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

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Message-ID: <9c53c755-a497-25f0-40ae-7e435be3269b@linux.vnet.ibm.com>
Date:   Mon, 20 Jul 2020 14:20:35 +0530
From:   Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>
To:     Joe Lawrence <joe.lawrence@...hat.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 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.

Regards,

-- 
Kamalesh