Linux Kernel Debugging Advanced Operating Systems 2020/2021 Vlastimil Babka Linux Kernel Developer, SUSE Labs
[email protected] Agenda – Debugging Scenarios • Debugging production kernels – Post-mortem analysis: interpreting kernel oops/panic output, creating and analyzing kernel crash dumps – Kernel observability – dynamic debug, tracing, alt-sysrq dumps, live crash session • Debugging during individual kernel development – Debug prints – printk() facilitiy – Debugger (gdb) support • Finding (latent) bugs during collaborative development – Optional runtime checks configurable during build – Testing and fuzzing – Static analysis 2 Enterprise Linux Distro and Bugs (incl. Kernel) • The software installation (e.g. ISO) itself is free (and open source, obviously) • Customers pay for support subscription – Delivery of (tested by QA) package updates – fixing known bugs, CVE’s… but not more! – Getting reported bugs fixed • Bugs specific to customer’s workload, hardware, “luck”, large number of machines… • Upstream will also fix reported bugs, but only with latest kernel and no effort guarantees • Dealing with kernel bugs, e.g. crashes – Find out the root cause (buggy code) with limited possibilities (compared to local development) • Typically no direct access to customer’s system or workload • Long turnarounds for providing a modified debug kernel and reproducing the bug – Write and deliver a fix (upstream first!) or workaround; fix goes to next update • Possibly a livepatch in some cases – Is a lot of fun ;-) 3 Kernel Oops - The Real World Example