linux-kernel - Re: [PATCH 2/2] start_kernel: omit prevent_tail_call

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Date:   Wed, 12 Apr 2023 15:04:08 -0700
From:   Nathan Chancellor <nathan@...nel.org>
To:     ndesaulniers@...gle.com
Cc:     "Borislav Petkov (AMD)" <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Miguel Ojeda <ojeda@...nel.org>, Tom Rix <trix@...hat.com>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH 2/2] start_kernel: omit prevent_tail_call_optimization
 for newer toolchains

On Wed, Apr 12, 2023 at 11:32:13AM -0700, ndesaulniers@...gle.com wrote:
> prevent_tail_call_optimization was added in
> commit a9a3ed1eff36 ("x86: Fix early boot crash on gcc-10, third try")
> to work around stack canaries getting inserted into functions that would
> initialize the stack canary in the first place.
> 
> Now that we have no_stack_protector function attribute (gcc-11+,
> clang-7+) and use it on start_kernel, remove the call to
> prevent_tail_call_optimization such that we may one day remove it
> outright.
> 
> Signed-off-by: Nick Desaulniers <ndesaulniers@...gle.com>

Reviewed-by: Nathan Chancellor <nathan@...nel.org>

> ---
>  init/main.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/init/main.c b/init/main.c
> index 213baf7b8cb1..c8503d02dfa6 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1152,7 +1152,12 @@ void start_kernel(void)
>  	/* Do the rest non-__init'ed, we're now alive */
>  	arch_call_rest_init();
>  
> +	/* Avoid stack canaries in callers of boot_init_stack_canary for gcc-10
> +	 * and older.
> +	 */
> +#if !__has_attribute(__no_stack_protector__)
>  	prevent_tail_call_optimization();
> +#endif
>  }
>  
>  /* Call all constructor functions linked into the kernel. */
> 
> -- 
> 2.40.0.577.gac1e443424-goog
>