.\" $NetBSD: ptrace.2,v 1.79.2.1 2019/10/15 19:13:22 martin Exp $ .\" .\" This file is in the public domain. .Dd October 9, 2019 .Dt PTRACE 2 .Os .Sh NAME .Nm ptrace .Nd process tracing and debugging .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In sys/types.h .In sys/ptrace.h .Ft int .Fn ptrace "int request" "pid_t pid" "void *addr" "int data" .Sh DESCRIPTION .Fn ptrace provides tracing and debugging facilities. It allows one process (the .Em tracing process) to control another (the .Em traced process). Most of the time, the traced process runs normally, but when it receives a signal .Po see .Xr sigaction 2 .Pc , it stops. The tracing process is expected to notice this via .Xr wait 2 or the delivery of a .Dv SIGCHLD signal .Po see .Xr siginfo 2 .Pc , examine the state of the stopped process, and cause it to terminate or continue as appropriate. .Fn ptrace is the mechanism by which all this happens. .Pp When a process that is traced by a debugger requests and calls .Xr execve 2 or any of the routines built on it .Po such as .Xr execv 3 .Pc , it will stop before executing the first instruction of the new image and emit .Dv SIGTRAP with .Dv si_code set to .Dv TRAP_EXEC . If a traced program calls .Xr execve 2 any setuid or setgid bits on the executable being executed will be ignored. .Pp Program (software) breakpoints are reported with .Dv SIGTRAP and the .Dv si_code value set to .Dv TRAP_BKPT . These breakpoints are machine specific instructions that interrupt the process. In order to put a trap by a tracer into the tracee's program, debugger must violate the .Dv PaX MPROTECT restrictions. For details check the .Dv security.pax.mprotect.ptrace option described in .Xr sysctl 7 . When a tracee is interrupted by a trap, the trap is not removed by the kernel and it must be handled by a debugger. .Pp If a program is traced with single steps .Dv ( PT_STEP ) it reports each step with .Dv SIGTRAP with .Dv si_code set to .Dv TRAP_TRACE . This event is not maskable .Dv PT_SET_EVENT_MASK . .Pp Child program traps are reported with .Dv SIGTRAP and the .Dv si_code value set to .Dv TRAP_CHLD . These events are by default disabled and can be configured with .Dv PT_SET_EVENT_MASK . If this event occurs, check with .Dv PT_GET_PROCESS_STATE the details of the process state associated with this event. .Pp Design choices for Debug Register accessors .Bl -dash .It .Fn exec .Dv ( TRAP_EXEC event) must remove debug registers from LWP .It debug registers are only per-LWP, not per-process globally .It debug registers must not be inherited after (v)forking a process .It debug registers must not be inherited after forking a thread .It a debugger is responsible to set global watchpoints/breakpoints with the debug registers, to achieve this .Dv PTRACE_LWP_CREATE / .Dv PTRACE_LWP_EXIT event monitoring function is designed to be used .It debug register traps must generate .Dv SIGTRAP with .Dv si_code .Dv TRAP_DBREG .It debugger is responsible to retrieve debug register state to distinguish the exact debug register trap .It kernel must not remove debug register traps after triggering a trap event; a debugger is responsible to detach this trap with appropriate .Dv PT_SETDBREGS call .It debug registers must not be exposed in mcontext .It userland must not be allowed to set a trap on the kernel .El .Pp A debugger might reuse port specific symbols, to help writing portable code as described in the port specific part of the .In sys/ptrace.h header. Among these symbols, there are: .Bl -dash .It .Dv PTRACE_REG_PC .It .Dv PTRACE_REG_SET_PC .It .Dv PTRACE_REG_SP .It .Dv PTRACE_REG_INTRV .It .Dv PTRACE_BREAKPOINT .It .Dv PTRACE_BREAKPOINT_SIZE .It .Dv PTRACE_BREAKPOINT_ADJ .El .Pp The .Fa request argument of .Nm 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 .Nm calls are made by the tracing process, and the .Fa pid argument specifies the process ID of the traced process. .Fa request can be: .Bl -tag -width 12n .It Dv 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 . .Pp This call does not stop the process neither emit .Dv SIGSTOP to parent. .It Dv PT_READ_I , Dv PT_READ_D These requests read a single .Vt int of data from the traced process' 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, .Dv PT_READ_I reads from the instruction space and .Dv PT_READ_D reads from the data space. In the current .Nx implementation, these two requests are completely identical. The .Fa 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 .Eo \& .Fn ptrace .Ec . .Pp These operations return success on incomplete and cancelled byte transfers. New software shall use .Dv PT_IO as it allows to check whether a byte transfer was completed. .It Dv PT_WRITE_I , Dv PT_WRITE_D These requests parallel .Dv PT_READ_I and .Dv PT_READ_D , except that they write rather than read. The .Fa data argument supplies the value to be written. .Pp New software shall use .Dv PT_IO as it allows to check whether an operation was completed. .It Dv PT_CONTINUE The traced process continues execution. .Fa addr is an address specifying the place where execution is to be resumed (a new value for the program counter), or .Li (void *)1 to indicate that execution is to pick up where it left off. .Fa 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. If a negative value is supplied, that is the negative of the LWP ID of the thread to be resumed, and only that thread executes. .It Dv PT_KILL The traced process terminates, as if .Dv PT_CONTINUE has been used with .Dv SIGKILL given as the signal to be delivered. However, unlike .Dv PT_CONTINUE , .Dv PT_KILL can be used on a non-stopped tracee. The .Fa addr and .Fa data arguments are ignored. .It Dv PT_STOP The traced process stops, as if .Xr kill has been used with .Dv SIGSTOP given as the signal to be delivered. .Xr wait 2 will report the child (again) as stopped even if it was stopped before. The .Fa addr and .Fa data arguments are ignored. Unlike .Dv PT_CONTINUE call with .Dv SIGSTOP , .Dv PT_STOP works both on running and stopped processes. .It Dv 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.) .Pp The tracing process will see the newly-traced process stop and may then control it as if it had been traced all along. It means that the .Dv SIGSTOP signal is emitted to tracer. It is different behavior to the one from .Dv PT_TRACE_ME . .Pp 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 .Xr init 8 . Third, if a process has its root directory set with .Xr chroot 2 , it may not trace another process unless that process' root directory is at or below the tracing process' root. .It Dv PT_DETACH This request is like PT_CONTINUE, except that after it succeeds, the traced process is no longer traced and continues execution normally. .It Dv PT_IO This request is a more general interface that can be used instead of .Dv PT_READ_D , .Dv PT_WRITE_D , .Dv PT_READ_I , and .Dv PT_WRITE_I . The I/O request is encoded in a .Vt struct ptrace_io_desc defined as: .Bd -literal -offset indent struct ptrace_io_desc { int piod_op; void *piod_offs; void *piod_addr; size_t piod_len; }; .Ed .Pp where .Fa piod_offs is the offset within the traced process where the I/O operation should take place, .Fa piod_addr is the buffer in the tracing process, and .Fa piod_len is the length of the I/O request. The .Fa piod_op field specifies which type of I/O operation to perform. Possible values are: .Pp .Bl -tag -width 18n -offset indent -compact .It Dv PIOD_READ_D .It Dv PIOD_WRITE_D .It Dv PIOD_READ_I .It Dv PIOD_WRITE_I .It Dv PIOD_READ_AUXV .El .Pp See the description of .Dv PT_READ_I for the difference between I and D spaces. .Pp The .Dv PIOD_READ_AUXV operation can be used to read from the ELF auxiliary vector. The .Fa piod_offs argument sets the offset within the tracee's vector. To read from the beginning of it, this value must be set to 0 and cast to .Dv (void *) . .Pp A pointer to the I/O descriptor is passed in the .Fa addr argument to .Fn ptrace . On return, the .Fa 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, .Fn ptrace will return \-1 and set .Va errno . .Pp This interface returns success for partial and cancelled byte transfers. For an interrupted transfer, a user shall check whether occurred at least a single of the following two conditions: .Dv piod_len == 0 and set .Va errno . Successful but incomplete byte transfers shall be restarted in the place where they were stopped. .It Dv PT_DUMPCORE Makes the process specified in the .Fa pid pid generate a core dump. The .Fa addr argument should contain the name of the core file to be generated and the .Fa data argument should contain the length of the core filename. .It Dv PT_LWPINFO Returns information about a thread from the list of threads for the process specified in the .Fa pid argument. The .Fa addr argument should contain a .Vt struct ptrace_lwpinfo defined as: .Bd -literal -offset indent struct ptrace_lwpinfo { lwpid_t pl_lwpid; int pl_event; }; .Ed .Pp where .Fa pl_lwpid contains a thread LWP ID. Information is returned for the thread following the one with the specified ID in the process thread list, or for the first thread if .Fa pl_lwpid is 0. Upon return .Fa pl_lwpid contains the LWP ID of the thread that was found, or 0 if there is no thread after the one whose LWP ID was supplied in the call. .Fa pl_event contains the event that stopped the thread. Possible values are: .Pp .Bl -tag -width 30n -offset indent -compact .It Dv PL_EVENT_NONE .It Dv PL_EVENT_SIGNAL .It Dv PL_EVENT_SUSPENDED .El .Pp The .Fa data argument should contain .Dq Li sizeof(struct ptrace_lwpinfo) . .It Dv PT_SYSCALL Stops a process before and after executing each system call. Otherwise this operation is the same as .Dv PT_CONTINUE . .It Dv PT_SYSCALLEMU Intercept and ignore a system call before it has been executed, for use with .Dv PT_SYSCALL . This operation shall be called for syscall entry trap from .Dv PT_SYSCALL . To resume execution after intercepting the system call, another .Dv PT_SYSCALL shall be used. .It Dv PT_SET_EVENT_MASK 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 .Vt struct ptrace_event defined as: .Bd -literal -offset indent typedef struct ptrace_event { int pe_set_event; } ptrace_event_t; .Ed .Pp .Fa pe_set_event is the set of events to be reported. This set is formed by OR'ing together the following values: .Bl -tag -width 18n .It PTRACE_FORK Report .Xr fork 2 . .It PTRACE_VFORK Report .Xr vfork 2 . .It PTRACE_VFORK_DONE Report parent resumed after .Xr vfork 2 . .It PTRACE_LWP_CREATE Report thread birth. .It PTRACE_LWP_EXIT Report thread termination. .It PTRACE_POSIX_SPAWN Report .Xr posix_spawn 3 . .El .Pp The .Xr fork 2 and .Xr vfork 2 events can occur with .Xr clone 2 . The .Dv PTRACE_FORK value means that process gives birth to its child without pending on its termination or .Xr execve 2 operation. If enabled, the child is also traced by the debugger and .Dv SIGTRAP is generated twice, first for the parent and second for the child. The .Dv PTRACE_VFORK event is the same as .Dv PTRACE_FORK , but the parent blocks after giving birth to the child. The .Dv PTRACE_VFORK_DONE event can be used to report unblocking of the parent. .Pp .Fn posix_spawn on .Nx directly creates the child process without intermediate fork. The .Dv PTRACE_POSIX_SPAWN event semantics are the same as .Dv PTRACE_FORK , but the child is reported with implied execution of a file. .Pp A pointer to this structure is passed in .Fa addr . The .Fa data argument should be set to .Li sizeof(struct ptrace_event) . .It Dv PT_GET_EVENT_MASK This request can be used to determine which events in the traced process will be reported. The information is read into the .Vt struct ptrace_event pointed to by .Fa addr . The .Fa data argument should be set to .Li sizeof(struct ptrace_event) . .It Dv PT_GET_PROCESS_STATE This request reads the state information associated with the event that stopped the traced process. The information is reported in a .Vt struct ptrace_state defined as: .Bd -literal -offset indent typedef struct ptrace_state { int pe_report_event; pid_t pe_other_pid; } ptrace_state_t; .Ed .Pp A pointer to this structure is passed in .Fa addr . The .Fa data argument should be set to .Li sizeof(struct ptrace_state) . .It Dv PT_SET_SIGINFO This request can be used to specify signal information emitted to tracee. This signal information is specified in .Vt struct ptrace_siginfo defined as: .Bd -literal -offset indentq typedef struct ptrace_siginfo { siginfo_t psi_siginfo; lwpid_t psi_lwpid; } ptrace_siginfo_t; .Ed .Pp Where .Fa psi_siginfo is the set to signal information structure. The .Fa psi_lwpid field describes LWP address of the signal. Value .Dv 0 means the whole process (route signal to all LWPs). .Pp A pointer to this structure is passed in .Fa addr . The .Fa data argument should be set to .Li sizeof(struct ptrace_siginfo) . .Pp In order to pass faked signal to the tracee, the signal type must match the signal passed to the process with .Dv PT_CONTINUE or .Dv PT_SYSCALL . .It Dv PT_GET_SIGINFO This request can be used to determine signal information that was received by a debugger .Po see .Xr siginfo 2 .Pc . The information is read into the .Vt struct ptrace_siginfo pointed to by .Fa addr . The .Fa data argument should be set to .Li sizeof(struct ptrace_siginfo) . .It Dv PT_RESUME Allow execution of a specified thread, change its state from suspended to continued. The .Fa addr argument is unused. The .Fa data argument specifies the LWP ID. .Pp This call is equivalent to .Xr _lwp_continue 2 called by a traced process. This call does not change the general process state from stopped to continued. .It Dv PT_SUSPEND Prevent execution of a specified thread, change its state from continued to suspended. The .Fa addr argument is unused. The .Fa data argument specifies the requested LWP ID. .Pp This call is equivalent to .Xr _lwp_suspend 2 called by a traced process. This call does not change the general process state from continued to stopped. .El .Pp Additionally, the following requests exist but are not available on all machine architectures. The file .In machine/ptrace.h lists which requests exist on a given machine. .Bl -tag -width 12n .It Dv PT_STEP Execution continues as in request PT_CONTINUE; however as soon as possible after execution of at least one instruction, execution stops again. If the .Fa data argument is greater than 0, it contains the LWP ID of the thread to be stepped, and any other threads are continued. If the .Fa data argument is less than zero, it contains the negative of the LWP ID of the thread to be stepped, and only that thread executes. .It Dv PT_SETSTEP This request will turn on single stepping of the specified thread. .Fa addr is unused. .Fa data specifies the LWP ID of the thread to be stepped. The execution does not continue until .Dv PT_CONTINUE is issued. This request permits combining single-stepping with sending signals and .Dv PT_SYSCALL . .It Dv PT_CLEARSTEP This request will turn off single stepping of the specified thread. .Fa addr is unused. .Fa data specifies the LWP ID of the thread to disable single-stepping. .It Dv PT_GETREGS This request reads the traced process' machine registers into the .Vt struct reg (defined in .In machine/reg.h ) pointed to by .Fa addr . The .Fa data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read. .It Dv PT_SETREGS This request is the converse of .Dv PT_GETREGS ; it loads the traced process' machine registers from the .Vt struct reg (defined in .In machine/reg.h ) pointed to by .Fa addr . The .Fa data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written. .It Dv PT_GETFPREGS This request reads the traced process' floating-point registers into the .Vt struct fpreg (defined in .In machine/reg.h ) pointed to by .Fa addr . The .Fa data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read. .It Dv PT_SETFPREGS This request is the converse of .Dv PT_GETFPREGS ; it loads the traced process' floating-point registers from the .Vt struct fpreg (defined in .In machine/reg.h ) pointed to by .Fa addr . The .Fa data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written. .It Dv PT_GETDBREGS This request reads the traced process' debug registers into the .Vt struct dbreg (defined in .In machine/reg.h ) pointed to by .Fa addr . The .Fa data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read. .It Dv PT_SETDBREGS This request is the converse of .Dv PT_GETDBREGS ; it loads the traced process' debug registers from the .Vt struct dbreg (defined in .In machine/reg.h ) pointed to by .Fa addr . The .Fa data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written. .It Dv PT_GETXMMREGS This request reads the traced process' XMM registers into the .Vt struct xmmregs (defined in .In machine/reg.h ) pointed to by .Fa addr . The .Fa data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read. .It Dv PT_SETXMMREGS This request is the converse of .Dv PT_GETXMMREGS ; it loads the traced process' XMM registers from the .Vt struct xmmregs (defined in .In machine/reg.h ) pointed to by .Fa addr . The .Fa data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written. .It Dv PT_GETVECREGS This request reads the traced process' vector registers into the .Vt struct vreg (defined in .In machine/reg.h ) pointed to by .Fa addr . The .Fa data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read. .It Dv PT_SETVECREGS This request is the converse of .Dv PT_GETVECREGS ; it loads the traced process' vector registers from the .Vt struct vreg (defined in .In machine/reg.h ) pointed to by .Fa addr . The .Fa data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written. .It Dv PT_GETXSTATE This request reads the traced process' FPU extended state into the .Vt struct xstate (defined in .In machine/cpu_extended_state.h ) . .Fa addr should be a pointer to .Vt struct iovec (defined in .In sys/uio.h ) specifying the pointer to the aforementioned struct as .Fa iov_base and its size as .Fa iov_len . The .Fa data argument contains the LWP ID of the thread whose registers are to be read. If zero is supplied, the first thread of the process is read. The struct will be filled up to the specified .Fa iov_len . The caller needs to check the .Fa xs_rfbm bitmap in order to determine which fields were provided by the CPU, and may check .Fa xs_xstate_bv to determine which component states were changed from the initial state. .It Dv PT_SETXSTATE This request is the converse of .Dv PT_GETXSTATE ; it loads the traced process' extended FPU state from the .Vt struct xstate (defined in .In machine/cpu_extended_state.h ) . .Fa addr should be a pointer to .Vt struct iovec (defined in .In sys/uio.h ) specifying the pointer to the aforementioned struct as .Fa iov_base and its size as .Fa iov_len . The .Fa data argument contains the LWP ID of the thread whose registers are to be written. If zero is supplied, the first thread of the process is written. The .Fa xs_rfbm field of the supplied xstate specifies which state components are to be updated. Other components (fields) will be ignored. The .Fa xs_xstate_bv field specifies whether component state should be set to provided values (when 1) or reset to unitialized (when 0). The request will fail if .Fa xs_xstate_bv is not a subset of .Fa xs_rfbm , or any of the specified components is not supported by the CPU or kernel (i.e., not returned by .Dv PT_GETXSTATE ) . .El .Sh ERRORS Some requests can cause .Fn ptrace to return \-1 as a non-error value; to disambiguate, .Va errno can be set to 0 before the call and checked afterwards. The possible errors are: .Bl -tag -width "[EINVAL]" .It Bq Er EAGAIN Process is currently exec'ing and cannot be traced. .It Bq Er EBUSY .Bl -bullet -compact .It .Dv PT_ATTACH was attempted on a process that was already being traced. .It A request attempted to manipulate a process that was being traced by some process other than the one making the request. .It A request (other than .Dv PT_ATTACH ) specified a process that wasn't stopped. .El .It Bq Er EDEADLK An attempt to unstop a process with locked threads. .It Bq Er EINVAL .Bl -bullet -compact .It A process attempted to use .Dv PT_ATTACH on itself. .It The .Fa request was not a legal request on this machine architecture. .It The signal number (in .Fa data ) to .Dv PT_CONTINUE was neither 0 nor a legal signal number. .It .Dv PT_GETREGS , .Dv PT_SETREGS , .Dv PT_GETFPREGS , .Dv PT_SETFPREGS , .Dv PT_GETXSTATE , or .Dv PT_SETXSTATE was attempted on a process with no valid register set. (This is normally true only of system processes.) .It A process attempted to set Program Counter to 0 in .Dv PT_CONTINUE , .Dv PT_SYSCALL or .Dv PT_DETACH with .Dv vm.user_va0_disable set to 1. .It .Dv PT_SETXSTATE attempted to set state components not supported by the kernel, or .Dv xs_xstate_bv was not a subset of .Dv xs_rfbm . .El .It Bq Er EPERM .Bl -bullet -compact .It A request (other than .Dv PT_ATTACH ) attempted to manipulate a process that wasn't being traced at all. .It An attempt was made to use .Dv PT_ATTACH on a process in violation of the requirements listed under .Dv PT_ATTACH above. .El .It Bq Er ESRCH No process having the specified process ID exists. .El .Sh SEE ALSO .Xr sigaction 2 , .Xr signal 7 .Sh HISTORY The .Fn ptrace function appeared in .At v7 . .Sh BUGS On the SPARC, the PC is set to the provided PC value for .Dv PT_CONTINUE and similar calls, but the NPC is set willy-nilly to 4 greater than the PC value. Using .Dv PT_GETREGS and .Dv PT_SETREGS to modify the PC, passing .Li (void *)1 to .Eo \& .Fn ptrace .Ec , should be able to sidestep this. .Pp .Dv PT_SET_SIGINFO , .Dv PT_RESUME and .Dv PT_SUSPEND can change the image of process returned by .Dv PT_LWPINFO .