linux-kernel - Re: [PATCH v4 4/9] kentry: Simplify the common syscall API

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Date:   Fri, 19 Mar 2021 17:09:53 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>, x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Brian Gerst <brgerst@...il.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH v4 4/9] kentry: Simplify the common syscall API

On Wed, Mar 17 2021 at 11:12, Andy Lutomirski wrote:
> The new common syscall API had a large and confusing API surface.  Simplify
> it.  Now there is exactly one way to use it.  It's a bit more verbose than
> the old way for the simple x86_64 native case, but it's much easier to use
> right,

and therefore much easier to get wrong...

>  __visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
>  {
> -	nr = syscall_enter_from_user_mode(regs, nr);
> -
> +	kentry_enter_from_user_mode(regs);
> +	local_irq_enable();

... That needs to be _after_ instrumentation_begin(). If you fiddle
with this then please make sure that objtool validates noinstr...

> +	kentry_enter_from_user_mode(regs);
> +	local_irq_enable();

Ditto

> +	instrumentation_begin();
>  static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
>  {
>  	unsigned int nr = syscall_32_enter(regs);
> +	bool ret;
>  	int res;
>  
> -	/*
> -	 * This cannot use syscall_enter_from_user_mode() as it has to
> -	 * fetch EBP before invoking any of the syscall entry work
> -	 * functions.
> -	 */
> -	syscall_enter_from_user_mode_prepare(regs);
> -
> +	kentry_enter_from_user_mode(regs);
> +	local_irq_enable();

...

>  	instrumentation_begin();

Thanks,

        tglx