Re: [PATCH 6/6] powerpc/livepatch: Add live patching support on ppc64le

On Thu, Mar 24, 2016 at 10:04:05PM +1100, Michael Ellerman wrote:

+livepatch_handler:
+	CURRENT_THREAD_INFO(r12, r1)

[...]

+	/* Put ctr in r12 for global entry and branch there */
+	mfctr	r12
+	bctrl

            ^
I like this piece. No need to fiddle out the return helper address.

+	/*
+	 * Now we are returning from the patched function to the original
+	 * caller A. We are free to use r0 and r12, and we can use r2 until we
+	 * restore it.
+	 */
+
+	CURRENT_THREAD_INFO(r12, r1)
+
+	/* Save stack pointer into r0 */
+	mr	r0, r1
+
+	ld	r1, TI_livepatch_sp(r12)
+
+	/* Check stack marker hasn't been trashed */
+	lis     r2,  STACK_END_MAGIC@h
+	ori     r2,  r2, STACK_END_MAGIC@l
+	ld	r12, -8(r1)
+1:	tdne	r12, r2
+	EMIT_BUG_ENTRY 1b, __FILE__, __LINE__ - 1, 0

This however worries me a bit. Sure, in the end, a stack overflow is
a stack overflow, and if all the information does not fit there,
there's little you can do.

But wouldn't it be better to kmalloc that area and realloc in
klp_arch_set_pc when it's full? Maybe along with a warning message?
That way a live patched kernel will not run into stack size problems
any earlier than an unpatched kernel would.
Just a thought.

Anyway, patch 5+6

Reviewed-by: Torsten Duwe <redacted>

	Torsten