From: Nathan Chancellor <nathan@kernel.org>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org, linux@armlinux.org.uk,
arnd@arndb.de, ndesaulniers@google.com, keescook@chromium.org
Subject: Re: [PATCH] ARM: Thumb2: avoid __builtin_thread_pointer() on Clang
Date: Thu, 28 Oct 2021 09:32:29 -0700 [thread overview]
Message-ID: <YXrQnQNRGaE7anWy@archlinux-ax161> (raw)
In-Reply-To: <20211028083527.1057158-1-ardb@kernel.org>
On Thu, Oct 28, 2021 at 10:35:27AM +0200, Ard Biesheuvel wrote:
> If available, we use the __builtin_thread_pointer() helper to get the
> value of the TLS register, to help the compiler understand that it
> doesn't need to reload it every time we access 'current'.
>
> Unfortunately, Clang fails to emit the MRC system register read
> directly when building for Thumb2, and instead, it issues a call to the
> __aeabi_read_tp helper, which the kernel does not provide, and so this
> result in link failures at build time.
>
> So create a special case for this, and emit the MRC directly using an
> asm() block, just like we do when the helper is not available to begin
> with.
>
> Link: https://github.com/ClangBuiltLinux/linux/issues/1485
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
> ---
> arch/arm/include/asm/current.h | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/arch/arm/include/asm/current.h b/arch/arm/include/asm/current.h
> index 1d472fa7697b..6bf0aad672c3 100644
> --- a/arch/arm/include/asm/current.h
> +++ b/arch/arm/include/asm/current.h
> @@ -26,12 +26,17 @@ static inline struct task_struct *get_current(void)
> {
> struct task_struct *cur;
>
> -#if __has_builtin(__builtin_thread_pointer)
> +#if __has_builtin(__builtin_thread_pointer) && \
> + !(defined(CONFIG_THUMB2_KERNEL) && \
> + defined(CONFIG_CC_IS_CLANG) && CONFIG_CLANG_VERSION < 130001)
> /*
> * Use the __builtin helper when available - this results in better
> * code, especially when using GCC in combination with the per-task
> * stack protector, as the compiler will recognize that it needs to
> * load the TLS register only once in every function.
> + *
> + * Clang < 13.0.1 gets this wrong for Thumb2 builds:
> + * https://github.com/ClangBuiltLinux/linux/issues/1485
> */
> cur = __builtin_thread_pointer();
> #else
> --
> 2.30.2
>
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next prev parent reply other threads:[~2021-10-28 16:33 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-28 8:35 [PATCH] ARM: Thumb2: avoid __builtin_thread_pointer() on Clang Ard Biesheuvel
2021-10-28 16:25 ` Nick Desaulniers
2021-10-28 16:32 ` Nathan Chancellor [this message]
2021-10-28 16:52 ` Kees Cook
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