March 12, 2007 PTRACE 2

### NAME

ptrace - process tracing and debugging

### LIBRARY

Standard C Library (libc, -lc)

### SYNOPSIS

```int` ptrace(```int request``, pid_t pid``, void *addr``, int data`)

### DESCRIPTION

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 execv(3) (such as,) 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.

```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:

  ```PIOD_READ_D`

  ```PIOD_WRITE_D`

  ```PIOD_READ_I`

  ```PIOD_WRITE_I`

  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 the specific thread from 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 the thread for which to get info. Upon return ```pl_event` contains the event that stopped the thread. Possible values are:

  ```PL_EVENT_NONE`

  ```PL_EVENT_SIGNAL`

  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.

```PT_GETREGS`
  This request reads the traced process' machine registers into the `````struct reg`'' (defined in <```machine/reg.h`>) pointed to by ```addr`.

```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`.

```PT_GETFPREGS`
  This request reads the traced process' floating-point registers into the `````struct fpreg`'' (defined in <```machine/reg.h`>) pointed to by ```addr`.

```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`.

```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.

### ERRORS

Some requests can cause ptrace(``) to return ```-1` as a non-error value; to disambiguate, errno can be set to 0 before the call and checked afterwards. The possible errors are:

[```EAGAIN`]
  Process is currently exec'ing and cannot be traced.

[```ESRCH`]
  No process having the specified process ID exists.

[```EINVAL`]
  - A process attempted to use ```PT_ATTACH` on itself.
  - The ```request` was not a legal request on this machine architecture.
  - The signal number (in ```data`) to ```PT_CONTINUE` was neither 0 nor a legal signal number.
  - ```PT_GETREGS`, ```PT_SETREGS`, ```PT_GETFPREGS`, or ```PT_SETFPREGS` was attempted on a process with no valid register set. (This is normally true only of system processes.)

[```EBUSY`]
  - ```PT_ATTACH` was attempted on a process that was already being traced.
  - A request attempted to manipulate a process that was being traced by some process other than the one making the request.
  - A request (other than ```PT_ATTACH`) specified a process that wasn't stopped.

[```EPERM`]
  - A request (other than ```PT_ATTACH`) attempted to manipulate a process that wasn't being traced at all.
  - An attempt was made to use ```PT_ATTACH` on a process in violation of the requirements listed under ```PT_ATTACH` above.

### SEE ALSO

sigaction(2), signal(7)

### BUGS

On the SPARC, the PC is set to the provided PC value for ```PT_CONTINUE` and similar calls, but the NPC is set willy-nilly to 4 greater than the PC value. Using ```PT_GETREGS` and ```PT_SETREGS` to modify the PC, passing ```(caddr_t)1` to ptrace(``) should be able to sidestep this.