Kernel Debugging Techniques

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Often the pivotal factor in achieving development timetables comes down to one's efficiency in finding and fixing bugs. Debugging inside the Linux kernel can be quite challenging. No matter how you approach it, kernel debugging will always be complex. This chapter examines some of the complexities and presents ideas and methods to improve your debugging skills inside the kernel and device drivers.

Debugging Optimized Kernel Code

At the start of this chapter, we said that one of the challenges identified in debugging kernel code results from compiler optimization. We noted that the Linux kernel is compiled by default with optimization level -O2. In the examples up to this point, we used -O1 optimization to simplify the debugging task. Here we illustrate one of the many ways optimization can complicate debugging.

The related Internet mail lists are strewn with questions related to what appear to be broken tools. Sometimes the poster reports that his debugger is singlestepping backward or that his line numbers do not line up with his source code. Here we present an example to illustrate the complexities that optimizing compilers bring to source-level debugging. In this example, the line numbers that gdb reports when a breakpoint is hit do not match up with the line numbers in our source file due to function inlining.

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