linux-kernel - Re: [PATCH] Revert "kbuild: use -flive-patching when CONFIG

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Message-ID: <940c041f-84e8-337b-ae98-1086119ddf9a@redhat.com>
Date:   Mon, 20 Jul 2020 07:47:43 -0400
From:   Joe Lawrence <joe.lawrence@...hat.com>
To:     Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        live-patching@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] Revert "kbuild: use -flive-patching when CONFIG_LIVEPATCH
 is enabled"

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