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Mon, 20 Jul 2020 11:47:44 +0000 (UTC) Received: from [10.10.114.255] (ovpn-114-255.rdu2.redhat.com [10.10.114.255]) by smtp.corp.redhat.com (Postfix) with ESMTPS id B39112DE64; Mon, 20 Jul 2020 11:47:43 +0000 (UTC) Subject: Re: [PATCH] Revert "kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled" To: Kamalesh Babulal , Josh Poimboeuf , live-patching@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Randy Dunlap References: <696262e997359666afa053fe7d1a9fb2bb373964.1595010490.git.jpoimboe@redhat.com> <9c53c755-a497-25f0-40ae-7e435be3269b@linux.vnet.ibm.com> From: Joe Lawrence Message-ID: <940c041f-84e8-337b-ae98-1086119ddf9a@redhat.com> Date: Mon, 20 Jul 2020 07:47:43 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0 MIME-Version: 1.0 In-Reply-To: <9c53c755-a497-25f0-40ae-7e435be3269b@linux.vnet.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 7/20/20 4:50 AM, Kamalesh Babulal wrote: > On 20/07/20 9:05 am, Joe Lawrence wrote: >> On 7/17/20 2:29 PM, Josh Poimboeuf wrote: >>> Use of the new -flive-patching flag was introduced with the following >>> commit: >>> >>>    43bd3a95c98e ("kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled") >>> >>> This flag has several drawbacks: >>> >>> [ ... snip ... ] >>> >>> - While there *is* a distro which relies on this flag for their distro >>>    livepatch module builds, there's not a publicly documented way to >>>    create safe livepatch modules with it.  Its use seems to be based on >>>    tribal knowledge.  It serves no benefit to those who don't know how to >>>    use it. >>> >>>    (In fact, I believe the current livepatch documentation and samples >>>    are misleading and dangerous, and should be corrected.  Or at least >>>    amended with a disclaimer.  But I don't feel qualified to make such >>>    changes.) >> >> FWIW, I'm not exactly qualified to document source-based creation either, however I have written a few of the samples and obviously the kselftest modules. >> >> The samples should certainly include a disclaimer (ie, they are only for API demonstration purposes!) and eventually it would be great if the kselftest modules could guarantee their safety as well.  I don't know quite yet how we can automate that, but perhaps some kind of post-build sanity check could verify that they are in fact patching what they intend to patch. >> >> As for a more general, long-form warning about optimizations, I grabbed Miroslav's LPC slides from a few years back and poked around at some IPA-optimized disassembly... Here are my notes that attempt to capture some common cases: >> >> http://file.bos.redhat.com/~jolawren/klp-compiler-notes/livepatch/compiler-considerations.html > > Hi Joe, > > The notes link you shared is not accessible. > Oops, lets try that again: http://people.redhat.com/~jolawren/klp-compiler-notes/livepatch/compiler-considerations.html -- Joe