Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64

[Peter Zijlstra]

On Mon, Nov 26, 2018 at 11:56:24AM -0600, Josh Poimboeuf wrote:
> diff --git a/arch/x86/kernel/static_call.c b/arch/x86/kernel/static_call.c
> index d3869295b88d..8fd6c8556750 100644
> --- a/arch/x86/kernel/static_call.c
> +++ b/arch/x86/kernel/static_call.c
> @@ -7,24 +7,19 @@
>  
>  #define CALL_INSN_SIZE 5
>  
> +unsigned long bp_handler_call_return_addr;
>  
> +static void static_call_bp_handler(struct pt_regs *regs)
> +{
>  #ifdef CONFIG_HAVE_STATIC_CALL_INLINE
> +     /*
> +      * Push the return address on the stack so the "called" function will
> +      * return to immediately after the call site.
> +      */
> +     regs->sp -= sizeof(long);
> +     *(unsigned long *)regs->sp = bp_handler_call_return_addr;
>  #endif
> +}
>  
>  void arch_static_call_transform(void *site, void *tramp, void *func)
>  {
> @@ -52,14 +47,12 @@ void arch_static_call_transform(void *site, void *tramp, 
> void *func)
>       opcodes[0] = insn_opcode;
>       memcpy(&opcodes[1], &dest_relative, CALL_INSN_SIZE - 1);
>  
>       if (IS_ENABLED(CONFIG_HAVE_STATIC_CALL_INLINE))
> +             bp_handler_call_return_addr = insn + CALL_INSN_SIZE;
>  
>       /* Patch the call site: */
>       text_poke_bp((void *)insn, opcodes, CALL_INSN_SIZE,
> -                  static_call_bp_handler);
> +                  static_call_bp_handler, func);
>  
>  done:
>       mutex_unlock(&text_mutex);


like maybe something along the lines of:

struct sc_data {
        unsigned long ret;
        unsigned long ip;
};

void sc_handler(struct pt_regs *regs, void *data)
{
        struct sc_data *scd = data;

        regs->sp -= sizeof(long);
        *(unsigned long *)regs->sp = scd->ret;
        regs->ip = scd->ip;
}

arch_static_call_transform()
{
        ...

        scd = (struct sc_data){
                .ret = insn + CALL_INSN_SIZE,
                .ip = (unsigned long)func,
        };

        text_poke_bp((void *)insn, opcodes, CALL_INSN_SIZE,
                        sc_handler, (void *)&scd);

        ...
}