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
