linux-kernel - Re: [RFC PATCH 2/8] jump label v4 - x86: Introduce generic jump patching without stop

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Date:	Thu, 14 Jan 2010 10:32:19 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu,
	mathieu.desnoyers@...ymtl.ca, hpa@...or.com, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	mhiramat@...hat.com, Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC PATCH 2/8] jump label v4 - x86: Introduce generic jump
 patching without stop_machine

On Tue, 2010-01-12 at 11:26 -0500, Jason Baron wrote:

> +/**
> + * text_poke_fixup() -- cross-modifying kernel text with fixup address.
> + * @addr:	Modifying address.
> + * @opcode:	New instruction.
> + * @len:	length of modifying bytes.
> + * @fixup:	Fixup address.
> + *
> + * Note: You must backup replaced instructions before calling this,
> + * if you need to recover it.
> + * Note: Must be called under text_mutex.
> + */
> +void *__kprobes text_poke_fixup(void *addr, const void *opcode, size_t len,
> +				void *fixup)
> +{
> +	static const unsigned char int3_insn = BREAKPOINT_INSTRUCTION;
> +	static const int int3_size = sizeof(int3_insn);
> +
> +	/* Replacing 1 byte can be done atomically. */
> +	if (unlikely(len <= 1))
> +		return text_poke(addr, opcode, len);

This part bothers me. The text_poke just writes over the text directly
(using a separate mapping). But if that memory is in the pipeline of
another CPU, I think this could cause a GPF.

-- Steve

> +
> +	/* Preparing */
> +	patch_fixup_addr = fixup;
> +	wmb();
> +	patch_fixup_from = (u8 *)addr + int3_size; /* IP address after int3 */
> +
> +	/* Cap by an int3 */
> +	text_poke(addr, &int3_insn, int3_size);
> +	sync_core_all();
> +
> +	/* Replace tail bytes */
> +	text_poke((char *)addr + int3_size, (const char *)opcode + int3_size,
> +		  len - int3_size);
> +	sync_core_all();
> +
> +	/* Replace int3 with head byte */
> +	text_poke(addr, opcode, int3_size);
> +	sync_core_all();
> +
> +	/* Cleanup */
> +	patch_fixup_from = NULL;
> +	wmb();
> +	return addr;
> +}
> +

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