linux-kernel - Re: [PATCH v6 0/9] ftrace with regs + live patching for ppc64 LE (ABI v2)

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Date:	Thu, 28 Jan 2016 12:19:10 +0100
From:	Torsten Duwe <duwe@....de>
To:	Michael Ellerman <mpe@...erman.id.au>
Cc:	Balbir Singh <bsingharora@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Jiri Kosina <jkosina@...e.cz>, linuxppc-dev@...ts.ozlabs.org,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org
Subject: Re: [PATCH v6 0/9] ftrace with regs + live patching for ppc64 LE
	(ABI v2)

On Thu, Jan 28, 2016 at 02:31:58PM +1100, Michael Ellerman wrote:
> 
> Looking at GCC history it looks like the fix is in 4.9.0 and anything later.

Good. But 4.8.5 has a buggy -mprofile-kernel, and there will be no 4.8.6, Bad.

> But a version check doesn't work with patched distro/vendor toolchains. So we
> probably need some sort of runtime check.

Agreed.

/bin/echo -e '#include <linux/compiler.h>\nnotrace int func() { return 0; }' |
 gcc -D__KERNEL__ -Iinclude -p -mprofile-kernel -x c -O2 - -S -o - | grep mcount

should be empty. If it yields "bl _mcount" your compiler is buggy.
I haven't looked at the kernel's "autoconf" yet, but it's probably capable
of testing this.

	Torsten