[lld][ELF] Error when deplibs adds new input file after LTO

lld/ELF/Driver.cpp

@@ -2966,6 +2966,7 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
   // With this the symbol table should be complete. After this, no new names
   // except a few linker-synthesized ones will be added to the symbol table.
   const size_t numObjsBeforeLTO = ctx.objectFiles.size();
+  const size_t numInputFilesBeforeLTO = ctx.driver.files.size();
   compileBitcodeFiles<ELFT>(skipLinkedOutput);
 
   // Symbol resolution finished. Report backward reference problems,
@@ -2990,6 +2991,20 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
   for (const DuplicateSymbol &d : ctx.duplicates)
     reportDuplicate(*d.sym, d.file, d.section, d.value);
 
+  // ELF dependent libraries may have introduced new input files after LTO has
+  // completed. This is an error if the files haven't already been parsed, since
+  // changing the symbol table could break the semantic assumptions of LTO.
+  auto newInputFiles = ArrayRef(ctx.driver.files).slice(numInputFilesBeforeLTO);
+  if (!newInputFiles.empty()) {
+    DenseSet<StringRef> oldFilenames;
+    for (InputFile *f :
+         ArrayRef(ctx.driver.files).slice(0, numInputFilesBeforeLTO))
+      oldFilenames.insert(f->getName());
+    for (InputFile *newFile : newInputFiles)
+      if (!oldFilenames.contains(newFile->getName()))
+        errorOrWarn("input file '" + newFile->getName() + "' added after LTO");
+  }
+
   // Handle --exclude-libs again because lto.tmp may reference additional
   // libcalls symbols defined in an excluded archive. This may override
   // versionId set by scanVersionScript().

lld/test/ELF/deplibs-lto.s

@@ -0,0 +1,37 @@
+# REQUIRES: aarch64
+
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=aarch64 -o deplibs.o deplibs.s
+# RUN: llvm-mc -filetype=obj -triple=aarch64 -o foo.o foo.s
+# RUN: llvm-as -o lto.o lto.ll
+# RUN: llvm-ar rc libdeplibs.a deplibs.o
+# RUN: llvm-ar rc libfoo.a foo.o
+
+## LTO emits a libcall (__aarch64_ldadd4_relax) that is resolved using a
+## library (libdeplibs.a) that contains a .deplibs section pointing to a file
+## (libfoo.a) not yet added to the link.
+# RUN: not ld.lld lto.o -u a -L. -ldeplibs 2>&1 | FileCheck %s
+# CHECK: error: input file 'foo.o' added after LTO
+
+## Including the file before LTO prevents the issue.
+# RUN: ld.lld lto.o -u a -L. -ldeplibs -lfoo
+
+#--- foo.s
+.global foo
+foo:
+#--- deplibs.s
+.global __aarch64_ldadd4_relax
+__aarch64_ldadd4_relax:
+    b foo
+.section ".deplibs","MS",@llvm_dependent_libraries,1
+    .asciz "foo"
+#--- lto.ll
+target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
+target triple = "aarch64"
+
+define void @a(i32* nocapture %0) #0 {
+  %2 = atomicrmw add i32* %0, i32 1 monotonic, align 4
+  ret void
+}
+
+attributes #0 = { "target-features"="+outline-atomics" }