[PATCH] D41105: [ELF] Prevent crash in writing an .ARM.exidx sentinel entry.

[Igor Kudrin via Phabricator via llvm-commits]

ikudrin updated this revision to Diff 126704.
ikudrin retitled this revision from "[ELF] Fix placement of a sentinel entry in the .ARM.exidx section." to "[ELF] Prevent crash in writing an .ARM.exidx sentinel entry.".
ikudrin edited the summary of this revision.
ikudrin added a comment.

- Limited the patch to fix only the crash in `ARMExidxSentinelSection::writeTo()`.


https://reviews.llvm.org/D41105

Files:
  ELF/SyntheticSections.cpp
  test/ELF/linkerscript/arm-exidx-sentinel-and-assignment.s


Index: test/ELF/linkerscript/arm-exidx-sentinel-and-assignment.s
===================================================================
--- /dev/null
+++ test/ELF/linkerscript/arm-exidx-sentinel-and-assignment.s
@@ -0,0 +1,12 @@
+# REQUIRES: arm
+# RUN: llvm-mc -filetype=obj -triple=armv7a-none-linux-gnueabi %s -o %t.o
+# RUN: echo "SECTIONS {                                 \
+# RUN:         .ARM.exidx : { *(.ARM.exidx*) foo = .; } \
+# RUN:       }" > %t.script
+## We used to crash in the case when the last output section command
+## for .ARM.exidx was anything but an input section description.
+# RUN: ld.lld -T %t.script %t.o -shared -o %t.so
+
+.fnstart
+.cantunwind
+.fnend
Index: ELF/SyntheticSections.cpp
===================================================================
--- ELF/SyntheticSections.cpp
+++ ELF/SyntheticSections.cpp
@@ -2569,16 +2569,22 @@
 void ARMExidxSentinelSection::writeTo(uint8_t *Buf) {
   // The Sections are sorted in order of ascending PREL31 address with the
   // sentinel last. We need to find the InputSection that precedes the
-  // sentinel. By construction the Sentinel is in the last
-  // InputSectionDescription as the InputSection that precedes it.
+  // sentinel.
   OutputSection *C = getParent();
-  auto ISD =
-      std::find_if(C->SectionCommands.rbegin(), C->SectionCommands.rend(),
-                   [](const BaseCommand *Base) {
-                     return isa<InputSectionDescription>(Base);
-                   });
-  auto L = cast<InputSectionDescription>(*ISD);
-  InputSection *Highest = L->Sections[L->Sections.size() - 2];
+  InputSection *Highest = nullptr;
+  int Skip = 1;
+  for (const BaseCommand *Base : llvm::reverse(C->SectionCommands)) {
+    if (!isa<InputSectionDescription>(Base))
+      continue;
+    auto L = cast<InputSectionDescription>(Base);
+    if (Skip >= L->Sections.size()) {
+      Skip -= L->Sections.size();
+      continue;
+    }
+    Highest = L->Sections[L->Sections.size() - Skip - 1];
+    break;
+  }
+  assert(Highest);
   InputSection *LS = Highest->getLinkOrderDep();
   uint64_t S = LS->getParent()->Addr + LS->getOffset(LS->getSize());
   uint64_t P = getVA();


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