ptrace (2)

Leading comments

 $FreeBSD: releng/11.0/lib/libc/sys/ptrace.2 297355 2016-03-28 16:48:28Z trasz $
	$NetBSD: ptrace.2,v 1.2 1995/02/27 12:35:37 cgd Exp $

 This file is in the public domain.

(The comments found at the beginning of the groff file "man2/ptrace.2freebsd".)

NAME

ptrace - process tracing and debugging

LIBRARY

Lb libc

SYNOPSIS

In sys/types.h In sys/ptrace.h Ft int Fn ptrace int request pid_t pid caddr_t addr int data

DESCRIPTION

The Fn ptrace system call provides tracing and debugging facilities. It allows one process (the tracing process) to control another (the traced process). The tracing process must first attach to the traced process, and then issue a series of Fn ptrace system calls to control the execution of the process, as well as access process memory and register state. For the duration of the tracing session, the traced process will be ``re-parented'' with its parent process ID (and resulting behavior) changed to the tracing process. It is permissible for a tracing process to attach to more than one other process at a time. When the tracing process has completed its work, it must detach the traced process; if a tracing process exits without first detaching all processes it has attached, those processes will be killed.

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. The signal may be a normal process signal, generated as a result of traced process behavior, or use of the kill(2) system call; alternatively, it may be generated by the tracing facility as a result of attaching, system calls, or stepping by the tracing process. The tracing process may choose to intercept the signal, using it to observe process behavior (such as SIGTRAP ) or forward the signal to the process if appropriate. The Fn ptrace system call is the mechanism by which all this happens.

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 or a corresponding thread ID. The Fa request argument 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 Fn ptrace . ) When a process has used this request and calls execve(2) or any of the routines built on it (such as execv(3)), 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. If the child was created by vfork(2) system call or rfork(2) call with the RFMEM flag specified, the debugging events are reported to the parent only after the execve(2) is executed.

PT_READ_I , PT_READ_D

These requests read a single Vt int of data from the traced process's 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, PT_READ_I reads from the instruction space and PT_READ_D reads from the data space. In the current Fx implementation, these two requests are completely identical. The Fa addr argument specifies the address (in the traced process's 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 Fn 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 Fa data argument supplies the value to be written.

PT_IO

This request allows reading and writing arbitrary amounts of data in the traced process's address space. The Fa addr argument specifies a pointer to a Vt struct ptrace_io_desc , which is defined as follows:

struct ptrace_io_desc {
        int     piod_op;        /* I/O operation */
        void    *piod_offs;     /* child offset */
        void    *piod_addr;     /* parent offset */
        size_t  piod_len;       /* request length */
};

/*
 * Operations in piod_op.
 */
#define PIOD_READ_D     1       /* Read from D space */
#define PIOD_WRITE_D    2       /* Write to D space */
#define PIOD_READ_I     3       /* Read from I space */
#define PIOD_WRITE_I    4       /* Write to I space */

The Fa data argument is ignored. The actual number of bytes read or written is stored in piod_len upon return.

PT_CONTINUE

The traced process continues execution. The Fa addr argument is an address specifying the place where execution is to be resumed (a new value for the program counter), or Po Vt caddr_t Pc Ns 1 to indicate that execution is to pick up where it left off. The Fa data argument 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_STEP

The traced process is single stepped one instruction. The Fa addr argument should be passed Po Vt caddr_t Pc Ns 1 . The Fa data argument 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, 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.

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.

PT_GETREGS

This request reads the traced process's machine registers into the Do Vt struct reg Dc (defined in In machine/reg.h ) pointed to by Fa addr .

PT_SETREGS

This request is the converse of PT_GETREGS it loads the traced process's machine registers from the Do Vt struct reg Dc (defined in In machine/reg.h ) pointed to by Fa addr .

PT_GETFPREGS

This request reads the traced process's floating-point registers into the Do Vt struct fpreg Dc (defined in In machine/reg.h ) pointed to by Fa addr .

PT_SETFPREGS

This request is the converse of PT_GETFPREGS it loads the traced process's floating-point registers from the Do Vt struct fpreg Dc (defined in In machine/reg.h ) pointed to by Fa addr .

PT_GETDBREGS

This request reads the traced process's debug registers into the Do Vt struct dbreg Dc (defined in In machine/reg.h ) pointed to by Fa addr .

PT_SETDBREGS

This request is the converse of PT_GETDBREGS it loads the traced process's debug registers from the Do Vt struct dbreg Dc (defined in In machine/reg.h ) pointed to by Fa addr .

PT_LWPINFO

This request can be used to obtain information about the kernel thread, also known as light-weight process, that caused the traced process to stop. The Fa addr argument specifies a pointer to a Vt struct ptrace_lwpinfo , which is defined as follows:

struct ptrace_lwpinfo {
        lwpid_t pl_lwpid;
        int     pl_event;
        int     pl_flags;
        sigset_t pl_sigmask;
        sigset_t pl_siglist;
        siginfo_t pl_siginfo;
        char    pl_tdname[MAXCOMLEN + 1];
        int     pl_child_pid;
};

The Fa data argument is to be set to the size of the structure known to the caller. This allows the structure to grow without affecting older programs.

The fields in the Vt struct ptrace_lwpinfo have the following meaning:

pl_lwpid

LWP id of the thread

pl_event

Event that caused the stop. Currently defined events are

PL_EVENT_NONE

No reason given

PL_EVENT_SIGNAL

Thread stopped due to the pending signal

pl_flags

Flags that specify additional details about observed stop. Currently defined flags are:

PL_FLAG_SCE

The thread stopped due to system call entry, right after the kernel is entered. The debugger may examine syscall arguments that are stored in memory and registers according to the ABI of the current process, and modify them, if needed.

PL_FLAG_SCX

The thread is stopped immediately before syscall is returning to the usermode. The debugger may examine system call return values in the ABI-defined registers and/or memory.

PL_FLAG_EXEC

When PL_FLAG_SCX is set, this flag may be additionally specified to inform that the program being executed by debuggee process has been changed by successful execution of a system call from the Fn execve 2 family.

PL_FLAG_SI

Indicates that pl_siginfo member of Vt struct ptrace_lwpinfo contains valid information.

PL_FLAG_FORKED

Indicates that the process is returning from a call to Fn fork 2 that created a new child process. The process identifier of the new process is available in the pl_child_pid member of Vt struct ptrace_lwpinfo .

PL_FLAG_CHILD

The flag is set for first event reported from a new child, which is automatically attached due to PT_FOLLOW_FORK enabled.

pl_sigmask

The current signal mask of the LWP

pl_siglist

The current pending set of signals for the LWP. Note that signals that are delivered to the process would not appear on an LWP siglist until the thread is selected for delivery.

pl_siginfo

The siginfo that accompanies the signal pending. Only valid for PL_EVENT_SIGNAL stop when PL_FLAG_SI is set in pl_flags

pl_tdname

The name of the thread.

pl_child_pid

The process identifier of the new child process. Only valid for a PL_EVENT_SIGNAL stop when PL_FLAG_FORKED is set in pl_flags

PT_GETNUMLWPS

This request returns the number of kernel threads associated with the traced process.

PT_GETLWPLIST

This request can be used to get the current thread list. A pointer to an array of type Vt lwpid_t should be passed in Fa addr , with the array size specified by Fa data . The return value from Fn ptrace is the count of array entries filled in.

PT_SETSTEP

This request will turn on single stepping of the specified process.

PT_CLEARSTEP

This request will turn off single stepping of the specified process.

PT_SUSPEND

This request will suspend the specified thread.

PT_RESUME

This request will resume the specified thread.

PT_TO_SCE

This request will trace the specified process on each system call entry.

PT_TO_SCX

This request will trace the specified process on each system call exit.

PT_SYSCALL

This request will trace the specified process on each system call entry and exit.

PT_FOLLOW_FORK

This request controls tracing for new child processes of a traced process. If Fa data is non-zero, then new child processes will enable tracing and stop before executing their first instruction. If Fa data is zero, then new child processes will execute without tracing enabled. By default, tracing is not enabled for new child processes. Child processes do not inherit this property. The traced process will set the PL_FLAG_FORKED flag upon exit from a system call that creates a new process.

PT_VM_TIMESTAMP

This request returns the generation number or timestamp of the memory map of the traced process as the return value from Fn ptrace . This provides a low-cost way for the tracing process to determine if the VM map changed since the last time this request was made.

PT_VM_ENTRY

This request is used to iterate over the entries of the VM map of the traced process. The Fa addr argument specifies a pointer to a Vt struct ptrace_vm_entry , which is defined as follows:

struct ptrace_vm_entry {
        int             pve_entry;
        int             pve_timestamp;
        u_long          pve_start;
        u_long          pve_end;
        u_long          pve_offset;
        u_int           pve_prot;
        u_int           pve_pathlen;
        long            pve_fileid;
        uint32_t        pve_fsid;
        char            *pve_path;
};

The first entry is returned by setting pve_entry to zero. Subsequent entries are returned by leaving pve_entry unmodified from the value returned by previous requests. The pve_timestamp field can be used to detect changes to the VM map while iterating over the entries. The tracing process can then take appropriate action, such as restarting. By setting pve_pathlen to a non-zero value on entry, the pathname of the backing object is returned in the buffer pointed to by pve_path provided the entry is backed by a vnode. The pve_pathlen field is updated with the actual length of the pathname (including the terminating null character). The pve_offset field is the offset within the backing object at which the range starts. The range is located in the VM space at pve_start and extends up to pve_end (inclusive).

The Fa data argument is ignored.

Additionally, machine-specific requests can exist.

RETURN VALUES

Some requests can cause Fn ptrace to return -1 as a non-error value; to disambiguate, errno can be set to 0 before the call and checked afterwards.

ERRORS

The Fn ptrace system call may fail if:

Bq Er ESRCH

• No process having the specified process ID exists.

Bq Er EINVAL

• A process attempted to use PT_ATTACH on itself.
• The Fa request argument was not one of the legal requests.
• The signal number (in Fa data ) to PT_CONTINUE was neither 0 nor a legal signal number.
• PT_GETREGS PT_SETREGS PT_GETFPREGS PT_SETFPREGS PT_GETDBREGS or PT_SETDBREGS was attempted on a process with no valid register set. (This is normally true only of system processes.)
• PT_VM_ENTRY was given an invalid value for Fa pve_entry . This can also be caused by changes to the VM map of the process.
• The size (in Fa data ) provided to PT_LWPINFO was less than or equal to zero, or larger than the Vt ptrace_lwpinfo structure known to the kernel.

Bq Er 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 was not stopped.

Bq Er EPERM

• A request (other than PT_ATTACH attempted to manipulate a process that was not 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.

Bq Er ENOENT

• PT_VM_ENTRY previously returned the last entry of the memory map. No more entries exist.

Bq Er ENAMETOOLONG

• PT_VM_ENTRY cannot return the pathname of the backing object because the buffer is not big enough. Fa pve_pathlen holds the minimum buffer size required on return.

SEE ALSO

execve(2), sigaction(2), wait(2), execv(3), i386_clr_watch3, i386_set_watch3

HISTORY

The Fn ptrace function appeared in AT&T System v7 .