AArch64: add BTI hint to *fcontext trampolines - fixes BTI crash on Graviton4

[zaydr-al]

Summary

When Boost.Context is built with PAC + BTI hardening (-mbranch-protection=standard), the first indirect jump into any trampoline raises SIGILL on BTI-enforcing CPUs (Graviton 4).
Cause: the trampolines are missing the mandatory btic hint.

LLVM is currently fixing the analogous omission for compiler-generated landing pads in llvm.llvm-project #149267 (PR llvm/llvm-project#149680).
Hand-written assembly still needs the hint.

Minimal repro (Amazon Linux 2023, Boost develop, Clang 15):

#1. clone Boost super-repo
git clone --branch develop --recursive https://github.com/boostorg/boost.git boost
cd boost

#2. build Boost.Context only, with PAC+BTI
./bootstrap.sh >/dev/null
./b2 -j$"(nproc)" -q --with-context toolset=clang \
        cxxflags="-O2 -fPIC -mbranch-protection=standard" \
        asmflags="-O2 -fPIC -mbranch-protection=standard" \
        linkflags="-Wl,-z,force-bti" \
        install --prefix=stage

#3. small test that throws inside a fiber
cat > test.cpp <<'CPP'
#include <boost/context/fiber.hpp>
#include <iostream>
int main(){
    boost::context::fiber f([](boost::context::fiber&& s){
        try { throw 1; } catch(int){ std::cout<<"caught\n"; }
        return std::move(s);
    }); 
    f = std::move(f).resume();
}
CPP

clang++ -target aarch64-linux-gnu -mbranch-protection=standard \
        -I$(pwd)/stage/include \
        test.cpp -Lstage/lib -lboost_context \
        -Wl,-z,force-bti \
        -Wl,-rpath,$ORIGIN/../lib -Wl,-rpath,$(pwd)/stage/lib \
        -o test_fiber

# 4. run on Arm host that enforces BTI (Graviton 4)
./test_fiber

Proposed Fix

Insert at the top of each AArch64 trampoline:

#if defined(__ARM_FEATURE_BTI_DEFAULT) && (__ARM_FEATURE_BTI_DEFAULT == 1)
/* bti c */
      hint        #34 
#endif

Files

src/asm/jump_arm64_aapcs_elf_gas.S
src/asm/make_arm64_aapcs_elf_gas.S
src/asm/ontop_arm64_aapcs_elf_gas.S

Adds one 4-byte NOP-class instruction only when BTI is requested.

Environment

• Boost develop
• AmazonLinux2023
• Clang15, Clang 18
• Hardware: AWSGraviton4

[ozbenh]

You also need to add the .note.gnu.property section to indicate BTI support in every one of those .S files. Afaik, without that, the linker will not put it in the final binary (it should require it to be present in all the .o's) and thus the boost .so will not be mapped with PROT_BTI.

In your example, you "work around" it by using force-bti, but that shouldn't be necessary if you added the section.

[zaydr-al]

Thanks, that makes sense. I encountered the warning (all inputs do not have BTI in note section) because I was forcing BTI at link time. Emitting the .note.gnu.property at the object level in each trampoline should indeed address this.

[olk]

Please provide an PR