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Add section on removing static local references. 

This was discovered when trying to patch a netfilter issue and resulted in the following issue:
https://github.com/dynup/kpatch/issues/599

This patch adds some documentation in case this situation is encountered in the future.
This commit is contained in:
Chris J Arges 2016-07-01 09:47:59 -05:00 committed by Chris J Arges
parent 03ef5a2ff4
commit b28c2b0638
1 changed files with 15 additions and 0 deletions
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doc/patch-author-guide.md
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@ -143,3 +143,18 @@ applied. The patch may require a load hook function which detects whether such
init code has run, and which rewrites or changes the original initialization to
force it into the desired state. Some changes involving hardware init are
inherently incompatible with live patching.
Removing Static Local References
--------------------------------
Removing references to static locals will fail to patch unless extra steps are taken.
Static locals are basically global variables because they outlive the function's
scope. They need to be correlated so that the new function will use the old static
local via a dynrela. That way patching the function doesn't inadvertently reset the
variable to zero; instead the variable keeps its old value.
To work around this limitation one needs to retain the reference to the static local.
This might be as simple as adding the variable back in the patched function in a
non-functional way and ensuring the compiler doesn't optimize it away.