linux-kernel - Re: Remove __end_entry_SYSENTER

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Message-ID: <20170712084803.ar6v3fiks65v25lc@gmail.com>
Date:   Wed, 12 Jul 2017 10:48:03 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Andy Lutomirski <luto@...capital.net>, x86-ml <x86@...nel.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: Remove __end_entry_SYSENTER_compat?


* Borislav Petkov <bp@...en8.de> wrote:

> Anyone think this is an OK-ish idea?
> 
> It saves us the global symbol but requires the two functions to remained
> glued together. :-\
> 
> ---
> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
> index e1721dafbcb1..262519da8661 100644
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -131,7 +131,6 @@ ENTRY(entry_SYSENTER_compat)
>  	pushq	$X86_EFLAGS_FIXED
>  	popfq
>  	jmp	.Lsysenter_flags_fixed
> -GLOBAL(__end_entry_SYSENTER_compat)
>  ENDPROC(entry_SYSENTER_compat)
>  
>  /*
> @@ -180,6 +179,9 @@ ENDPROC(entry_SYSENTER_compat)
>   * edi  arg5
>   * esp  user stack
>   * 0(%esp) arg6
> + *
> + * DO NOT! move this function and the above before adjusting
> + * is_sysenter_singlestep().
>   */
>  ENTRY(entry_SYSCALL_compat)
>  	/* Interrupts are off on entry. */
> diff --git a/arch/x86/include/asm/proto.h b/arch/x86/include/asm/proto.h
> index 8d3964fc5f91..afdef9f3f0f0 100644
> --- a/arch/x86/include/asm/proto.h
> +++ b/arch/x86/include/asm/proto.h
> @@ -21,7 +21,6 @@ void __end_SYSENTER_singlestep_region(void);
>  
>  #ifdef CONFIG_IA32_EMULATION
>  void entry_SYSENTER_compat(void);
> -void __end_entry_SYSENTER_compat(void);
>  void entry_SYSCALL_compat(void);
>  void entry_INT80_compat(void);
>  #endif
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index bf54309b85da..143902ffe9ff 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -668,7 +668,7 @@ static bool is_sysenter_singlestep(struct pt_regs *regs)
>  		(unsigned long)__begin_SYSENTER_singlestep_region;
>  #elif defined(CONFIG_IA32_EMULATION)
>  	return (regs->ip - (unsigned long)entry_SYSENTER_compat) <
> -		(unsigned long)__end_entry_SYSENTER_compat -
> +		(unsigned long)entry_SYSCALL_compat -
>  		(unsigned long)entry_SYSENTER_compat;
>  #else
>  	return false;

Hm, I'd argue that the old code is much clearer: we need both the start and the 
end of a function and have the properly named symbols for that.

That entry_SYSCALL_compat() happens to start just where 
__end_entry_SYSENTER_compat is an accident of placement.

Is it even true - doesn't ENTRY() imply an .align, in which case it might be that 
__end_entry_SYSENTER_compat != entry_SYSCALL_compat?

In fact that appears to be the case for my defconfig:

  ffffffff81942f90 T entry_SYSENTER_compat
  ffffffff81942feb T __end_entry_SYSENTER_compat
  ffffffff81942ff0 T entry_SYSCALL_compat

So unless there's some disadvantage beyond having one more symbol, I'd favor the 
old approach.

Thanks,

	Ingo