.\"	$NetBSD: ptrace.2,v 1.1 1994/11/14 06:26:46 deraadt Exp $
.\" This file is in the public domain.
.Dd November 7, 1994
.Dt PTRACE 2
.Os NetBSD 1.0BETA
.Sh NAME
.Nm ptrace
.Nd process tracing and debugging
.Sh SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/ptrace.h>
.Ft int
.Fn ptrace "int request" "pid_t pid" "caddr_t addr" "int data"
.Sh DESCRIPTION
.Fn ptrace
provides tracing and debugging facilities.  It allows one process (the
.Em tracing
process) to control another (the
.Em traced
process).  Most of the time, the traced process runs normally, but when
it receives a signal
.Po
see
.Xr sigaction 2
.Pc ,
it stops.  The tracing process is expected to notice this via
.Xr wait 2
or the delivery of a
.Dv SIGCHLD
signal, examine the state of the stopped process, and cause it to
terminate or continue as appropriate.
.Fn ptrace
is the mechanism by which all this happens.
.Pp
The
.Fa 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
.Fn ptrace
calls are made by the tracing process, and the
.Fa pid
argument specifies the process ID of the traced process.
.Fa request
can be:
.Bl -tag -width 12n
.It Dv 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
.Eo \&
.Fn ptrace
.Ec \&.)
When a process has used this request and calls
.Xr execve 2
or any of the routines built on it
.Po
such as
.Xr execv 3
.Pc ,
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.
.It Dv PT_READ_I , Dv PT_READ_D
These requests read a single
.Li int
of data from the traced process' address space.  Traditionally,
.Fn ptrace
has allowed for machines with distinct address spaces for instruction
and data, which is why there are two requests: conceptually,
.Dv PT_READ_I
reads from the instruction space and
.Dv PT_READ_D
reads from the data space.  In the current NetBSD implementation, these
two requests are completely identical.  The
.Fa 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
.Eo \&
.Fn ptrace
.Ec .
.It Dv PT_WRITE_I , Dv PT_WRITE_D
These requests parallel
.Dv PT_READ_I
and
.Dv PT_READ_D ,
except that they write rather than read.  The
.Fa data
argument supplies the value to be written.
.It Dv PT_READ_U
This request reads an
.Li int
from the traced process' user structure.  The
.Fa addr
argument specifies the location of the int relative to the base of the
user structure; it will usually be an integer value cast to
.Li caddr_t
either explicitly or via the presence of a prototype for
.Eo \&
.Fn ptrace
.Ec .
Unlike
.Dv PT_READ_I
and
.Dv PT_READ_D ,
.Fa addr
must be aligned on an
.Li int
boundary.  The value read is returned as the return value from
.Eo \&
.Fn ptrace
.Ec .
.It Dv PT_WRITE_U
This request writes an
.Li int
into the traced process' user structure.
.Fa addr
specifies the offset, just as for
.Dv PT_READ_U ,
and
.Fa data
specifies the value to be written, just as for
.Dv PT_WRITE_I
and
.Dv PT_WRITE_D .
.It Dv PT_CONTINUE
The traced process continues execution.
.Fa addr
is an address specifying the place where execution is to be resumed (a
new value for the program counter), or
.Li (caddr_t)1
to indicate that execution is to pick up where it left off.
.Fa 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.
.It Dv PT_KILL
The traced process terminates, as if
.Dv PT_CONTINUE
had been used with
.Dv SIGKILL
given as the signal to be delivered.
.It Dv 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,
.Fa 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.
.It Dv PT_DETACH
This request is like PT_CONTINUE, except that it does not allow
specifying an alternate place to continue execution, and after it
succeeds, the traced process is no longer traced and continues
execution normally.
.El
.Pp
Additionally, machine-specific requests can exist.  On the SPARC, these
are:
.Bl -tag -width 12n
.It Dv PT_GETREGS
This request reads the traced process' machine registers into the
.Dq Li "struct reg"
(defined in
.Aq Pa machine/reg.h )
pointed to by
.Fa addr .
.It Dv PT_SETREGS
This request is the converse of
.Dv PT_GETREGS ;
it loads the traced process' machine registers from the
.Dq Li "struct reg"
(defined in
.Aq Pa machine/reg.h )
pointed to by
.Fa addr .
.It Dv PT_GETFPREGS
This request reads the traced process' floating-point registers into
the
.Dq Li "struct fpreg"
(defined in
.Aq Pa machine/reg.h )
pointed to by
.Fa addr .
.It Dv PT_SETFPREGS
This request is the converse of
.Dv PT_GETFPREGS ;
it loads the traced process' floating-point registers from the
.Dq Li "struct fpreg"
(defined in
.Aq Pa machine/reg.h )
pointed to by
.Fa addr .
.It Dv PT_SYSCALL
This request is like
.Dv PT_CONTINUE
except that the process will stop next time it executes any system
call.  Information about the system call can be examined with
.Dv PT_READ_U
and potentially modified with
.Dv PT_WRITE_U
through the
.Li u_kproc.kp_proc.p_md
element of the user structure (see below).  If the process is continued
with another
.Dv PT_SYSCALL
request, it will stop again on exit from the syscall, at which point
the return values can be examined and potentially changed.  The
.Li u_kproc.kp_proc.p_md
element is of type
.Dq Li "struct mdproc" ,
which should be declared by including
.Aq Pa sys/param.h ,
.Aq Pa sys/user.h ,
and
.Aq Pa machine/proc.h ,
and contains the following fields (among others):
.Bl -item -compact -offset indent
.It
.Li syscall_num
.It
.Li syscall_nargs
.It
.Li syscall_args[8]
.It
.Li syscall_err
.It
.Li syscall_rv[2]
.El
When a process stops on entry to a syscall,
.Li syscall_num
holds the number of the syscall,
.Li syscall_nargs
holds the number of arguments it expects, and
.Li syscall_args
holds the arguments themselves.  (Only the first
.Li syscall_nargs
elements of
.Li syscall_args
are guaranteed to be useful.)  When a process stops on exit from a
syscall,
.Li syscall_num
is
.Eo \&
.Li -1
.Ec ,
.Li syscall_err
holds the error number
.Po
see
.Xr errno 2
.Pc ,
or 0 if no error occurred, and
.Li syscall_rv
holds the return values.  (If the syscall returns only one value, only
.Li syscall_rv[0]
is useful.)  The tracing process can modify any of these with
.Dv PT_WRITE_U ;
only some modifications are useful.
.Pp
On entry to a syscall,
.Li syscall_num
can be changed, and the syscall actually performed will correspond to
the new number (it is the responsibility of the tracing process to fill
in
.Li syscall_args
appropriately for the new call, but there is no need to modify
.Eo \&
.Li syscall_nargs
.Ec ).
If the new syscall number is 0, no syscall is actually performed;
instead,
.Li syscall_err
and
.Li syscall_rv
are passed back to the traced process directly (and therefore should be
filled in).  If the syscall number is otherwise out of range, a dummy
syscall which simply produces an
.Er ENOSYS
error is effectively performed.
.Pp
On exit from a syscall, only
.Li syscall_err
and
.Li syscall_rv
can usefully be changed; they are set to the values returned by the
syscall and will be passed back to the traced process by the normal
syscall return mechanism.
.El
.Sh ERRORS
Some requests can cause
.Fn ptrace
to return
.Li -1
as a non-error value; to disambiguate,
.Va errno
can be set to 0 before the call and checked afterwards.  The possible
errors are:
.Bl -tag -width 4n
.It Bq Er ESRCH
No process having the specified process ID exists.
.It Bq Er EINVAL
.Bl -bullet -compact
.It
A process attempted to use
.Dv PT_ATTACH
on itself.
.It
The
.Fa request
was not one of the legal requests.
.It
The
.Fa addr
to
.Dv PT_READ_U
or
.Dv PT_WRITE_U
was not
.Li int Ns \&-aligned.
.It
The signal number (in
.Fa data )
to
.Dv PT_CONTINUE
or
.Dv PT_SYSCALL
was neither 0 nor a legal signal number.
.It
.Dv PT_GETREGS ,
.Dv PT_SETREGS ,
.Dv PT_GETFPREGS ,
or
.Dv PT_SETFPREGS
was attempted on a process with no valid register set.  (This is
normally true only of system processes.)
.El
.It Bq Er EBUSY
.Bl -bullet -compact
.It
.Dv PT_ATTACH
was attempted on a process that was already being traced.
.It
A request attempted to manipulate a process that was being traced by
some process other than the one making the request.
.It
A request (other than
.Dv PT_ATTACH )
specified a process that wasn't stopped.
.El
.It Bq Er EPERM
.Bl -bullet -compact
.It
A request (other than
.Dv PT_ATTACH )
attempted to manipulate a process that wasn't being traced at all.
.It
An attempt was made to use
.Dv PT_ATTACH
on a process in violation of the requirements listed under
.Dv PT_ATTACH
above.
.El
.Sh BUGS
On the SPARC, the PC is set to the provided PC value for
.Dv PT_CONTINUE
and similar calls, but the NPC is set willy-nilly to 4 greater than the
PC value.  Using
.Dv PT_GETREGS
and
.Dv PT_SETREGS
to modify the PC, passing
.Li (caddr_t)1
to
.Eo \&
.Fn ptrace
.Ec ,
should be able to sidestep this.
.Pp
Single-stepping is not available.
.Pp
When using
.Dv PT_SYSCALL ,
there is no easy way to tell whether the traced process stopped because
it made a syscall or because a signal was sent at a moment that it just
happened to have valid-looking garbage in its
.Dq Li "struct mdproc" .