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

[MaskRay]

Summary

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 -mcmodel=large -no-pie -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 -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.

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.

Mips has an ABI issue (https://github.com/llvm/llvm-project/issues/5837) and we
don't implement GNU ld's DW_EH_PE_absptr conversion. mips64-eh-abs-reloc.s wants
a dynamic relocation, so keep the original behavior for EM_MIPS.

[peter.smith]

LGTM as avoiding text segment relocations when we can is probably better than trying to add dynamic relocations.

I'm not sure I fully understand why our sectionBase::getOffset isn't working for this particular case, I expect it would become clear if I traced through the code though. However I think the canonical PLT is good solution regardless.

Some suggestions for the language in the comments.

lld/ELF/Relocations.cpp

1082–1090

Use a simple -z notext rule that treats all sections except .eh_frame as writable. GNU ld does not produce dynamic relocations in .ehframe (and if we do, our sectionBase::getOffset will incorrectly adjust the offset).

lld/test/ELF/eh-frame-znotext.s

5

Typo and and.

5

There's a typo (and and), could be useful to restrict the canonical PLT to .ehframe. Probably implicit from the test name though.

suggest "try avoiding that in .ehframe (https://github.com/llvm/llvm-project/issues/60392) and use a canonical PLT entry instead.

[MaskRay]

comments

lld/ELF/Relocations.cpp

void RelocationScanner::processAux(RelExpr expr, RelType type, uint64_t offset,
   // choices, and we choose dynamic relocations to be consistent with the
   // handling of GOT-generating relocations.
   if (isStaticLinkTimeConstant(expr, type, sym, offset) ||
       (!config->isPic && sym.isUndefWeak())) {
     sec->addReloc({expr, type, offset, addend, &sym});
     return;
   }
 
-  bool canWrite = (sec->flags & SHF_WRITE) || !config->zText;
+  // Use a simple -z notext rule that treats all sections except .eh_frame as
+  // writable. GNU ld does not produce 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)) {
       addRelativeReloc<true>(*sec, offset, sym, addend, expr, type);
       return;
     } else if (rel != 0) {
       if (config->emachine == EM_MIPS && rel == target->symbolicRel)
         rel = target->relativeRel;

lld/test/ELF/eh-frame-znotext.s

+# REQUIRES: aarch64
+## While a symbolic relocation for -z notext in .eh_frame can emit a dynamic
+## relocation, we try avoiding that (https://github.com/llvm/llvm-project/issues/60392)
+## 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: