PTRACE(2)

NAME
ptrace - process trace

SYNOPSIS
#include <sys/types.h>
#include <sys/ptrace.h>
long ptrace(int req, pid_t pid, long addr, long data)

DESCRIPTION
The ptrace call provides a primitive means to trace (debug) another process. A process can submit itself to tracing using a T_OK ptrace request, or can be attached to by a tracer using a T_ATTACH request. From that point on, whenever a signal is sent to the traced process, the process will be stopped. Its tracer will be told about the signal causing the stop, via wait(2). The tracer can then inspect the traced process, and choose how to continue the process's execution and whether to pass on the signal to it.

In the current model, the tracer will be notified of the signal before any checks on ignore or block masks are made. A SIGKILL signal cannot be intercepted by the tracer, and will always kill the traced process.

When the traced process performs a successful execve(2) call, it will be stopped and a SIGTRAP will be generated for it. Set-uid and set-gid bits on the new executable are ignored.

The req parameter specifies the process trace request. The interpretation of the remaining parameters depends on the given request. For all requests except T_OK, the pid parameter specifies process ID of the target process. For all requests except T_OK and T_ATTACH, the process must be stopped. The following requests are supported:

T_OK
    Set the caller's parent to be its tracer. All other arguments are ignored. This request is typically made by the child fork of a debugger, before performing an execve(2) call.

T_GETINS, T_GETDATA
    Retrieve a value from the given process's instruction and data area, respectively, at the address given in addr.

T_SETINS, T_SETDATA
    Set the value from the given process's instruction and data area, respectively, at the address given in addr, to the value given in data.

T_GETUSER
    Retrieve the value at the zero-based offset given in addr from the process's struct proc kernel structure, followed by, aligned on long size boundary, its struct priv kernel structure.

T_SETUSER
    Set some of the given process's registers at the beginning of its struct proc kernel structure. The value in data will be written to the zero-based offset given in addr from the process's struct proc kernel structure.

T_RESUME
    Resume execution of the process. A nonzero data argument will be interpreted as a signal to pass to the process.

T_STEP
    Single-step an instruction. A nonzero data argument will be interpreted as a signal to pass to the process.

T_SYSCALL
    Resume execution with system call tracing. When the traced process makes a system call, a SIGTRAP signal will be generated. A subsequent T_SYSCALL request will then cause a SIGTRAP signal to be generated when the process leaves the system call. A nonzero data argument will be interpreted as a signal to pass to the process.

T_EXIT
    Terminate the traced process, with the exit code given in the data argument. This call will return once the process has exited.

T_ATTACH
    Attach to the given process. The process will be stopped with a SIGSTOP signal.

T_DETACH
    Detach from the given process. Any signals still pending for the tracer are passed on directly to the process. A nonzero data argument will be interpreted as an additional signal to pass to the process.

T_SETOPT
    Set the given process's trace options to the bit combination of flags given in the data argument.

T_GETRANGE
    Get a range of values from the address space of the traced process. The addr argument must be a pointer to a fully initialized struct ptrace_range structure.

T_SETRANGE
    Set a range of values in the address space of the traced process. The addr argument must be a pointer to a fully initialized struct ptrace_range structure.

The following option flags are currently supported for T_SETOPT:

TO_TRACEFORK
    When the traced process performs a fork(2), automatically attach to the new child as well. The child will be stopped with a SIGSTOP signal right after forking.

TO_ALTEXEC
    Send SIGSTOP instead of SIGTRAP upon a successful execve(2). This allows the tracer to disambiguate between this case and other traps.

TO_NOEXEC
    Do not send any signal upon a successful execve(2).

The default set of trace options when tracing is initiated with T_OK is 0. The default set of trace options after attaching with T_ATTACH is TO_NOEXEC.

The T_GETRANGE and T_SETRANGE calls use the following structure:

    struct ptrace_range {
    	int	pr_space;
    	long	pr_addr;
    	size_t	pr_size;
    	void	*pr_ptr;
    };

The pr_space field specifies the address space from which to retrieve or set the values. It must be set to either of the following values:

TS_INS
    Text space.

TS_DATA
    Data space.

The pr_addr field specifies the start address of the target area in the traced process. The pr_size field specifies the number of bytes to retrieve or set, and must be between 1 and LONG_MAX. The pr_ptr field must point to a buffer in the calling process that is used to store the retrieved values (for T_GETRANGE) or contains the values to set (for T_SETRANGE).

All addresses specified for the T_GETINS, T_GETDATA, T_GETUSER requests and their T_SET* counterparts must be aligned on long boundary. Similarly, only long sized values can be retrieved and set at a time.

RETURN VALUE
All but the T_GETINS, T_GETDATA, T_GETUSER requests return 0 upon successful completion. Otherwise, a value of -1 is returned and errno is set to indicate the error.

The T_GETINS, T_GETDATA, T_GETUSER requests return the resulting data. Here, -1 is a legitimate return value. To distinguish between this and an error, clear errno before the ptrace call, and check whether it is zero afterwards.

ERRORS
The functions will fail if any of the following errors occur:

EINVAL
    Invalid request, signal, space, or length given.

ESRCH
    The given process is not found, exiting, or not traced by the caller.

EBUSY
    The given process is not stopped, or already being traced.

EFAULT
    The given address is invalid, inaccessible, or not properly aligned.

EPERM
    Attaching is denied, because the caller equals the given process, or the caller is not root and does not match the given process's user or group ID, or the caller is not root and the given process is a system process, or the caller is a system process, or the given process may not be traced at all.

LIMITATIONS
Signals are not ordered. Attaching to a process guarantees that a SIGSTOP will arrive at the tracer, but it is not guaranteed that this will be the first signal to arrive. The same goes for automatically attached children of the traced process. Similarly, if the tracer wants to detach from a running process, it will typically send a SIGSTOP using kill(2) to the process to stop it, but there is no guarantee that this will be the first signal to arrive.

Signals not caused by the process itself (e.g. those caused by kill(2)) will arrive at the tracer while the process is in stopped state, but this does not imply that the process is in a stable state at that point. The process may still have a system call pending, and this means that registers and memory of the process may change almost arbitrarily after the tracer has been told about the arrival of the current signal. Implementers of debuggers are advised to make minimal assumptions about the conditions of the process when an unexpected signal arrives.

It is not possible to use T_SYSCALL to get a trap upon leaving of a system call, if T_SYSCALL was not used to get a trap upon entering that system call. This is in fact helpful: after attaching to a process, the first T_SYSCALL call will always cause a trap after entering the next system call. As the only exception, T_SYSCALL on a fork(2) call of a process with TO_TRACEFORK set, will result in two traps upon leaving: one for the parent, and one for the child. The child's SIGSTOP signal will always come before the SIGTRAP from its leaving the system call.

There is no way to reliably distinguish between real signals and signals generated for the tracer.

For system stability reasons, the PM and VM servers cannot be traced.

SEE ALSO
wait(2), kill(2), mdb(1)

AUTHOR
Manual page written by David van Moolenbroek <dcvmoole@cs.vu.nl>