Modifying Kernel Variables

You can use both kmdb and mdb to modify kernel variables or other kernel state. Kernel state modification with mdb should be done with care, because mdb does not stop the kernel before making modifications. Groups of modifications can be made atomically by using kmdb, because kmdb stops the kernel before allowing access by the user. The mdb debugger is capable of making single atomic modifications only.

Be sure to use the proper format specifier to perform the modification. The formats are:

• w – Writes the lowest two bytes of the value of each expression to the target beginning at the location specified by dot

• W – Writes the lowest 4 bytes of the value of each expression to the target beginning at the location specified by dot

• Z – Write the complete 8 bytes of the value of each expression to the target beginning at the location specified by dot

Use the ::sizeof dcmd to determine the size of the variable to be modified.

The following example overwrites the value of moddebug with the value 0x80000000.

Example 22–24 Modifying a Kernel Variable With a Debugger

> moddebug/W 0x80000000
    moddebug:       0 = 0x80000000