PTRACE(2) NetBSD Programmer's Manual PTRACE(2)
NAME
ptrace - process tracing and debugging
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <sys/types.h> #include <sys/ptrace.h> int ptrace(int request, pid_t pid, caddr_t addr, int data);
DESCRIPTION
ptrace() provides tracing and debugging facilities. It allows one pro- cess (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 par- ent. All the other arguments are ignored. (If the parent process does not expect to trace the child, it will proba- bly be rather confused by the results; once the traced pro- cess 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 (such as execv(3)), it will stop before executing the first instruc- tion 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 al- lowed for machines with distinct address spaces for in- struction 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 align- ment 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 sup- plies 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 indi- cate that execution is to pick up where it left off. data provides a signal number to be delivered to the traced pro- cess 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 other- wise 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 re- quest 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 ap- ply.) The tracing process will see the newly-traced pro- cess 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 directo- ry set with chroot(2), it may not trace another process un- less that process's root directory is at or below the trac- ing process's root. PT_DETACH This request is like PT_CONTINUE, except that after it suc- ceeds, 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. Additionally, the following requests exist but are not avaliable 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 instruc- tion, execution stops again. PT_GETREGS This request reads the traced process' machine registers into the ``struct reg'' (defined in <machine/reg.h>) point- ed 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 reg- isters 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.
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 after- wards. 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 architec- ture. + 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 at- tempted on a process with no valid register set. (This is nor- mally 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 pro- cess 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. NetBSD 1.6.1 November 7, 1994 3
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