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

[David Laight <david.laight.linux@gmail.com>]

On Thu,  8 Jan 2026 09:25:38 +0000
Ard Biesheuvel <ardb@kernel.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@kernel.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. */