From e018cbf7208b3d34f18997ddee84c66cee32fb1b Mon Sep 17 00:00:00 2001 From: Fangrui Song Date: Tue, 23 May 2023 09:49:57 -0700 Subject: [IR] Make stack protector symbol dso_local according to -f[no-]direct-access-external-data There are two motivations. `-fno-pic -fstack-protector -mstack-protector-guard=global` created `__stack_chk_guard` is referenced directly on all ELF OSes except FreeBSD. This patch allows referencing the symbol indirectly with -fno-direct-access-external-data. Some Linux kernel folks want `-fno-pic -fstack-protector -mstack-protector-guard-reg=gs -mstack-protector-guard-symbol=__stack_chk_guard` created `__stack_chk_guard` to be referenced directly, avoiding R_X86_64_REX_GOTPCRELX (even if the relocation may be optimized out by the linker). https://github.com/llvm/llvm-project/issues/60116 Why they need this isn't so clear to me. --- Add module flag "direct-access-external-data" and set the dso_local property of the stack protector symbol. The module flag can benefit other LLVMCodeGen synthesized symbols that are not represented in LLVM IR. Nowadays, with `-fno-pic` being uncommon, ideally we should set "direct-access-external-data" when it is true. However, doing so would require ~90 clang/test tests to be updated, which are too much. As a compromise, we set "direct-access-external-data" only when it's different from the implied default value. Reviewed By: nickdesaulniers Differential Revision: https://reviews.llvm.org/D150841 --- llvm/lib/IR/Module.cpp | 12 ++++++++++++ 1 file changed, 12 insertions(+) (limited to 'llvm/lib/IR/Module.cpp') diff --git a/llvm/lib/IR/Module.cpp b/llvm/lib/IR/Module.cpp index a1f8cd0..73354a8 100644 --- a/llvm/lib/IR/Module.cpp +++ b/llvm/lib/IR/Module.cpp @@ -672,6 +672,18 @@ void Module::setRtLibUseGOT() { addModuleFlag(ModFlagBehavior::Max, "RtLibUseGOT", 1); } +bool Module::getDirectAccessExternalData() const { + auto *Val = cast_or_null( + getModuleFlag("direct-access-external-data")); + if (Val) + return cast(Val->getValue())->getZExtValue() > 0; + return getPICLevel() == PICLevel::NotPIC; +} + +void Module::setDirectAccessExternalData(bool Value) { + addModuleFlag(ModFlagBehavior::Max, "direct-access-external-data", Value); +} + UWTableKind Module::getUwtable() const { if (auto *Val = cast_or_null(getModuleFlag("uwtable"))) return UWTableKind(cast(Val->getValue())->getZExtValue()); -- cgit v1.1