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Date: Wed, 6 Mar 2024 11:15:31 -0800
From: Sami Tolvanen <samitolvanen@google.com>
To: Alice Ryhl <aliceryhl@google.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>,
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	rust-for-linux@vger.kernel.org
Subject: Re: [PATCH v2] rust: add flags for shadow call stack sanitizer
Message-ID: <20240306191531.GA454514@google.com>
References: <20240305-shadow-call-stack-v2-1-c7b4a3f4d616@google.com>
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In-Reply-To: <20240305-shadow-call-stack-v2-1-c7b4a3f4d616@google.com>

On Tue, Mar 05, 2024 at 11:58:45AM +0000, Alice Ryhl wrote:
> Add flags to support the shadow call stack sanitizer, both in the
> dynamic and non-dynamic modes.
> 
> Right now, the compiler will emit the warning "unknown feature specified
> for `-Ctarget-feature`: `reserve-x18`". However, the compiler still
> passes it to the codegen backend, so the flag will work just fine. Once
> rustc starts recognizing the flag (or provides another way to enable the
> feature), it will stop emitting this warning. See [1] for the relevant
> issue.
> 
> Currently, the compiler thinks that the aarch64-unknown-none target
> doesn't support -Zsanitizer=shadow-call-stack, so the build will fail if
> you enable shadow call stack in non-dynamic mode. However, I still think
> it is reasonable to add the flag now, as it will at least fail the build
> when using an invalid configuration, until the Rust compiler is fixed to
> list -Zsanitizer=shadow-call-stack as supported for the target. See [2]
> for the feature request to add this.
> 
> I have tested this change with Rust Binder on an Android device using
> CONFIG_DYNAMIC_SCS. Without the -Ctarget-feature=+reserve-x18 flag, the
> phone crashes immediately on boot, and with the flag, the phone appears
> to work normally.
> 
> This contains a TODO to add the -Zuse-sync-unwind=n flag. The flag
> defaults to n, so it isn't a problem today, but the flag is unstable, so
> the default could change in a future compiler release.
> 
> Link: https://github.com/rust-lang/rust/issues/121970 [1]
> Link: https://github.com/rust-lang/rust/issues/121972 [2]
> Signed-off-by: Alice Ryhl <aliceryhl@google.com>
> ---
> This patch raises the question of whether we should change the Rust
> aarch64 support to use a custom target.json specification. If we do
> that, then we can fix both the warning for dynamic SCS and the
> build-failure for non-dynamic SCS without waiting for a new version of
> rustc with the mentioned issues fixed.
> ---
> Changes in v2:
> - Add -Cforce-unwind-tables flag.
> - Link to v1: https://lore.kernel.org/r/20240304-shadow-call-stack-v1-1-f055eaf40a2c@google.com

Reviewed-by: Sami Tolvanen <samitolvanen@google.com>

Sami