[llvm-branch-commits] [lld] 16cb791 - [ELF] --emit-relocs: fix a crash if .rela.dyn is an empty output section

[Fangrui Song via llvm-branch-commits]

Author: Fangrui Song
Date: 2020-12-16T08:59:38-08:00
New Revision: 16cb7910f51f0c2570b1f3406bcd8d4069e52a3e

URL: https://github.com/llvm/llvm-project/commit/16cb7910f51f0c2570b1f3406bcd8d4069e52a3e
DIFF: https://github.com/llvm/llvm-project/commit/16cb7910f51f0c2570b1f3406bcd8d4069e52a3e.diff

LOG: [ELF] --emit-relocs: fix a crash if .rela.dyn is an empty output section

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.

Reviewed By: grimar

Differential Revision: https://reviews.llvm.org/D93367

Added: 
    lld/test/ELF/linkerscript/emit-relocs-rela-dyn.s

Modified: 
    lld/ELF/OutputSections.cpp

Removed: 
    


################################################################################
diff  --git a/lld/ELF/OutputSections.cpp b/lld/ELF/OutputSections.cpp
index 5eea3bc8f6a51..9fd226fbba2c8 100644
--- a/lld/ELF/OutputSections.cpp
+++ b/lld/ELF/OutputSections.cpp
@@ -418,7 +418,11 @@ void OutputSection::finalize() {
   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;

diff  --git a/lld/test/ELF/linkerscript/emit-relocs-rela-dyn.s b/lld/test/ELF/linkerscript/emit-relocs-rela-dyn.s
new file mode 100644
index 0000000000000..a6c627fb18ce3
--- /dev/null
+++ b/lld/test/ELF/linkerscript/emit-relocs-rela-dyn.s
@@ -0,0 +1,17 @@
+# 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 %t
+# RUN: llvm-readelf -S %t | FileCheck %s
+
+## Note, sh_link of such an empty .rela.dyn is 0.
+# CHECK: Name      Type Address          Off    Size   ES Flg Lk Inf Al
+# CHECK: .rela.dyn RELA 0000000000000000 001000 000000 18   A  0   0  8
+
+SECTIONS {
+  .rela.dyn : { *(.rela*) }
+  __rela_offset = ABSOLUTE(ADDR(.rela.dyn));
+}