linux-kernel - Re: [RFC/RFT PATCH 11/19] x86/rethook: Use RIP-relative reference for fake return address

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Message-ID: <20260108120829.6ea6aa0a@pumpkin>
Date: Thu, 8 Jan 2026 12:08:29 +0000
From: David Laight <david.laight.linux@...il.com>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, Thomas Gleixner
 <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov
 <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin"
 <hpa@...or.com>, Josh Poimboeuf <jpoimboe@...nel.org>, Peter Zijlstra
 <peterz@...radead.org>, Kees Cook <kees@...nel.org>, Uros Bizjak
 <ubizjak@...il.com>, Brian Gerst <brgerst@...il.com>,
 linux-hardening@...r.kernel.org
Subject: Re: [RFC/RFT PATCH 11/19] x86/rethook: Use RIP-relative reference
 for fake return address

On Thu,  8 Jan 2026 09:25:38 +0000
Ard Biesheuvel <ardb@...nel.org> wrote:

> Pushing an immediate absolute address to the stack is not permitted when
> linking x86_64 code in PIE mode. Usually, the address can be taken using
> a RIP-relative LEA instruction, but this is not possible here as there
> are no available registers.
> 
> So instead, take the address into a static global, and push it onto the
> stack using a RIP-relative memory operand.

The comment implies the address is 'fake'.
Does that mean it could just be a constant?
Clearly the unwinder would need the same change.

	David

> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/x86/kernel/rethook.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/rethook.c b/arch/x86/kernel/rethook.c
> index 85e2f2d16a90..50812ac718b0 100644
> --- a/arch/x86/kernel/rethook.c
> +++ b/arch/x86/kernel/rethook.c
> @@ -11,6 +11,10 @@
>  
>  __visible void arch_rethook_trampoline_callback(struct pt_regs *regs);
>  
> +#ifdef CONFIG_X86_64
> +static __used void * const __arch_rethook_trampoline = &arch_rethook_trampoline;
> +#endif
> +
>  #ifndef ANNOTATE_NOENDBR
>  #define ANNOTATE_NOENDBR
>  #endif
> @@ -27,7 +31,7 @@ asm(
>  #ifdef CONFIG_X86_64
>  	ANNOTATE_NOENDBR "\n"	/* This is only jumped from ret instruction */
>  	/* Push a fake return address to tell the unwinder it's a rethook. */
> -	"	pushq $arch_rethook_trampoline\n"
> +	"	pushq __arch_rethook_trampoline(%rip)\n"
>  	UNWIND_HINT_FUNC
>  	"       pushq $" __stringify(__KERNEL_DS) "\n"
>  	/* Save the 'sp - 16', this will be fixed later. */