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

On 08/03/16 21:45, Torsten Duwe wrote:

On Tue, Mar 08, 2016 at 06:33:57PM +1100, Balbir Singh wrote:

quoted

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!

Yes, I did. We did think about documenting that limitation, but the big concern
was that the system can be panic'd with a simple test case. We expect live patches
to be tested and signed so we should be OK, but there still is a window

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.

Yes, if you care to read the changelog
"

I tested the sample in the livepatch and an additional sample
that patches int_to_scsilun. I'll post out that sample if there
is interest later. I also tested ftrace functionality on the
command line to check for breakage"

I've tested patching calls from module to module
(ibmvscsi to scsi_mod), within the kernel (livepatch-sample/
proc_cmdline_show). I must admit there is more testing to
be done

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, the magic will be in the patched function? Please share the test/demonstrator

quoted

+	 * 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)

[...]

quoted

+ * 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 [off-list ref]

Why? For using CR+4 or removing the frame? Or you believe there is a better way to
handle this that work, IOW what is broken?

	Torsten

Balbir Singh