AArch64 BTI support - No BTI after setjmp

[llvmbot]

Bugzilla Link	49544
Version	11.0
OS	other
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor,@zygoloid,@smithp35

Extended Description

When enabling the -mbranch-protection={bti,standard} option in Clang, BTI/PAC instruction are well inserted at each function entry, which works well.
However, the setjmp/longjmp API is not supported while it is in the GNU GCC compiler (see https://gcc.gnu.org/legacy-ml/gcc-patches/2018-11/msg02472.html).
To work properly, a BTI instruction must be added after each call to setjmp function. I have not found any reference to similar in Clang.

While this code is under fixing, is it possible to build a Clang/LLVM module to add instruction right after these setjmp calls ?

[smithp35]

As I understand it there is a trade off between adding a BTI after a call to setjmp which permits setjmp to return with an indirect jump, but also means there are more gadgets in the program, the address in the jump_buf could be retrieved and branched to.

For example the Bionic implementation of longjump returns with a ret which means a BTI is not needed https://android.googlesource.com/platform/bionic/+/refs/heads/master/libc/arch-arm64/bionic/setjmp.S

Having said that, if the compiler doesn't know what libc is being used it should assume that it might need a BTI and insert one after setjmp. If it does happen to know, such as when Android is used, then it won't need to.

So I agree we should do this. I couldn't give you a commitment of when it will happen though.

[DavidSpickett]

I've just started to look at doing this. https://reviews.llvm.org/D112427 did the same thing for Arm 32 bit M profile targets so I will base it on that. Then we can have the clang side drivers set it if we know what that platform's C lib does.

Showing the difference: https://godbolt.org/z/PzMhvo6WM

[llvmbot]

@llvm/issue-subscribers-backend-AArch64

[DavidSpickett]

OP-TEE workaround for future reference: OP-TEE/optee_os@30e743f

I now have a working patch just need to see how to add GlobalIsel support as well.

[DavidSpickett]

https://reviews.llvm.org/D121707