dt4

Usage

Overview

System and application tracing can happen in the kernel. It has to be configured and enabled through the ioctl(2) interface exposed by the pseudo-device /dev/dt.

This device can only be opened when the kern.allowdt sysctl(2) variable is set.

Ioctl interface

The ioctl(2) command codes below are defined in ⟨dev/dt/dtvar.h⟩.

DTIOCGPLIST struct dtioc_probe *dtpr

Get available probe entries.

struct dtioc_probe {
	size_t		 	 dtpr_size;
	struct dtioc_probe_info	*dtpr_probes;
};

If dtpr_size is non-zero, as many probes as possible that can fit into this size will be copied into the supplied buffer. On exit, dtpr_size is always set to the total size required to hold all probe entries (i.e., it is set to sizeof(struct dtioc_probe_info) * dt_nprobes).

DTIOCGSTATS struct dtioc_stat *dtst

Get statistics for current recording.

struct dtioc_stat {
	uint64_t		 dtst_readevt;
	uint64_t		 dtst_dropevt;
};

DTIOCRECORD int on

Start or stop recording.

DTIOCPRBENABLE struct dtioc_req *dtrq

Enable the given probe for recording.

struct dtioc_req {
	uint32_t		 dtrq_pbn;
	struct dt_filter	 dtrq_filter;
	uint32_t		 dtrq_rate;
	uint64_t		 dtrq_evtflags;
};

DTIOCRDVNODE struct *dtioc_rdvn

Resolve the instruction pointer from the traced process to the ELF object.

struct dtioc_rdvn {
	pid_t			 dtrv_pid;
	int			 dtrv_fd;
	caddr_t			 dtrv_va;
	caddr_t			 dtrv_offset;
	caddr_t			 dtrv_start;
	size_t			 dtrv_len;
};

The caller populates dtrv_pid with the process ID of the traced process and dtrv_va with the program counter address to look up.

On success, the remaining members are populated. dtrv_fd is the file descriptor to the DSO file that dtrv_va belongs to. dtrv_len is the length of the DSO file. dtrv_offset and dtrv_start are the relative offset of dtrv_va to the start of the text segment and the base address of that segment respectively.

To calculate the relative offset of dtrv_va in the DSO the following calculation can be used:

iOffset = dtrv_va - dtrv_start + dtrv_offset

The segment can be found by walking the ELF program headers and looking for the mapping where iOffset is included in the range [GElf_Phdr.p_offset, GElf_Phdr.p_offset + GElf_Phdr.p_filesz)

Using the GElf_Phdr.p_vaddr the base address of the segment can be calculated by

lbase = dtrv_start - GElf_Phdr.p_vaddr

For symbol lookups the offset can now be calculated as

symOffset = dtrv_va - lbase

Files

/dev/dt

dynamic tracing device.

Errors

An open of /dev/dt will fail if:

[EPERM]

the kern.allowdt sysctl(2) variable wasn't set.

[ENOMEM]

not enough memory space was available.