239 lines
8.9 KiB
Groff
239 lines
8.9 KiB
Groff
.TH PTRACE 2 "September 27, 2009"
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.UC 4
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.SH NAME
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ptrace \- process trace
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.SH SYNOPSIS
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.nf
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.ft B
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#include <sys/types.h>
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#include <sys/ptrace.h>
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long ptrace(int \fIreq\fP, pid_t \fIpid\fP, void *\fIaddr\fP, long \fIdata\fP)
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.ft R
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.fi
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.SH DESCRIPTION
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The \fBptrace\fP call provides a primitive means to trace (debug) another
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process. A process can submit itself to tracing using a \fBT_OK\fP ptrace
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request, or can be attached to by a tracer using a \fBT_ATTACH\fP request.
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From that point on, whenever a signal is sent to the traced process,
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the process will be stopped. Its tracer will be told about the signal
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causing the stop, via
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.BR wait (2).
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The tracer can then inspect the traced process, and choose how to continue the
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process's execution and whether to pass on the signal to it.
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.PP
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In the current model, the tracer will be notified of the signal before any
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checks on ignore or block masks are made. A \fBSIGKILL\fP signal cannot be
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intercepted by the tracer, and will always kill the traced process.
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.PP
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When the traced process performs a successful
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.BR execve (2)
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call, it will be stopped and a \fBSIGTRAP\fP will be generated for it.
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Set-uid and set-gid bits on the new executable are ignored.
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.PP
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The \fIreq\fP parameter specifies the process trace request. The interpretation
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of the remaining parameters depends on the given request. For all requests
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except \fBT_OK\fP, the \fIpid\fP parameter specifies process ID of the target
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process. For all requests except \fBT_OK\fP and \fBT_ATTACH\fP, the process
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must be stopped. The following requests are supported:
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.TP 2
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.B T_OK
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Set the caller's parent to be its tracer. All other arguments are ignored.
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This request is typically made by the child fork of a debugger,
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before performing an
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.BR execve (2)
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call.
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.TP
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.B T_GETINS, T_GETDATA
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Retrieve a value from the given process's instruction and data area,
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respectively, at the address given in \fIaddr\fP.
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.TP
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.B T_SETINS, T_SETDATA
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Set the value from the given process's instruction and data area, respectively,
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at the address given in \fIaddr\fP, to the value given in \fIdata\fP.
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.TP
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.B T_GETUSER
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Retrieve the value at the zero-based offset given in \fIaddr\fP from the
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process's \fBstruct proc\fP kernel structure, followed by, aligned on
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\fBlong\fP size boundary, its \fBstruct priv\fP kernel structure.
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.TP
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.B T_SETUSER
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Set some of the given process's registers at the beginning of its
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\fBstruct proc\fP kernel structure. The value in \fIdata\fP will be written to
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the zero-based offset given in \fIaddr\fP from the process's \fBstruct proc\fP
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kernel structure.
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.TP
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.B T_RESUME
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Resume execution of the process. A nonzero \fIdata\fP argument will be
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interpreted as a signal to pass to the process.
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.TP
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.B T_STEP
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Single-step an instruction. A nonzero \fIdata\fP argument will be interpreted
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as a signal to pass to the process.
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.TP
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.B T_SYSCALL
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Resume execution with system call tracing. When the traced process makes a
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system call, a \fBSIGTRAP\fP signal will be generated. A subsequent
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\fBT_SYSCALL\fP request will then cause a \fBSIGTRAP\fP signal to be generated
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when the process leaves the system call. A nonzero \fIdata\fP argument will be
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interpreted as a signal to pass to the process.
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.TP
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.B T_EXIT
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Terminate the traced process, with the exit code given in the \fIdata\fP
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argument. This call will return once the process has exited.
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.TP
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.B T_ATTACH
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Attach to the given process. The process will be stopped with a \fBSIGSTOP\fP
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signal.
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.TP
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.B T_DETACH
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Detach from the given process. Any signals still pending for the tracer are
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passed on directly to the process. A nonzero \fIdata\fP argument will be
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interpreted as an additional signal to pass to the process.
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.TP
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.B T_SETOPT
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Set the given process's trace options to the bit combination of flags given
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in the \fIdata\fP argument.
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.TP
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.B T_GETRANGE
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Get a range of values from the address space of the traced process. The
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\fIaddr\fP argument must be a pointer to a fully initialized
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\fBstruct ptrace_range\fP structure.
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.TP
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.B T_SETRANGE
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Set a range of values in the address space of the traced process. The
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\fIaddr\fP argument must be a pointer to a fully initialized
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\fBstruct ptrace_range\fP structure.
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.PP
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The following option flags are currently supported for \fBT_SETOPT\fP:
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.TP 2
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.B TO_TRACEFORK
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When the traced process performs a
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.BR fork (2),
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automatically attach to the new child as well.
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The child will be stopped with a \fBSIGSTOP\fP signal right after forking.
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.TP
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.B TO_ALTEXEC
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Send \fBSIGSTOP\fP instead of \fBSIGTRAP\fP upon a successful
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.BR execve (2).
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This allows the tracer to disambiguate between this case and other traps.
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.TP
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.B TO_NOEXEC
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Do not send any signal upon a successful
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.BR execve (2).
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.PP
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The default set of trace options when tracing is initiated with \fBT_OK\fP is
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\fB0\fP.
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The default set of trace options after attaching with \fBT_ATTACH\fP is
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\fBTO_NOEXEC\fP.
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.PP
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The \fBT_GETRANGE\fP and \fBT_SETRANGE\fP calls use the following structure:
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.PP
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.RS
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.nf
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.ft B
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.ta +4n +8n
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struct ptrace_range {
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int pr_space;
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long pr_addr;
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size_t pr_size;
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void *pr_ptr;
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};
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.ft R
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.fi
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.RE
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.PP
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The \fBpr_space\fP field specifies the address space from which to retrieve
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or set the values. It must be set to either of the following values:
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.PP
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.TP 10
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.B TS_INS
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Text space.
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.TP
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.B TS_DATA
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Data space.
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.PP
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The \fBpr_addr\fP field specifies the start address of the target area in the
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traced process. The \fBpr_size\fP field specifies the number of bytes to
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retrieve or set, and must be between 1 and LONG_MAX. The \fBpr_ptr\fP field
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must point to a buffer in the calling process that is used to store the
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retrieved values (for \fBT_GETRANGE\fP) or contains the values to set (for
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\fBT_SETRANGE\fP).
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.PP
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All addresses specified for the \fBT_GETINS\fP, \fBT_GETDATA\fP,
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\fBT_GETUSER\fP requests and their \fBT_SET\fP* counterparts must be
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aligned on \fBlong\fP boundary. Similarly, only \fBlong\fP sized values can be
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retrieved and set at a time.
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.SH "RETURN VALUE"
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All but the \fBT_GETINS\fP, \fBT_GETDATA\fP, \fBT_GETUSER\fP requests return 0
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upon successful completion.
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Otherwise, a value of -1 is returned and \fIerrno\fP is set to indicate the
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error.
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.PP
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The \fBT_GETINS\fP, \fBT_GETDATA\fP, \fBT_GETUSER\fP requests return the
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resulting data. Here, -1 is a legitimate return value.
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To distinguish between this and an error, clear \fIerrno\fP
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before the \fBptrace\fP call, and check whether it is zero afterwards.
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.SH ERRORS
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The functions will fail if any of the following errors occur:
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.TP 10
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.B EINVAL
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Invalid request, signal, space, or length given.
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.TP
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.B ESRCH
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The given process is not found, exiting, or not traced by the caller.
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.TP
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.B EBUSY
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The given process is not stopped, or already being traced.
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.TP
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.B EFAULT
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The given address is invalid, inaccessible, or not properly aligned.
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.TP
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.B EPERM
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Attaching is denied, because the caller equals the given process,
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or the caller is not root and does not match the given process's
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user or group ID, or the caller is not root and the given process
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is a system process, or the caller is a system process,
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or the given process may not be traced at all.
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.SH LIMITATIONS
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Signals are not ordered. Attaching to a process guarantees that a \fBSIGSTOP\fP
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will arrive at the tracer, but it is not guaranteed that this will be the first
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signal to arrive. The same goes for automatically attached children of the
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traced process. Similarly, if the tracer wants to detach from a running
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process, it will typically send a \fBSIGSTOP\fP using
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.BR kill (2)
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to the process to stop it, but there is no guarantee that this will be the
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first signal to arrive.
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.PP
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Signals not caused by the process itself (e.g. those caused by
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.BR kill (2))
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will arrive at the tracer while the process is in stopped state, but this does
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not imply that the process is in a stable state at that point. The process may
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still have a system call pending, and this means that registers and memory of
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the process may change almost arbitrarily after the tracer has been told about
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the arrival of the current signal. Implementers of debuggers are advised to
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make minimal assumptions about the conditions of the process when an unexpected
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signal arrives.
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.PP
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It is not possible to use \fBT_SYSCALL\fP to get a trap upon leaving of a
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system call, if \fBT_SYSCALL\fP was not used to get a trap upon entering that
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system call. This is in fact helpful: after attaching to a process, the first
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\fBT_SYSCALL\fP call will always cause a trap after entering the next system
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call. As the only exception, \fBT_SYSCALL\fP on a
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.BR fork (2)
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call of a process with \fBTO_TRACEFORK\fP set, will result in two traps upon
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leaving: one for the parent, and one for the child. The child's \fBSIGSTOP\fP
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signal will always come before the \fBSIGTRAP\fP from its leaving the system
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call.
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.PP
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There is no way to reliably distinguish between real signals and signals
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generated for the tracer.
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.PP
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For system stability reasons, the PM and VM servers cannot be traced.
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.SH "SEE ALSO"
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.BR wait (2),
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.BR kill (2),
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.BR mdb (1)
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.SH AUTHOR
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Manual page written by David van Moolenbroek <dcvmoole@cs.vu.nl>
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