393 lines
9.6 KiB
Groff
393 lines
9.6 KiB
Groff
.\" $FreeBSD$
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.\" $NetBSD: ptrace.2,v 1.2 1995/02/27 12:35:37 cgd Exp $
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.\"
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.\" This file is in the public domain.
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.Dd November 9, 2006
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.Dt PTRACE 2
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.Os
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.Sh NAME
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.Nm ptrace
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.Nd process tracing and debugging
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.Sh LIBRARY
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.Lb libc
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.Sh SYNOPSIS
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.In sys/types.h
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.In sys/ptrace.h
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.Ft int
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.Fn ptrace "int request" "pid_t pid" "caddr_t addr" "int data"
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.Sh DESCRIPTION
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The
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.Fn ptrace
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system call
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provides tracing and debugging facilities.
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It allows one process
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(the
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.Em tracing
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process)
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to control another
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(the
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.Em traced
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process).
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The tracing process must first attach to the traced process, and then
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issue a series of
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.Fn ptrace
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system calls to control the execution of the process, as well as access
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process memory and register state.
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For the duration of the tracing session, the traced process will be
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.Dq re-parented ,
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with its parent process ID (and resulting behavior)
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changed to the tracing process.
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It is permissible for a tracing process to attach to more than one
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other process at a time.
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When the tracing process has completed its work, it must detach the
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traced process; if a tracing process exits without first detaching all
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processes it has attached, those processes will be killed.
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.Pp
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Most of the time, the traced process runs normally, but when it
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receives a signal
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(see
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.Xr sigaction 2 ) ,
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it stops.
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The tracing process is expected to notice this via
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.Xr wait 2
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or the delivery of a
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.Dv SIGCHLD
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signal, examine the state of the stopped process, and cause it to
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terminate or continue as appropriate.
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The signal may be a normal process signal, generated as a result of
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traced process behavior, or use of the
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.Xr kill 2
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system call; alternatively, it may be generated by the tracing facility
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as a result of attaching, system calls, or stepping by the tracing
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process.
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The tracing process may choose to intercept the signal, using it to
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observe process behavior (such as
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.Dv SIGTRAP ) ,
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or forward the signal to the process if appropriate.
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The
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.Fn ptrace
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system call
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is the mechanism by which all this happens.
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.Pp
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The
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.Fa request
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argument specifies what operation is being performed; the meaning of
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the rest of the arguments depends on the operation, but except for one
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special case noted below, all
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.Fn ptrace
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calls are made by the tracing process, and the
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.Fa pid
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argument specifies the process ID of the traced process.
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The
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.Fa request
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argument
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can be:
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.Bl -tag -width 12n
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.It Dv PT_TRACE_ME
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This request is the only one used by the traced process; it declares
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that the process expects to be traced by its parent.
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All the other arguments are ignored.
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(If the parent process does not expect to trace the child, it will
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probably be rather confused by the results; once the traced process
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stops, it cannot be made to continue except via
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.Fn ptrace . )
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When a process has used this request and calls
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.Xr execve 2
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or any of the routines built on it
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(such as
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.Xr execv 3 ) ,
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it will stop before executing the first instruction of the new image.
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Also, any setuid or setgid bits on the executable being executed will
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be ignored.
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.It Dv PT_READ_I , Dv PT_READ_D
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These requests read a single
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.Vt int
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of data from the traced process's address space.
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Traditionally,
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.Fn ptrace
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has allowed for machines with distinct address spaces for instruction
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and data, which is why there are two requests: conceptually,
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.Dv PT_READ_I
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reads from the instruction space and
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.Dv PT_READ_D
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reads from the data space.
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In the current
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.Fx
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implementation, these two requests are completely identical.
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The
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.Fa addr
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argument specifies the address
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(in the traced process's virtual address space)
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at which the read is to be done.
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This address does not have to meet any alignment constraints.
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The value read is returned as the return value from
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.Fn ptrace .
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.It Dv PT_WRITE_I , Dv PT_WRITE_D
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These requests parallel
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.Dv PT_READ_I
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and
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.Dv PT_READ_D ,
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except that they write rather than read.
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The
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.Fa data
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argument supplies the value to be written.
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.It Dv PT_IO
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This request allows reading and writing arbitrary amounts of data in
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the traced process's address space.
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The
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.Fa addr
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argument specifies a pointer to a
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.Vt "struct ptrace_io_desc" ,
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which is defined as follows:
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.Bd -literal
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struct ptrace_io_desc {
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int piod_op; /* I/O operation */
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void *piod_offs; /* child offset */
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void *piod_addr; /* parent offset */
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size_t piod_len; /* request length */
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};
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/*
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* Operations in piod_op.
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*/
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#define PIOD_READ_D 1 /* Read from D space */
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#define PIOD_WRITE_D 2 /* Write to D space */
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#define PIOD_READ_I 3 /* Read from I space */
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#define PIOD_WRITE_I 4 /* Write to I space */
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.Ed
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.Pp
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The
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.Fa data
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argument is ignored.
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The actual number of bytes read or written is stored in
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.Va piod_len
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upon return.
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.It Dv PT_CONTINUE
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The traced process continues execution.
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The
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.Fa addr
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argument
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is an address specifying the place where execution is to be resumed
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(a new value for the program counter),
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or
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.Po Vt caddr_t Pc Ns 1
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to indicate that execution is to pick up where it left off.
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The
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.Fa data
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argument
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provides a signal number to be delivered to the traced process as it
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resumes execution, or 0 if no signal is to be sent.
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.It Dv PT_STEP
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The traced process is single stepped one instruction.
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The
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.Fa addr
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argument
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should be passed
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.Po Vt caddr_t Pc Ns 1 .
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The
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.Fa data
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argument
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provides a signal number to be delivered to the traced process as it
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resumes execution, or 0 if no signal is to be sent.
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.It Dv PT_KILL
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The traced process terminates, as if
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.Dv PT_CONTINUE
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had been used with
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.Dv SIGKILL
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given as the signal to be delivered.
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.It Dv PT_ATTACH
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This request allows a process to gain control of an otherwise
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unrelated process and begin tracing it.
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It does not need any cooperation from the to-be-traced process.
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In
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this case,
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.Fa pid
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specifies the process ID of the to-be-traced process, and the other
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two arguments are ignored.
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This request requires that the target process must have the same real
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UID as the tracing process, and that it must not be executing a setuid
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or setgid executable.
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(If the tracing process is running as root, these restrictions do not
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apply.)
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The tracing process will see the newly-traced process stop and may
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then control it as if it had been traced all along.
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.It Dv PT_DETACH
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This request is like PT_CONTINUE, except that it does not allow
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specifying an alternate place to continue execution, and after it
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succeeds, the traced process is no longer traced and continues
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execution normally.
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.It Dv PT_GETREGS
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This request reads the traced process's machine registers into the
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.Do
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.Vt "struct reg"
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.Dc
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(defined in
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.In machine/reg.h )
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pointed to by
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.Fa addr .
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.It Dv PT_SETREGS
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This request is the converse of
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.Dv PT_GETREGS ;
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it loads the traced process's machine registers from the
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.Do
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.Vt "struct reg"
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.Dc
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(defined in
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.In machine/reg.h )
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pointed to by
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.Fa addr .
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.It Dv PT_GETFPREGS
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This request reads the traced process's floating-point registers into
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the
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.Do
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.Vt "struct fpreg"
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.Dc
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(defined in
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.In machine/reg.h )
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pointed to by
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.Fa addr .
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.It Dv PT_SETFPREGS
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This request is the converse of
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.Dv PT_GETFPREGS ;
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it loads the traced process's floating-point registers from the
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.Do
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.Vt "struct fpreg"
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.Dc
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(defined in
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.In machine/reg.h )
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pointed to by
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.Fa addr .
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.It Dv PT_GETDBREGS
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This request reads the traced process's debug registers into
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the
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.Do
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.Vt "struct dbreg"
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.Dc
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(defined in
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.In machine/reg.h )
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pointed to by
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.Fa addr .
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.It Dv PT_SETDBREGS
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This request is the converse of
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.Dv PT_GETDBREGS ;
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it loads the traced process's debug registers from the
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.Do
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.Vt "struct dbreg"
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.Dc
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(defined in
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.In machine/reg.h )
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pointed to by
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.Fa addr .
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.It Dv PT_LWPINFO
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This request can be used to obtain information about the kernel thread,
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also known as light-weight process, that caused the traced process to stop.
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The
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.Fa addr
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argument specifies a pointer to a
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.Vt "struct ptrace_lwpinfo" ,
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which is defined as follows:
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.Bd -literal
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struct ptrace_lwpinfo {
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lwpid_t pl_lwpid; /* LWP described. */
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int pl_event; /* Event received. */
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};
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.Ed
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.Pp
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The
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.Fa data
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argument is to be set to the size of the structure known to the caller.
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This allows the structure to grow without affecting older programs.
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.It PT_GETLWPLIST
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This request can be used to get the current thread list.
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.El
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.Pp
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Additionally, machine-specific requests can exist.
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.Sh RETURN VALUES
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Some requests can cause
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.Fn ptrace
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to return
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\-1
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as a non-error value; to disambiguate,
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.Va errno
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can be set to 0 before the call and checked afterwards.
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.Sh ERRORS
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The
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.Fn ptrace
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system call may fail if:
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.Bl -tag -width Er
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.It Bq Er ESRCH
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.Bl -bullet -compact
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.It
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No process having the specified process ID exists.
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.El
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.It Bq Er EINVAL
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.Bl -bullet -compact
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.It
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A process attempted to use
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.Dv PT_ATTACH
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on itself.
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.It
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The
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.Fa request
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argument
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was not one of the legal requests.
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.It
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The signal number
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(in
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.Fa data )
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to
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.Dv PT_CONTINUE
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was neither 0 nor a legal signal number.
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.It
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.Dv PT_GETREGS ,
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.Dv PT_SETREGS ,
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.Dv PT_GETFPREGS ,
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.Dv PT_SETFPREGS ,
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.Dv PT_GETDBREGS ,
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or
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.Dv PT_SETDBREGS
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was attempted on a process with no valid register set.
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(This is normally true only of system processes.)
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.El
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.It Bq Er EBUSY
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.Bl -bullet -compact
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.It
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.Dv PT_ATTACH
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was attempted on a process that was already being traced.
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.It
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A request attempted to manipulate a process that was being traced by
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some process other than the one making the request.
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.It
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A request
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(other than
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.Dv PT_ATTACH )
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specified a process that was not stopped.
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.El
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.It Bq Er EPERM
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.Bl -bullet -compact
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.It
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A request
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(other than
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.Dv PT_ATTACH )
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attempted to manipulate a process that was not being traced at all.
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.It
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An attempt was made to use
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.Dv PT_ATTACH
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on a process in violation of the requirements listed under
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.Dv PT_ATTACH
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above.
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.El
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.El
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.Sh SEE ALSO
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.Xr execve 2 ,
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.Xr sigaction 2 ,
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.Xr wait 2 ,
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.Xr execv 3 ,
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.Xr i386_clr_watch 3 ,
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.Xr i386_set_watch 3
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.Sh HISTORY
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The
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.Fn ptrace
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function appeared in
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.At v7 .
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