In the evolution of an operating system there is a continuing tension between the need to develop and test new features, and the need to provide a stable and secure execution environment to users. A compromise, adopted by the developers of the Linux kernel, is to release new versions, including bug fixes and new features, frequently, while maintaining some older "longterm" versions. This strategy raises the problem of how to identify bug fixing patches that are submitted to the current version but should be applied to the longterm versions as well. The current approach is to rely on the individual subsystem maintainers to forward patches that seem relevant to the maintainers of the longterm kernels. The reactivity and diligence of the maintainers, however, varies, and thus many important patches could be missed by this approach. In this paper, we propose an approach that automatically identifies bug fixing patches based on the changes and commit messages recorded in code repositories. We compare our approach with the keyword-based approach for identifying bug-fixing patches used in the literature, in the context of the Linux kernel. The results show that our approach can achieve a 53.19% improvement in recall as compared to keyword-based approaches, with similar precision.