linux-kernel - Re: [tip: x86/core] x86/ibt: Implement FineIBT-BHI mitigation

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Message-ID: <CANiq72mi62AkrKzre254DDd_VwUsZzEMqNuXpFeY_4AjObrNVw@mail.gmail.com>
Date: Wed, 12 Mar 2025 12:36:42 +0100
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Ramon de C Valle <rcvalle@...gle.com>, Matthew Maurer <mmaurer@...gle.com>, linux-kernel@...r.kernel.org, 
	ojeda@...nel.org, linux-tip-commits@...r.kernel.org, 
	Scott Constable <scott.d.constable@...el.com>, Ingo Molnar <mingo@...nel.org>, 
	Kees Cook <kees@...nel.org>, x86@...nel.org, Sami Tolvanen <samitolvanen@...gle.com>, 
	Alice Ryhl <aliceryhl@...gle.com>, Nathan Chancellor <nathan@...nel.org>
Subject: Re: [tip: x86/core] x86/ibt: Implement FineIBT-BHI mitigation

On Wed, Mar 12, 2025 at 10:16 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> The rust-log-analyzer seems to suggest the nightly build failed?
>
> Suppose it didn't fail, where do I find it?

Ah, sorry for the confusion -- by "nightly" here I meant the toolchain
that gets built and distributed by the Rust project.

To get one, we need the PR to land first, and then we can use the new
flag. It is easy to land such a PR/feature because it lands as an
"unstable feature", i.e. meant for testing and so on only. So we can
get quickly "proper" toolchains (i.e. tested and built like the
full/normal releases are), and if everything checks out, then upstream
Rust can commit to stabilize the feature later on.

That is why I suggested to wait for that, since the PR seemed
straightforward to land to me, and thus it could land soon.

I don't know if there may be a way to pick up the toolchains that
their CI built for testing PRs, though. It is not too hard to build
from scratch anyway if you want to do so -- I can also build it for
you if you want to test right away before it lands. Otherwise, I can
ping you when the nightly toolchain is ready.

> I normally build llvm toolchains using tc-build, but it seems rust is
> not actually part of llvm?

Yeah, Rust uses LLVM but is not part of the LLVM project.

But I think we could have support to build Rust there easily -- I
discussed it with Nathan (Cc'd) in the past. Currently, for the
LLVM+Rust toolchains he provides in kernel.org, he just bundles the
upstream Rust binaries AFAIR -- and IIRC he uses tc-build to drive
that build, so if we do that we could also "easily" get the full chain
in kernel.org too ("easily" if we ignore doing the PGO dance for the
Rust side etc. and assuming the building time/resources makes it
doable, which I don't know about).

If that is correct, I could take a look into adding a simple Rust
build to tc-build (i.e. without PGO etc.).

Cheers,
Miguel