Messages in this thread | | | Subject | Re: [PATCH] Revert "kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled" | From | Joe Lawrence <> | Date | Sun, 19 Jul 2020 23:35:20 -0400 |
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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
It's not complete and I lost steam about 80% of the way through today. :) But if it looks useful enough to add to Documentation/livepatch, we can work on it on-list and try to steer folks into using the automated kpatch-build, objtool (eventually) or a source-based safety checklist. The source-based steps have been posted on-list a few times, but I think it only needs to be formalized in a doc.
-- Joe
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