linux-kernel - Re: [PATCH v2 03/13] uprobes: allow arch access to xol slot

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Message-ID: <20131019163627.GA7837@redhat.com>
Date:	Sat, 19 Oct 2013 18:36:27 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Long <dave.long@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, Rabin Vincent <rabin@....in>,
	"Jon Medhurst (Tixy)" <tixy@...aro.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 03/13] uprobes: allow arch access to xol slot

On 10/15, David Long wrote:
>
> Allow arches to customize how the instruction is filled into the xol
> slot.  ARM will use this to insert an undefined instruction after the
> real instruction in order to simulate a single step of the instruction
> without hardware support.

OK, but

> +void __weak arch_uprobe_xol_copy(struct arch_uprobe *auprobe, void *vaddr)
> +{
> +	memcpy(vaddr, auprobe->insn, MAX_UINSN_BYTES);
> +}
> +
>  /*
>   * xol_get_insn_slot - allocate a slot for xol.
>   * Returns the allocated slot address or 0.
> @@ -1246,6 +1251,7 @@ static unsigned long xol_get_insn_slot(struct uprobe *uprobe)
>  {
>  	struct xol_area *area;
>  	unsigned long xol_vaddr;
> +	void *kaddr;
>  
>  	area = get_xol_area();
>  	if (!area)
> @@ -1256,7 +1262,9 @@ static unsigned long xol_get_insn_slot(struct uprobe *uprobe)
>  		return 0;
>  
>  	/* Initialize the slot */
> -	copy_to_page(area->page, xol_vaddr, uprobe->arch.insn, MAX_UINSN_BYTES);
> +	kaddr = kmap_atomic(area->page);
> +	arch_uprobe_xol_copy(&uprobe->arch, kaddr + (xol_vaddr & ~PAGE_MASK));
> +	kunmap_atomic(kaddr);

This looks a bit strange and defeats the purpose of generic helper...

How about

	void __weak arch_uprobe_xol_copy(...)
	{
		copy_to_page(...);
	}

then just

	- copy_to_page(...);
	+ arch_uprobe_xol_copy(...);

?

Or, I am just curious, can't we have an empty "__weak arch_uprobe_xol_copy" if
we call it right after copy_to_page() ?

Oleg.

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