linux-kernel - Re: [v5][PATCH] livepatch/ppc: Enable livepatching on powerpc

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Date:	Tue, 8 Mar 2016 11:45:52 +0100
From:	Torsten Duwe <duwe@....de>
To:	Balbir Singh <bsingharora@...il.com>
Cc:	linuxppc-dev@...abs.org, pmladek@...e.com, jeyu@...hat.com,
	jkosina@...e.cz, jikos@...nel.org, linux-kernel@...r.kernel.org,
	rostedt@...dmis.org, kamalesh@...ux.vnet.ibm.com,
	live-patching@...r.kernel.org, mbenes@...e.cz
Subject: Re: [v5][PATCH] livepatch/ppc: Enable livepatching on powerpc

On Tue, Mar 08, 2016 at 06:33:57PM +1100, Balbir Singh wrote:
> Changelog v5:
> 	1. Removed the mini-stack frame created for klp_return_helper.
> 	   As a result of the mini-stack frame, function with > 8
> 	   arguments could not be patched

Did you get my previous mails? Those functions only require special
care, the _can_ be patched. In general, writing replacement functions
always requires attention!

Have you *tested* this patch? Replacing a function in the kernel?
Replacing a function in a module? For local calls? For global calls?
I strongly doubt so because it does not work this way.

To be fair, my last mail still was not 100% correct, but the conclusion
that the mini frame is not needed at all is invalid. Please leave it as it
was, I'm working on a test / demonstrator for how to handle these.

> +	 * Why do we need this?
> +	 * After patching we need to return to a trampoline return function
> +	 * that guarantees that we restore the TOC and return to the correct
> +	 * caller back
> +	 */
> +	std	r2, 24(r1)	/* save TOC now, unconditionally. */
> +	subf	r0, r2, r0	/* Calculate offset from current TOC */
> +	stw	r0, 12(r1)	/* Of the final LR and save it in CR+4 */
> +	bl	5f
> +5:	mflr	r12
> +	addi	r12, r12, (klp_return_helper + 4 - .)@l
> +	std	r12, LRSAVE(r1)
[...]
> + * maybe inserting a klp_return_helper frame or not.
> +*/
> +klp_return_helper:
> +	ld	r2, 24(r1)	/* restore TOC (saved by ftrace_caller) */
> +	lwa	r0, 12(r1)	/* Load from CR+4, offset of LR w.r.t TOC */
> +	add	r0, r0, r2	/* Add the offset to current TOC */
> +	std	r0, LRSAVE(r1)	/* save the real return address */
> +	mtlr	r0
> +	blr
> +#endif

NAKed-by: Torsten Duwe <duwe@...e.de>

	Torsten