linux-kernel - Re: [PATCH] x86: rust: Disable entry padding with Rust

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Message-ID: <ca0f2db8-e353-4f52-aaa1-35042e52ad91@intel.com>
Date: Wed, 31 Jan 2024 12:31:55 -0800
From: Dave Hansen <dave.hansen@...el.com>
To: Matthew Maurer <mmaurer@...gle.com>, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, Miguel Ojeda <ojeda@...nel.org>,
 Alex Gaynor <alex.gaynor@...il.com>,
 Wedson Almeida Filho <wedsonaf@...il.com>
Cc: "H. Peter Anvin" <hpa@...or.com>, Boqun Feng <boqun.feng@...il.com>,
 Gary Guo <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, Benno Lossin <benno.lossin@...ton.me>,
 Andreas Hindborg <a.hindborg@...sung.com>, Alice Ryhl
 <aliceryhl@...gle.com>, linux-kernel@...r.kernel.org,
 rust-for-linux@...r.kernel.org
Subject: Re: [PATCH] x86: rust: Disable entry padding with Rust

On 12/15/23 11:39, Matthew Maurer wrote:
> +config HAVE_ENTRY_PADDING
> +	def_bool CC_HAS_ENTRY_PADDING && !RUST

My only worry with this is that we need HAVE_ENTRY_PADDING for:

> config HAVE_CALL_THUNKS
>         def_bool y
>         depends on CC_HAS_ENTRY_PADDING && RETHUNK && OBJTOOL
..
> config CALL_DEPTH_TRACKING
>         bool "Mitigate RSB underflow with call depth tracking"
>         depends on CPU_SUP_INTEL && HAVE_CALL_THUNKS

so if they turn on RUST, they'll end up turning off CALL_DEPTH_TRACKING.
 I'm wondering if it might be better to do this instead (temporarily of
course):

config RUST
        bool "Rust support"
        depends on HAVE_RUST
        depends on RUST_IS_AVAILABLE
        depends on !MODVERSIONS
        depends on !GCC_PLUGINS
        depends on !RANDSTRUCT
        depends on !DEBUG_INFO_BTF || PAHOLE_HAS_LANG_EXCLUDE
+	depends on !CALL_THUNKS

That way, someone who is using CALL_DEPTH_TRACKING doesn't accidentally
lose it by turning on RUST.  To turn on RUST, they'd first need to go
turn off the things that are selecting CALL_THUNKS.