[ELF] PROVIDE: fix spurious "symbol not found"

[MaskRay]

When archive member extraction involving ENTRY happens after
addScriptReferencedSymbolsToSymTable,
addScriptReferencedSymbolsToSymTable may fail to define some PROVIDE
symbols used by ENTRY. This is an edge case that regressed after #84512.
(The interaction with PROVIDE and ENTRY-in-archive was not considered
before).

While here, also ensure that --undefined-glob extracted object files
are parsed before addScriptReferencedSymbolsToSymTable.

Fixes: ebb326a

[llvmbot]

@llvm/pr-subscribers-lld

@llvm/pr-subscribers-lld-elf

Author: Fangrui Song (MaskRay)

Changes

When archive member extraction involving ENTRY happens after
addScriptReferencedSymbolsToSymTable,
addScriptReferencedSymbolsToSymTable may fail to define some PROVIDE
symbols used by ENTRY. This is an edge case that regressed after #84512.
(The interaction with PROVIDE and ENTRY-in-archive was not considered
before).

Fixes: ebb326a

---

Full diff: https://github.com/llvm/llvm-project/pull/87530.diff

2 Files Affected:

• (modified) lld/ELF/Driver.cpp (+10-7)
• (modified) lld/test/ELF/linkerscript/symbolreferenced.s (+25)

diff --git a/lld/ELF/Driver.cpp b/lld/ELF/Driver.cpp
index b43da7727e22ae..451c0770fb2e13 100644
--- a/lld/ELF/Driver.cpp
+++ b/lld/ELF/Driver.cpp
@@ -2727,13 +2727,6 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
   // Create dynamic sections for dynamic linking and static PIE.
   config->hasDynSymTab = !ctx.sharedFiles.empty() || config->isPic;
 
-  script->addScriptReferencedSymbolsToSymTable();
-
-  // Prevent LTO from removing any definition referenced by -u.
-  for (StringRef name : config->undefined)
-    if (Defined *sym = dyn_cast_or_null<Defined>(symtab.find(name)))
-      sym->isUsedInRegularObj = true;
-
   // If an entry symbol is in a static archive, pull out that file now.
   if (Symbol *sym = symtab.find(config->entry))
     handleUndefined(sym, "--entry");
@@ -2742,6 +2735,16 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
   for (StringRef pat : args::getStrings(args, OPT_undefined_glob))
     handleUndefinedGlob(pat);
 
+  // After potential archive member extraction involving ENTRY and
+  // -u/--undefined-glob, check whether PROVIDE symbols should be defined (the
+  // RHS may refer to definitions in just extracted object files).
+  script->addScriptReferencedSymbolsToSymTable();
+
+  // Prevent LTO from removing any definition referenced by -u.
+  for (StringRef name : config->undefined)
+    if (Defined *sym = dyn_cast_or_null<Defined>(symtab.find(name)))
+      sym->isUsedInRegularObj = true;
+
   // Mark -init and -fini symbols so that the LTO doesn't eliminate them.
   if (Symbol *sym = dyn_cast_or_null<Defined>(symtab.find(config->init)))
     sym->isUsedInRegularObj = true;
diff --git a/lld/test/ELF/linkerscript/symbolreferenced.s b/lld/test/ELF/linkerscript/symbolreferenced.s
index 6f583d20e27641..6848082690837f 100644
--- a/lld/test/ELF/linkerscript/symbolreferenced.s
+++ b/lld/test/ELF/linkerscript/symbolreferenced.s
@@ -50,6 +50,21 @@
 # RUN: not ld.lld -T chain2.t a.o 2>&1 | FileCheck %s --check-prefix=ERR --implicit-check-not=error:
 # ERR-COUNT-3: error: chain2.t:1: symbol not found: undef
 
+## _start in a lazy object file references PROVIDE symbols. We extract _start
+## earlier to avoid spurious "symbol not found" errors.
+# RUN: llvm-mc -filetype=obj -triple=x86_64 undef.s -o undef.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64 start.s -o start.o
+# RUN: ld.lld -T chain2.t undef.o --start-lib start.o --end-lib -o lazy
+# RUN: llvm-nm lazy | FileCheck %s --check-prefix=LAZY
+# RUN: ld.lld -e 0 -T chain2.t --undefined-glob '_start*' undef.o --start-lib start.o --end-lib -o lazy
+# RUN: llvm-nm lazy | FileCheck %s --check-prefix=LAZY
+
+# LAZY:      T _start
+# LAZY-NEXT: t f1
+# LAZY-NEXT: T f2
+# LAZY-NEXT: T newsym
+# LAZY-NEXT: T unde
+
 #--- a.s
 .global _start
 _start:
@@ -89,3 +104,13 @@ PROVIDE(newsym = f1);
 PROVIDE(f2 = undef);
 PROVIDE_HIDDEN(f1 = f2);
 PROVIDE(newsym = f1);
+
+#--- undef.s
+.globl undef
+undef: ret
+
+#--- start.s
+.globl _start
+_start: ret
+.data
+.quad newsym

[MaskRay]

@partaror

[partaror]

Thank you for the quick fix!

[statham-arm]

I've checked this against the real cases where we were having link failures, and confirmed that it solves those as well as my 3-line reduced examples in #84512.

LGTM. Thanks for the very quick fix!

[MaskRay]

Landed as dcc45fa.

[vitalybuka]

This on another ELF related commit breaks https://lab.llvm.org/buildbot/#/builders/5/builds/42341