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PTRACE(2) System Calls Manual PTRACE(2)

ptrace — process tracing and debugging

#include <sys/types.h>
#include <sys/ptrace.h>

int
ptrace(int request, pid_t pid, caddr_t addr, int data);

() provides tracing and debugging facilities. It allows one process (the process) to control another (the 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. ptrace() is only available on kernels compiled with the PTRACE option.

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 () calls are made by the tracing process, and the pid argument specifies the process ID of the traced process. request can be:

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 (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.
, 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 OpenBSD implementation, these two requests operate in the same address space. 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_D
These requests parallel PT_READ_I and PT_READ_D, except that they write rather than read. PT_WRITE_I may be necessary to ensure that instruction caches are flushed appropriately. The data argument supplies the value to be written.
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.
The traced process terminates, as if PT_CONTINUE had been used with SIGKILL given as the signal to be delivered.
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 set-user-ID or set-group-ID executable. Additionally, if the kern.global_ptrace sysctl is 0, then the target process must be a descendant of the tracing process. (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.
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.
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 be made, piod_addr is the buffer in the parent and piod_len is the length of the I/O request. The piod_op member specifies what operation needs to be done. Possible values are:

PIOD_READ_D
 
PIOD_WRITE_D
 
PIOD_READ_I
 
PIOD_WRITE_I
 
PIOD_READ_AUXV
 

See also the description of PT_READ_I for the difference between D and I spaces. The PIOD_READ_AUXV operation can be used to read from the ELF auxiliary vector. A pointer to the descriptor is passed in addr. On return the piod_len field in the descriptor will be updated with the actual number of bytes transferred. If the requested I/O could not be successfully performed, () will return -1 and set errno.

This request can be used to specify which events in the traced process should be reported to the tracing process. These events are specified in a “struct ptrace_event” defined as: 
typedef struct ptrace_event {
	int	pe_set_event;
} ptrace_event_t;

Where pe_set_event is the set of events to be reported. This set is formed by OR'ing together the following values:

PTRACE_FORK
Report fork(2).

A pointer to this structure is passed in addr. The data argument should be set to sizeof(struct ptrace_event).

This request can be used to determine which events in the traced process will be reported. The information is read into the “struct ptrace_event” pointed to by addr. The data argument should be set to sizeof(struct ptrace_event).
This request reads the state information associated with the event that stopped the traced process. The information is reported in a “struct ptrace_state” defined as: 
typedef struct ptrace_state {
	int	pe_report_event;
	pid_t	pe_other_pid;
} ptrace_state_t;

Where pe_report_event is the event being reported. If the event being reported is PTRACE_FORK, pe_other_pid will be set to the process ID of the other end of the fork. A pointer to this structure is passed in addr. The data argument should be set to sizeof(struct ptrace_state).

This request reads the thread ID of the traced process' first thread into the “struct ptrace_thread_state” pointed to by addr. The data argument should be set to sizeof(struct ptrace_thread_state).
This request is just like PT_GET_THREAD_FIRST, except it returns the thread ID of the subsequent thread. The “struct ptrace_thread_state” pointed to by addr must be initialized by a previous PT_GET_THREAD_FIRST or PT_GET_THREAD_NEXT request.

Additionally, machine-specific requests can exist. Depending on the architecture, the following requests may be available under OpenBSD:

(all platforms)
This request reads the traced process' machine registers into the “struct reg” (defined in <machine/reg.h>) pointed to by addr.
(all platforms)
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.
(not available on sparc64)
The traced process continues execution, as in request PT_CONTINUE; however, execution stops as soon as possible after execution of at least one instruction (single-step).
(not available on luna88k or mips64)
This request reads the traced process' floating-point registers into the “struct fpreg” (defined in <machine/reg.h>) pointed to by addr.
(not available on luna88k or mips64)
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.
(i386 only)
This request reads the traced process' XMM registers into the “struct xmmregs” (defined in <machine/reg.h>) pointed to by addr.
(i386 only)
This request is the converse of PT_GETXMMREGS; it loads the traced process' XMM registers from the “struct xmmregs” (defined in <machine/reg.h>) pointed to by addr.
(sparc64 only)
This request reads the traced process' ‘window cookie’ into the int pointed to by addr. The window cookie needs to be ‘XOR'ed’ to stack-saved program counters.

Some requests can cause ptrace() to return -1 as a non-error value; to disambiguate, errno is set to zero and this should be checked. The possible errors are:

[]
No process having the specified process ID exists.
[]
  • A process attempted to use PT_ATTACH on itself.
  • The request was not one of the legal requests.
  • The signal number (in data) to PT_CONTINUE was neither 0 nor a legal signal number.
  • , 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.)
[]
  • 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 and waited for.
[]
  • 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.
  • An attempt was made to use PT_ATTACH on a system process.

The ptrace() system call first appeared in Version 6 AT&T UNIX.

On several RISC architectures (such as luna88k and sparc64), the PC is set to the provided PC value for PT_CONTINUE and similar calls, and the remainder of the execution pipeline registers are set to the following instructions, even if the instruction at PC is a branch instruction. Using PT_GETREGS and PT_SETREGS to modify the PC, passing (caddr_t)1 to ptrace(), should be able to sidestep this.

September 11, 2022 OpenBSD-7.6