CHAPTER 4 Debugging Techniques

Debugging Support in the Kernel
CONFIG_DEBUG_KERNEL
    This option just makes other debugging options available; it should be turned on
    but does not, by itself, enable any features.
CONFIG_DEBUG_SLAB
    This crucial option turns on several types of checks in the kernel memory alloca-
    tion functions; with these checks enabled, it is possible to detect a number of
    memory overrun and missing initialization errors. Each byte of allocated memory
    is set to 0xa5 before being handed to the caller and then set to 0x6b when it is
    freed. If you ever see either of those “poison” patterns repeating in output from
    your driver (or often in an oops listing), you’ll know exactly what sort of error to
    look for. When debugging is enabled, the kernel also places special guard values
    before and after every allocated memory object; if those values ever get changed,
    the kernel knows that somebody has overrun a memory allocation, and it com-
    plains loudly. Various checks for more obscure errors are enabled as well.
CONFIG_DEBUG_PAGEALLOC
    Full pages are removed from the kernel address space when freed. This option
    can slow things down significantly, but it can also quickly point out certain
    kinds of memory corruption errors.
CONFIG_DEBUG_SPINLOCK
    With this option enabled, the kernel catches operations on uninitialized spin-
    locks and various other errors (such as unlocking a lock twice).
CONFIG_DEBUG_SPINLOCK_SLEEP
    This option enables a check for attempts to sleep while holding a spinlock. In
    fact, it complains if you call a function that could potentially sleep, even if the
    call in question would not sleep.
CONFIG_INIT_DEBUG
    Items marked with __init (or __initdata) are discarded after system initializa-
    tion or module load time. This option enables checks for code that attempts to
    access initialization-time memory after initialization is complete.
CONFIG_DEBUG_INFO
    This option causes the kernel to be built with full debugging information
    included. You’ll need that information if you want to debug the kernel with gdb.
    You may also want to enable CONFIG_FRAME_POINTER if you plan to use gdb.
CONFIG_MAGIC_SYSRQ
    Enables the “magic SysRq” key. We look at this key in the section “System
    Hangs,” later in this chapter.
CONFIG_DEBUG_STACKOVERFLOW
CONFIG_DEBUG_STACK_USAGE
    These options can help track down kernel stack overflows. A sure sign of a stack
    overflow is an oops listing without any sort of reasonable back trace. The first
    option adds explicit overflow checks to the kernel; the second causes the kernel
    to monitor stack usage and make some statistics available via the magic SysRq
    key.
CONFIG_KALLSYMS
    This option (under “General setup/Standard features”) causes kernel symbol
    information to be built into the kernel; it is enabled by default. The symbol
    information is used in debugging contexts; without it, an oops listing can give
    you a kernel traceback only in hexadecimal, which is not very useful.
CONFIG_IKCONFIG
CONFIG_IKCONFIG_PROC
    These options (found in the “General setup” menu) cause the full kernel config-
    uration state to be built into the kernel and to be made available via /proc. Most
    kernel developers know which configuration they used and do not need these
    options (which make the kernel bigger). They can be useful, though, if you are
    trying to debug a problem in a kernel built by somebody else.
CONFIG_ACPI_DEBUG
    Under “Power management/ACPI.” This option turns on verbose ACPI
    (Advanced Configuration and Power Interface) debugging information, which
    can be useful if you suspect a problem related to ACPI.
CONFIG_DEBUG_DRIVER
    Under “Device drivers.” Turns on debugging information in the driver core,
    which can be useful for tracking down problems in the low-level support code.
    We’ll look at the driver core in Chapter 14.
CONFIG_SCSI_CONSTANTS
    This option, found under “Device drivers/SCSI device support,” builds in infor-
    mation for verbose SCSI error messages. If you are working on a SCSI driver, you
    probably want this option.
CONFIG_INPUT_EVBUG
    This option (under “Device drivers/Input device support”) turns on verbose log-
    ging of input events. If you are working on a driver for an input device, this
    option may be helpful. Be aware of the security implications of this option, how-
    ever: it logs everything you type, including your passwords.
CONFIG_PROFILING
    This option is found under “Profiling support.” Profiling is normally used for
    system performance tuning, but it can also be useful for tracking down some
    kernel hangs and related problems.

printk
There are eight possible loglevel strings, defined in the header ; we
list them in order of decreasing severity:
KERN_EMERG
      Used for emergency messages, usually those that precede a crash.
KERN_ALERT
      A situation requiring immediate action.
KERN_CRIT
      Critical conditions, often related to serious hardware or software failures.
KERN_ERR
      Used to report error conditions; device drivers often use KERN_ERR to report hard-
      ware difficulties.
KERN_WARNING
      Warnings about problematic situations that do not, in themselves, create seri-
      ous problems with the system.
KERN_NOTICE
      Situations that are normal, but still worthy of note. A number of security-related
      conditions are reported at this level.
KERN_INFO
      Informational messages. Many drivers print information about the hardware
      they find at startup time at this level.
KERN_DEBUG
      Used for debugging messages.
Each string (in the macro expansion) represents an integer in angle brackets. Inte-
gers range from 0 to 7, with smaller values representing higher priorities.

use macro for debug switch:
#undef PDEBUG              /* undef it, just in case */
#ifdef SCULL_DEBUG
# ifdef __KERNEL__
      /* This one if debugging is on, and kernel space */
#     define PDEBUG(fmt, args...) printk( KERN_DEBUG "scull: " fmt, ## args)
# else
      /* This one for user space */
#     define PDEBUG(fmt, args...) fprintf(stderr, fmt, ## args)
# endif
#else
# define PDEBUG(fmt, args...) /* not debugging: nothing */
#endif
#undef PDEBUGG
#define PDEBUGG(fmt, args...) /* nothing: it's a placeholder */