Design
Ksplice takes as input a unified diff and the original kernel source code, and it updates the running kernel in memory. Using Ksplice does not require any preparation before the system is originally booted, (the running kernel needs no special prior compiling, for example). In order to generate an update, Ksplice must determine what code within the kernel has been changed by the source code patch. Ksplice performs this analysis at the Executable and Linking Format (ELF) object code layer, rather than at the C source code layer.
To apply a patch, Ksplice first freezes execution of a computer so it is the only program running. The system verifies that no processors were in the middle of executing functions that will be modified by the patch. Ksplice modifies the beginning of changed functions so that they instead point to new, updated versions of those functions, and modifies data and structures in memory that need to be changed. Finally, Ksplice resumes each processor running where it left off.
To be fully automatic, Ksplice's design was originally limited to patches that did not introduce semantic changes to data structures, since most Linux kernel security patches do not make these kinds of changes. An evaluation against Linux kernel security patches from May 2005 to May 2008 found that Ksplice was able to apply fixes for all the 64 significant kernel vulnerabilities discovered in that interval. In 2009, major Linux vendors asked their customers to install a kernel update more than once per month. For patches that do introduce semantic changes to data structures, Ksplice requires a programmer to write a short amount of additional code to help apply the patch. This was necessary for 12% of the updates in that time period.
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