As explained earlier, the second half of each buftag contains extra information about the corresponding buffer. Some of this data is debugging information, and some is data private to the allocator. While this auxiliary data can take several different forms, it is collectively known as "Buffer Control" or bufctl data.
However, the allocator needs to know whether a buffer's bufctl pointer is valid, since this pointer might also have been corrupted by malfunctioning code. The allocator confirms the integrity of its auxiliary pointer by storing the pointer and an encoded version of that pointer, and then cross-checking the two versions.
As shown in Figure 6-5, these pointers are the bcp (buffer control pointer) and bxstat (buffer control XOR status). The allocator arranges bcp and bxstat so that the expression bcp XOR bxstat equals a well-known value.
In the event that one or both of these pointers becomes corrupted, the allocator can easily detect such corruption and panic the system. When a buffer is allocated, bcp XOR bxstat = 0xa110c8ed ("allocated"). When a buffer is free, bcp XOR bxstat = 0xf4eef4ee ("freefree").
You might find it helpful to re-examine the example provided in "Freed Buffer Checking: 0xdeadbeef", in order to confirm that the buftag pointers shown there are consistent.
In the event that the allocator finds a corrupt buftag, it panics the system and produces a message similar to the following:
kernel memory allocator: boundary tag corrupted bcp ^ bxstat = 0xffeef4ee, should be f4eef4ee
Remember, if bcp is corrupt, it is still possible to retrieve its value by taking the value of bxstat XOR 0xf4eef4ee or bxstat XOR 0xa110c8ed, depending on whether the buffer is allocated or free.