ptrace() provides tracing and debugging facilities. It allows one process (the
tracing process) to control another (the
traced process). Most of the time, the traced process runs normally, but when it receives a signal (see
sigaction(2)), it stops. The tracing process is expected to notice this via
wait(2) or the delivery of a
SIGCHLD signal, examine the state of the stopped process, and cause it to terminate or continue as appropriate.
ptrace() is the mechanism by which all this happens.
The
request argument specifies what operation is being performed; the meaning of the rest of the arguments depends on the operation, but except for one special case noted below, all
ptrace() calls are made by the tracing process, and the
pid argument specifies the process ID of the traced process.
request can be:
PT_TRACE_ME
This request is the only one used by the traced process; it declares that the process expects to be traced by its parent. All the other arguments are ignored. (If the parent process does not expect to trace the child, it will probably be rather confused by the results; once the traced process stops, it cannot be made to continue except via
ptrace().) When a process has used this request and calls
execve(2) or any of the routines built on it (such as
execv(3)), it will stop before executing the first instruction of the new image. Also, any setuid or setgid bits on the executable being executed will be ignored.
PT_READ_I, PT_READ_D
These requests read a single int of data from the traced process' address space. Traditionally, ptrace() has allowed for machines with distinct address spaces for instruction and data, which is why there are two requests: conceptually, PT_READ_I reads from the instruction space and PT_READ_D reads from the data space. In the current NetBSD implementation, these two requests are completely identical. The addr argument specifies the address (in the traced process' virtual address space) at which the read is to be done. This address does not have to meet any alignment constraints. The value read is returned as the return value from ptrace().
PT_WRITE_I, PT_WRITE_D
These requests parallel PT_READ_I and PT_READ_D, except that they write rather than read. The data argument supplies the value to be written.
PT_CONTINUE
The traced process continues execution. addr is an address specifying the place where execution is to be resumed (a new value for the program counter), or (caddr_t)1 to indicate that execution is to pick up where it left off. data provides a signal number to be delivered to the traced process as it resumes execution, or 0 if no signal is to be sent. If a negative value is supplied, that is the negative of the LWP ID of the thread to be resumed, and only that thread executes.
PT_KILL
The traced process terminates, as if PT_CONTINUE had been used with SIGKILL given as the signal to be delivered.
PT_ATTACH
This request allows a process to gain control of an otherwise unrelated process and begin tracing it. It does not need any cooperation from the to-be-traced process. In this case,
pid specifies the process ID of the to-be-traced process, and the other two arguments are ignored. This request requires that the target process must have the same real UID as the tracing process, and that it must not be executing a setuid or setgid executable. (If the tracing process is running as root, these restrictions do not apply.) The tracing process will see the newly-traced process stop and may then control it as if it had been traced all along.
Three other restrictions apply to all tracing processes, even those running as root. First, no process may trace a system process. Second, no process may trace the process running
init(8). Third, if a process has its root directory set with
chroot(2), it may not trace another process unless that process's root directory is at or below the tracing process's root.
PT_DETACH
This request is like PT_CONTINUE, except that after it succeeds, the traced process is no longer traced and continues execution normally.
PT_IO
This request is a more general interface that can be used instead of
PT_READ_D,
PT_WRITE_D,
PT_READ_I, and
PT_WRITE_I. The I/O request is encoded in a “
struct ptrace_io_desc” defined as:
struct ptrace_io_desc {
int piod_op;
void *piod_offs;
void *piod_addr;
size_t piod_len;
};
where
piod_offs is the offset within the traced process where the I/O operation should take place,
piod_addr is the buffer in the tracing process, and
piod_len is the length of the I/O request. The
piod_op field specifies which type of I/O operation to perform. Possible values are:
See the description of
PT_READ_I for the difference between I and D spaces. A pointer to the I/O descriptor is passed in the
addr argument to
ptrace(). On return, the
piod_len field in the I/O descriptor will be updated with the actual number of bytes transferred. If the requested I/O could not be successfully performed,
ptrace() will return
-1 and set
errno.
PT_DUMPCORE
Makes the process specified in the pid pid generate a core dump. The addr argument should contain the name of the core file to be generated and the data argument should contain the length of the core filename. This ptrace call currently does not stop the child process so it can generate inconsistent data.
PT_LWPINFO
Returns information about a thread from the list of threads for the process specified in the
pid argument. The
addr argument should contain a “
struct ptrace_lwpinfo” defined as:
struct ptrace_lwpinfo {
lwpid_t pl_lwpid;
int pl_event;
};
where
pl_lwpid contains a thread LWP ID. Information is returned for the thread following the one with the specified ID in the process thread list, or for the first thread if
pl_lwpid is 0. Upon return
pl_lwpid contains the LWP ID of the thread that was found, or 0 if there is no thread after the one whose LWP ID was supplied in the call.
pl_event contains the event that stopped the thread. Possible values are:
The
data argument should contain “
sizeof(struct ptrace_lwpinfo)”.
PT_SYSCALL
Stops a process before and after executing each system call.
Additionally, the following requests exist but are not available on all machine architectures. The file
<machine/ptrace.h> lists which requests exist on a given machine.
PT_STEP
Execution continues as in request PT_CONTINUE; however as soon as possible after execution of at least one instruction, execution stops again. If the data argument is greater than 0, it contains the LWP ID of the thread to be stepped, and any other threads are continued. If the data argument is less than zero, it contains the negative of the LWP ID of the thread to be stepped, and only that thread executes.
PT_GETREGS
This request reads the traced process' machine registers into the “
struct reg” (defined in
<machine/reg.h>) pointed to by
addr. The
data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read.
PT_SETREGS
This request is the converse of
PT_GETREGS; it loads the traced process' machine registers from the “
struct reg” (defined in
<machine/reg.h>) pointed to by
addr. The
data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written.
PT_GETFPREGS
This request reads the traced process' floating-point registers into the “
struct fpreg” (defined in
<machine/reg.h>) pointed to by
addr. The
data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read.
PT_SETFPREGS
This request is the converse of
PT_GETFPREGS; it loads the traced process' floating-point registers from the “
struct fpreg” (defined in
<machine/reg.h>) pointed to by
addr. The
data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written.
PT_DUMPCORE
Cause the traced process to dump core. If the
addr argument is not
NULL it is taken to be the pathname of the core file to be generated and the
data argument should contain the length of the pathname. The pathname may contain
% patterns that are expanded as described in
sysctl(8). If the
data argument is
NULL, the default core file path generation rules are followed.