[PATCH] D143136: [ELF] -z notext: avoid dynamic relocations in .eh_frame

[Fangrui Song via Phabricator via llvm-commits]

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Fix https://github.com/llvm/llvm-project/issues/60392

  // a.cc
  void raise() { throw 42; }
  bool foo() {
    try { raise(); } catch (int) { return true; }
    return false;
  }
  int main() { foo(); }



  clang++ --target=x86_64-linux-gnu -fno-pic -no-pie -mcmodel=large -fuse-ld=lld -z notext a.cc -o a && ./a
  clang++ --target=aarch64-linux-gnu -fno-pic -no-pie -fuse-ld=lld -Wl,--dynamic-linker=/usr/aarch64-linux-gnu/lib/ld-linux-aarch64.so.1 -Wl,-rpath=/usr/aarch64-linux-gnu/lib -fuse-ld=lld -z notext a.cc -o a && ./a

Both commands fail because we produce a dynamic relocation for
R_X86_64_64/R_AARCH64_ABS64 in .eh_frame which will be adjusted to a wrong
offset by `SectionBase::getOffset` after D122459 <https://reviews.llvm.org/D122459>.

Since GNU ld uses a canonical PLT entry instead of a dynamic relocation for
.eh_frame, we follow suit as well to avoid the issue.
mips64-eh-abs-reloc.s wants a dynamic relocation, so keep the original behavior
for EM_MIPS.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D143136

Files:
  lld/ELF/Relocations.cpp
  lld/test/ELF/eh-frame-znotext.s


Index: lld/test/ELF/eh-frame-znotext.s
===================================================================
--- /dev/null
+++ lld/test/ELF/eh-frame-znotext.s
@@ -0,0 +1,30 @@
+# REQUIRES: aarch64
+## While a symbolic relocation for -z notext can emit a dynamic relocation, we
+## try avoiding that (https://github.com/llvm/llvm-project/issues/60392) and
+## and use a canonical PLT entry instead.
+
+# RUN: rm -rf %t && split-file %s %t && cd %t
+# RUN: llvm-mc -filetype=obj -triple=aarch64 a.s -o a.o
+# RUN: llvm-mc -filetype=obj -triple=aarch64 abi.s -o abi.o
+# RUN: ld.lld -shared abi.o -o abi.so
+
+# RUN: ld.lld a.o abi.so -o a
+# RUN: llvm-readelf -r --dyn-syms a | FileCheck %s
+# RUN: ld.lld -z notext a.o abi.so -o a
+# RUN: llvm-readelf -r --dyn-syms a | FileCheck %s
+
+# CHECK: R_AARCH64_JUMP_SLOT {{.*}} __gxx_personality_v0 + 0
+
+# CHECK: 1: 00000000002{{.*}} 0 FUNC    GLOBAL DEFAULT  UND __gxx_personality_v0
+
+#--- a.s
+foo:
+.cfi_startproc
+.cfi_personality 0, __gxx_personality_v0
+  ret
+.cfi_endproc
+
+#--- abi.s
+.globl __gxx_personality_v0
+.type __gxx_personality_v0, @function
+__gxx_personality_v0:
Index: lld/ELF/Relocations.cpp
===================================================================
--- lld/ELF/Relocations.cpp
+++ lld/ELF/Relocations.cpp
@@ -1079,7 +1079,15 @@
     return;
   }
 
-  bool canWrite = (sec->flags & SHF_WRITE) || !config->zText;
+  // Use a simple -z notext rule that treats all sections except .eh_frame
+  // writable. GNU ld does not reproduce dynamic relocations in .eh_frame (and
+  // our SectionBase::getOffset would incorrectly adjust the offset).
+  //
+  // For MIPS, we don't implement GNU ld's DW_EH_PE_absptr to DW_EH_PE_pcrel
+  // conversion. We still emit a dynamic relocation.
+  bool canWrite = (sec->flags & SHF_WRITE) ||
+                  !(config->zText ||
+                    (isa<EhInputSection>(sec) && config->emachine != EM_MIPS));
   if (canWrite) {
     RelType rel = target->getDynRel(type);
     if (expr == R_GOT || (rel == target->symbolicRel && !sym.isPreemptible)) {


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