [PATCH] =?UTF-8?q?[=F0=9D=98=80=F0=9D=97=BD=F0=9D=97=BF]=20initia?=
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Created using spr 1.3.5
---
 lld/ELF/Config.h        | 2 ++
 lld/ELF/Driver.cpp      | 1 +
 lld/ELF/Relocations.cpp | 3 +--
 lld/ELF/Target.cpp      | 3 +--
 4 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/lld/ELF/Config.h b/lld/ELF/Config.h
index d9639b06ca4bf..958e5caaf0dfa 100644
--- a/lld/ELF/Config.h
+++ b/lld/ELF/Config.h
@@ -701,6 +701,8 @@ struct Ctx : CommonLinkerContext {
   std::unique_ptr<llvm::TarWriter> tar;
   // InputFile for linker created symbols with no source location.
   InputFile *internalFile = nullptr;
+  // Dummy Undefined for relocations without a symbol.
+  Undefined *dummySym = nullptr;
   // True if symbols can be exported (isExported) or preemptible.
   bool hasDynsym = false;
   // True if SHT_LLVM_SYMPART is used.
diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index 21d228eda6470..4dcf577ebcb16 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -3138,6 +3138,7 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
     ctx.symtab->insert(arg->getValue())->traced = true;
 
   ctx.internalFile = createInternalFile(ctx, "<internal>");
+  ctx.dummySym = make<Undefined>(ctx.internalFile, "", STB_LOCAL, 0, 0);
 
   // Handle -u/--undefined before input files. If both a.a and b.so define foo,
   // -u foo a.a b.so will extract a.a.
diff --git a/lld/ELF/Relocations.cpp b/lld/ELF/Relocations.cpp
index bd22fe2f1aa25..e847e85b060fe 100644
--- a/lld/ELF/Relocations.cpp
+++ b/lld/ELF/Relocations.cpp
@@ -1948,13 +1948,12 @@ void elf::postScanRelocations(Ctx &ctx) {
 
   GotSection *got = ctx.in.got.get();
   if (ctx.needsTlsLd.load(std::memory_order_relaxed) && got->addTlsIndex()) {
-    static Undefined dummy(ctx.internalFile, "", STB_LOCAL, 0, 0);
     if (ctx.arg.shared)
       ctx.mainPart->relaDyn->addReloc(
           {ctx.target->tlsModuleIndexRel, got, got->getTlsIndexOff()});
     else
       got->addConstant({R_ADDEND, ctx.target->symbolicRel,
-                        got->getTlsIndexOff(), 1, &dummy});
+                        got->getTlsIndexOff(), 1, ctx.dummySym});
   }
 
   assert(ctx.symAux.size() == 1);
diff --git a/lld/ELF/Target.cpp b/lld/ELF/Target.cpp
index ad7d57d30668d..4946484074d05 100644
--- a/lld/ELF/Target.cpp
+++ b/lld/ELF/Target.cpp
@@ -105,10 +105,9 @@ ErrorPlace elf::getErrorPlace(Ctx &ctx, const uint8_t *loc) {
     if (isecLoc <= loc && loc < isecLoc + isec->getSize()) {
       std::string objLoc = isec->getLocation(loc - isecLoc);
       // Return object file location and source file location.
-      Undefined dummy(ctx.internalFile, "", STB_LOCAL, 0, 0);
       ELFSyncStream msg(ctx, DiagLevel::None);
       if (isec->file)
-        msg << isec->getSrcMsg(dummy, loc - isecLoc);
+        msg << isec->getSrcMsg(*ctx.dummySym, loc - isecLoc);
       return {isec, objLoc + ": ", std::string(msg.str())};
     }
   }