linux-kernel - Re: [RFC 17/17] x86/traps: Use a new on_thread

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Date:   Tue, 12 Sep 2017 21:25:58 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC 17/17] x86/traps: Use a new on_thread_stack() helper to
 clean up an assertion

On 12/09/2017 21:11, Josh Poimboeuf wrote:
> On Wed, Sep 06, 2017 at 02:37:02PM -0700, Andy Lutomirski wrote:
>> Signed-off-by: Andy Lutomirski <luto@...nel.org>
>> ---
>>  arch/x86/include/asm/processor.h   | 17 +++++++++++++++++
>>  arch/x86/include/asm/thread_info.h | 11 -----------
>>  arch/x86/kernel/traps.c            |  3 +--
>>  3 files changed, 18 insertions(+), 13 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
>> index 4c137472f530..b6f8dc11c222 100644
>> --- a/arch/x86/include/asm/processor.h
>> +++ b/arch/x86/include/asm/processor.h
>> @@ -534,6 +534,23 @@ static inline unsigned long current_top_of_stack(void)
>>  #endif
>>  }
>>  
>> +static inline unsigned long current_stack_pointer(void)
>> +{
>> +	unsigned long sp;
>> +#ifdef CONFIG_X86_64
>> +	asm("mov %%rsp,%0" : "=g" (sp));
>> +#else
>> +	asm("mov %%esp,%0" : "=g" (sp));
>> +#endif
>> +	return sp;
>> +}
> I know you're just moving the function, but this function could also be
> cleaned up by using _ASM_SP and getting rid of the ifdef.
>

For GCC, there is a rather more efficient way of doing this, which
allows the compiler much more flexibility than forcing the use of a mov
instruction.

http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/include/asm-x86/current.h;h=89849929ebf98e9ba04659f59b5521982a718b2d;hb=HEAD#l47

I don't know how much you care in Linux, but in Xen it makes a massive
code-volume difference, as that construct is the core of current
(because we can't use %fs/%gs).

~Andrew