Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 13/10/2021 13:22, Peter Zijlstra wrote:
> +/*
> + * Rewrite the compiler generated retpoline thunk calls.
> + *
> + * For spectre_v2=off (!X86_FEATURE_RETPOLINE), rewrite them into immediate
> + * indirect instructions, avoiding the extra indirection.
> + *
> + * For example, convert:
> + *
> + *   CALL __x86_indirect_thunk_\reg
> + *
> + * into:
> + *
> + *   CALL *%\reg
> + *
> + */
> +static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
> +{
> +	void (*target)(void);
> +	int reg, i = 0;
> +
> +	if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
> +		return -1;
> +
> +	target = addr + insn->length + insn->immediate.value;
> +	reg = (target - &__x86_indirect_thunk_rax) /
> +	      (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);

This is equal measures beautiful and terrifying.

Something around here really wants to BUG_ON(reg == 4), because
literally nothing good can come from selecting %rsp.

Also, it might be a good idea (previous patch perhaps) to have some
linker assertions to confirm that the symbols are laid out safely to do
this calculation.

~Andrew