[PATCH] D93367: [ELF] --emit-relocs: fix a crash if .rela.dyn is an output section description

[Fangrui Song via Phabricator via llvm-commits]

MaskRay created this revision.
MaskRay added reviewers: grimar, psmith, ruiu.
Herald added subscribers: arichardson, emaste.
Herald added a reviewer: espindola.
MaskRay requested review of this revision.
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Fix PR48357: If .rela.dyn appears as an output section description, its type may
be SHT_RELA (due to the empty synthetic .rela.plt) while there is no input
section. The empty .rela.dyn may be retained due to a reference in a linker
script. Don't crash.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D93367

Files:
  lld/ELF/OutputSections.cpp
  lld/test/ELF/linkerscript/emit-relocs-rela-dyn.s


Index: lld/test/ELF/linkerscript/emit-relocs-rela-dyn.s
===================================================================
--- /dev/null
+++ lld/test/ELF/linkerscript/emit-relocs-rela-dyn.s
@@ -0,0 +1,12 @@
+# REQUIRES: x86
+## PR48357: If .rela.dyn appears as an output section description, its type may
+## be SHT_RELA (due to the empty synthetic .rela.plt) while there is no input
+## section. The empty .rela.dyn may be retained due to a reference. Don't crash.
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64 /dev/null -o %t.o
+# RUN: ld.lld -shared --emit-relocs -T %s %t.o -o /dev/null
+
+SECTIONS {
+  .rela.dyn : { *(.rela*) }
+  __rela_offset = ABSOLUTE(ADDR(.rela.dyn));
+}
Index: lld/ELF/OutputSections.cpp
===================================================================
--- lld/ELF/OutputSections.cpp
+++ lld/ELF/OutputSections.cpp
@@ -418,7 +418,11 @@
   if (!config->copyRelocs || (type != SHT_RELA && type != SHT_REL))
     return;
 
-  if (isa<SyntheticSection>(first))
+  // Skip if 'first' is synthetic, i.e. not a section created by --emit-relocs.
+  // Normally 'type' was changed by 'first' so 'first' should be non-null.
+  // However, if the output section is .rela.dyn, 'type' can be set by the empty
+  // synthetic .rela.plt and first can be null.
+  if (!first || isa<SyntheticSection>(first))
     return;
 
   link = in.symTab->getParent()->sectionIndex;


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