[lld] [ELF] Make shouldAddProvideSym values consistent when demoted to Undefined (PR #111945)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-lld-elf

Author: Fangrui Song (MaskRay)

<details>
<summary>Changes</summary>

Case: `PROVIDE(f1 = bar);` when both f1 and bar are in separate sections
that would be discarded by GC.

Due to demoteDefined, `shouldAddProvideSym(f1)` may initially return
false (when Defined) and then return true (been demoted to Undefined).

```
addScriptReferencedSymbolsToSymTable
  shouldAddProvideSym(f1): false
  // the RHS (bar) is not added to `referencedSymbols` and may be GCed
declareSymbols
  shouldAddProvideSym(f1): false
markLive
demoteSymbolsAndComputeIsPreemptible
  // demoted f1 to Undefined
processSymbolAssignments
  addSymbol
    shouldAddProvideSym(f1): true
```

When `cmd->expression()` in `addSymbol` is evaluated, this could lead to
`symbol not found: bar` errors (#<!-- -->111478).

Fix this by adding a `sym->isUsedInRegularObj` condition, making
`shouldAddProvideSym(f1)` values consistent. In addition, we need a
`sym->exportDynamic` condition to keep provide-shared.s working.


---
Full diff: https://github.com/llvm/llvm-project/pull/111945.diff


2 Files Affected:

- (modified) lld/ELF/LinkerScript.cpp (+6-1) 
- (added) lld/test/ELF/linkerscript/provide-defined.s (+33) 


``````````diff
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index f3f95ec589bd82..2247d592c8bca3 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -1811,6 +1811,11 @@ void LinkerScript::addScriptReferencedSymbolsToSymTable() {
 }
 
 bool LinkerScript::shouldAddProvideSym(StringRef symName) {
+  // A Defined may be demoted to Undefined but the return value must stay the
+  // same. Use isUsedInRegularObj to ensure this. The exportDynamic condition,
+  // while not so accurate, allows PROVIDE to define a symbol referenced by a
+  // DSO.
   Symbol *sym = elf::ctx.symtab->find(symName);
-  return sym && !sym->isDefined() && !sym->isCommon();
+  return sym && !sym->isDefined() && !sym->isCommon() &&
+         (sym->isUsedInRegularObj || sym->exportDynamic);
 }
diff --git a/lld/test/ELF/linkerscript/provide-defined.s b/lld/test/ELF/linkerscript/provide-defined.s
new file mode 100644
index 00000000000000..6ebfd706c16e1b
--- /dev/null
+++ b/lld/test/ELF/linkerscript/provide-defined.s
@@ -0,0 +1,33 @@
+# REQUIRES: x86
+## Test the GC behavior when the PROVIDE symbol is defined by a relocatable file.
+
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=x86_64 a.s -o a.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64 b.s -o b.o
+# RUN: ld.lld -T a.t --gc-sections a.o b.o -o a
+# RUN: llvm-readelf -s a | FileCheck %s
+
+# CHECK:     1: {{.*}}               0 NOTYPE  GLOBAL DEFAULT     1 _start
+# CHECK-NEXT:2: {{.*}}               0 NOTYPE  GLOBAL DEFAULT     2 f3
+# CHECK-NOT: {{.}}
+
+#--- a.s
+.global _start, f1, f2, f3, bar
+_start:
+  call f3
+
+.section .text.f1,"ax"; f1:
+.section .text.f2,"ax"; f2:
+.section .text.f3,"ax"; f3:
+.section .text.bar,"ax"; bar:
+
+#--- b.s
+  call f2
+
+#--- a.t
+SECTIONS {
+  . = . + SIZEOF_HEADERS;
+  PROVIDE(f1 = bar+1);
+  PROVIDE(f2 = bar+2);
+  PROVIDE(f3 = bar+3);
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/111945