[ELF] - Remove unused synthetic sections correctly.

[grimar]

Summary

This is PR35740 which now crashes
because we remove unused synthetic sections incorrectly.

We can keep input section description and corresponding output
section live even if it must be empty and dead.
This results in a crash because SHF_LINK_ORDER handling code
tries to access first section which is nullptr in this case.

It is based on D37520 which tried to fix the same issue.

[grimar]

• Addressed review comment.

[grimar]

LGTMed by Rafael.

[grimar]

Ping. Rui ?

[ruiu]

LGTM

ELF/Writer.cpp

static void removeUnusedSyntheticSections() {
   for (InputSectionBase *S : llvm::reverse(InputSections)) {
     SyntheticSection *SS = dyn_cast<SyntheticSection>(S);
     if (!SS)
       return;
     OutputSection *OS = SS->getParent();
     if (!OS || !SS->empty())
       continue;
 
-    std::vector<BaseCommand *>::iterator Empty = OS->SectionCommands.end();
-    for (auto I = OS->SectionCommands.begin(), E = OS->SectionCommands.end();
-         I != E; ++I) {
-      BaseCommand *B = *I;
-      if (auto *ISD = dyn_cast<InputSectionDescription>(B)) {
+    // If we reach here, then SS is an unused synthetic section and we want to
+    // remove it from corresponding input section description of output section.
+    for (BaseCommand *B : OS->SectionCommands)
+      if (auto *ISD = dyn_cast<InputSectionDescription>(B))
         llvm::erase_if(ISD->Sections,
                        [=](InputSection *IS) { return IS == SS; });
-        if (ISD->Sections.empty())
-          Empty = I;
-      }
-    }
-    if (Empty != OS->SectionCommands.end())
-      OS->SectionCommands.erase(Empty);
 
-    // If there are no other sections in the output section, remove it from the
-    // output.
-    if (OS->SectionCommands.empty())
+    // If there are no other alive sections or commands left in the output
+    // section description, we remove it from the output.
+    bool IsEmpty = llvm::all_of(OS->SectionCommands, [](BaseCommand *B) {
+      if (auto *ISD = dyn_cast<InputSectionDescription>(B))
+        return ISD->Sections.empty();
+      return false;
+    });
+    if (IsEmpty)
       OS->Live = false;
   }
 }
 
 // Returns true if a symbol can be replaced at load-time by a symbol
 // with the same name defined in other ELF executable or DSO.
 static bool computeIsPreemptible(const Symbol &B) {
   assert(!B.isLocal());

test/ELF/linkerscript/unused-synthetic.s

 # REQUIRES: x86
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t.o
 # RUN: echo "SECTIONS { \
-# RUN:    .got  : { *(.got) } \
+# RUN:    .got  : { *(.got) *(.got) } \
 # RUN:    .plt  : { *(.plt) } \
 # RUN:    .text : { *(.text) } \
 # RUN:  }" > %t.script
 # RUN: ld.lld -shared -o %t.so --script %t.script %t.o
 
 # RUN: llvm-objdump -section-headers %t.so | FileCheck %s
 # CHECK-NOT:  .got
 # CHECK-NOT:  .plt

test/ELF/linkerscript/unused-synthetic2.s

+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=armv7-unknown-linux-gnueabi %s -o %t.o
+# RUN: echo "SECTIONS { .trap : { *(.ARM.exidx) *(.dummy) } }" > %t.script
+
+## We incorrectly removed unused synthetic sections and crashed before.
+## Check we do not crash and do not produce .trap output section.
+# RUN: ld.lld -shared -o %t.so --script %t.script %t.o
+# RUN: llvm-objdump -section-headers %t.so | FileCheck %s
+# CHECK-NOT: .trap