linux-kernel - Re: [patch 5/8] x86/entry/common: Provide trace/kprobe safe exit to user space functions

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Date:   Thu, 27 Feb 2020 16:53:00 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, Steven Rostedt <rostedt@...dmis.org>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Arnd Bergmann <arnd@...db.de>
Subject: Re: [patch 5/8] x86/entry/common: Provide trace/kprobe safe exit to user space functions

Alexandre Chartre <alexandre.chartre@...cle.com> writes:

> On 2/25/20 11:08 PM, Thomas Gleixner wrote:
>> Split prepare_enter_to_user_mode() and mark it notrace/noprobe so the irq
>> flags tracing on return can be put into it.
>
> This splits prepare_exit_to_usermode() not prepare_enter_to_user_mode().

Ooops

>>   /* Called with IRQs disabled. */
>> -__visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
>> +static inline void __prepare_exit_to_usermode(struct pt_regs *regs)
>>   {
>>   	struct thread_info *ti = current_thread_info();
>>   	u32 cached_flags;
>> @@ -241,6 +241,12 @@ static void exit_to_usermode_loop(struct
>>   	mds_user_clear_cpu_buffers();
>>   }
>>   
>> +__visible inline notrace void prepare_exit_to_usermode(struct pt_regs *regs)
>> +{
>> +	__prepare_exit_to_usermode(regs);
>> +}
>> +NOKPROBE_SYMBOL(prepare_exit_to_usermode);
>
>
> Would it be worth grouping local_irq_disable() and prepare_exit_to_usermode()
> (similarly to what was done entry with syscall_entry_fixups()) and then put
> trace_hardirqs_on() there too. For example:
>
> static __always_inline void syscall_exit_fixups(struct pt_regs *regs)
> {
>          local_irq_disable();
>          prepare_exit_to_usermode(regs);
>          trace_hardirqs_on();
> }
>
> Or is this planned once prepare_exit_from_usermode() is not used by idtentry
> anymore?

This should be consolidated at the very end when all the interrupt muck
is done, but maybe I forgot.