Fail to apply aarch64, v8.5a, BTI (branch target identification) feature

help
1

Hello,

When I apply bti (v8.5a) feature to make branch protection, the assembly code seen by objdump does not show BTI instruction.

Anyone has experience adapting 'BTI' feature over aarch64 target, please help.

  • feature
    target=aarch64-unknown-none
    target-feature=+v8.5a,+bti

  • toolchain
    installed toolchains
    nightly-x86_64-unknown-linux-gnu (default)
    ...
    installed targets for active toolchain
    aarch64-linux-android
    aarch64-unknown-linux-gnu
    aarch64-unknown-none
    x86_64-unknown-linux-gnu
    ...
    active toolchain
    nightly-x86_64-unknown-linux-gnu (default)
    rustc 1.55.0-nightly (e8cb1a4a5 2021-06-27)

  • rustc support
    $ rustc --target=aarch64-unknown-none --print target-features
    ...
    bti - Enable Branch Target Identification.
    ...
    v8.5a - Support ARM v8.5a instructions.

2

You could try -Cllvm-args=-mbranch-protection=bti.

3

I'v tried it, but llvm args has no support for it.

rustc -Cllvm-args="..." with: Unknown command line argument '-mbranch-protection=bti'. Try: 'rustc -Cllvm-args="..." with --help'
rustc -Cllvm-args="..." with: Did you mean '--tail-predication=bti'?

rustc -Cllvm-args="--help-list" shows its supporting list. and I could not found branch-protection.

4

-Cllvm-args=--tail-predication=bti may be the correct argument then. The LLVM argument names don't always exactly match clang.

5

Unfortunately, tail-predication does not accept bti.

rustc -Cllvm-args="..." with: for the --tail-predication option: Cannot find option named 'bti'!

--tail-predication= - MVE tail-predication pass options
=disabled - Don't tail-predicate loops
=enabled-no-reductions - Enable tail-predication, but not for reduction loops
=enabled - Enable tail-predication, including reduction loops
=force-enabled-no-reductions - Enable tail-predication, but not for reduction loops, and force this which might be unsafe
=force-enabled - Enable tail-predication, including reduction loops, and force this which might be unsafe

It may be the different feature.

6

Yeah, my bad.

7

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