Книга: Embedded Linux Primer: A Practical, Real-World Approach

14.2.3. Useful Kernel Breakpoints

14.2.3. Useful Kernel Breakpoints

We have now established a debug connection with the kernel on our target board. When we issue the gdb continue (c) command, the kernel proceeds to boot, and if there are no problems, the boot process completes. There is one minor limitation of using KGDB on many architectures and processors. An engineering trade-off was made between the need to support very early kernel debugging (for example, before a full-blown interrupt-driven serial port driver is installed) and the desire to keep the complexity of the KGDB debug engine itself very simple and, therefore, robust and portable. KGDB uses a simple polled serial driver that has zero overhead when the kernel is running. As a drawback to this implementation, the traditional Ctl-C or Break sequence on the serial port will have no effect. Therefore, it will be impossible to stop execution of the running kernel unless a breakpoint or other fault is encountered.

For this reason, it has become common practice to define some system-wide breakpoints, which provide the capability to halt the current thread of execution. Two of the most common are highlighted in Listing 14-3.

Listing 14-3. Common Kernel Breakpoints

(gdb) b panic
Breakpoint 1 at 0xc0016b18: file kernel/panic.c, line 74.
(gdb) b sys_sync
Breakpoint 2 at 0xc005a8c8: file fs/buffer.c, line 296.
(gdb)

Using the gdb breakpoint command, again using its abbreviated version, we enter two breakpoints. One is at panic() and the other is at the sync system call entry sys_sync(). The former allows the debugger to be invoked if a later event generates a panic. This enables examination of the system state at the time of the panic. The second is a useful way to halt the kernel and trap into the debugger from user space by entering the sync command from a terminal running on your target hardware.

We are now ready to proceed with our debugging session. We have a KGDB-enabled kernel running on our target, paused at a KGDB-defined early breakpoint. We established a connection to the target with our host-based cross debuggerin this case, invoked as ppc_4xx-gdb and we have entered a pair of useful system breakpoints. Now we can direct our debugging activities to the task at hand.

One caveat: By definition, we cannot use KGDB for stepping through code before the breakpoint() function in .../arch/ppc/setup.c used to establish the connection between a KGDB-enabled kernel and cross-gdb on our host. Figure 14-3 is a rough guide to the code that executes before KGDB gains control. Debugging this early code requires the use of a hardware-assisted debug probe. We cover this topic shortly in Section 14.4, "Hardware-Assisted Debugging."

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