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

David Laight
2026-01-08T12:08:38Z

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. */