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SubjectRe: [PATCH] Revert "kbuild: use -flive-patching when CONFIG_LIVEPATCH is enabled"
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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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