The buffer control (bufctl) pointer contained in the buftag region can have different meanings, depending on the cache's kmem_flags. The behavior toggled by the KMF_AUDIT flag is of particular interest: when the KMF_AUDIT flag is not set, the kernel memory allocator allocates a kmem_bufctl_t structure for each buffer. This structure contains some minimal accounting information about each buffer. When the KMF_AUDIT flag is set, the allocator instead allocates a kmem_bufctl_audit_t, an extended version of the kmem_bufctl_t.
This section presumes the KMF_AUDIT flag is set. For caches that do not have this bit set, the amount of available debugging information is reduced.
The kmem_bufctl_audit_t (bufctl_audit for short) contains additional information about the last transaction that occurred on this buffer. The following example shows how to apply the bufctl_audit macro to examine an audit record. The buffer shown is the example buffer used in "Detecting Memory Corruption":
> 0x70a9ae00,5/KKn 0x70a9ae00: 5 4ef83 0 0 1 bbddcafe feedface 4fffed 70ae3200 d1befaed
Using the techniques presented above, it is easy to see that 0x70ae3200 points to the bufctl_audit record: it is the first pointer following the redzone. To examine the bufctl_audit record it points to, apply the bufctl_audit macro:
> 0x70ae3200$<bufctl_audit 0x70ae3200: next addr slab 70378000 70a9ae00 707c86a0 0x70ae320c: cache timestamp thread 70039928 e1bd0e26afe 70aac4e0 0x70ae321c: lastlog contents stackdepth 7011c7c0 7018a0b0 4 0x70ae3228: kmem_zalloc+0x30 pid_assign+8 getproc+0x68 cfork+0x60
The 'addr' field is the address of the buffer corresponding to this bufctl_audit record. This is the original address: 0x70a9ae00. The 'cache' field points at the kmem_cache that allocated this buffer. You can use the ::kmem_cache dcmd to examine it as follows:
> 0x70039928::kmem_cache ADDR NAME FLAG CFLAG BUFSIZE BUFTOTL 70039928 kmem_alloc_24 020f 000000 24 612
The 'timestamp' field represents the time this transaction occurred. This time is expressed in the same manner as gethrtime(3C).
'thread' is a pointer to the thread that performed the last transaction on this buffer. The 'lastlog' and 'contents' pointers point to locations in the allocator's transaction logs. These logs are discussed in detail in "Allocator Logging Facility".
Typically, the most useful piece of information provided by bufctl_audit is the stack trace recorded at the point at which the transaction took place. In this case, the transaction was an allocation called as part of executing fork(2).