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.Dd November 7, 1994 .Dt PTRACE 2 .Os NetBSD 1.0BETA .Sh NAME .Nm ptrace .Nd process tracing and debugging .Sh SYNOPSIS .Fd #include <sys/types.h> .Fd #include <sys/ptrace.h> .Ft int .Fn ptrace "int request" "pid_t pid" "caddr_t 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
o see .Xr sigaction 2
c , it stops. The tracing process is expected to notice this via .Xr wait 2 or the delivery of a .Dv SIGCHLD signal, 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.
p 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. .Fa request can be: l -tag -width 12n t 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 .Eo \& .Fn ptrace .Ec .) When a process has used this request and calls .Xr execve 2 or any of the routines built on it
o such as .Xr execv 3
c , 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. t Dv PT_READ_I , Dv PT_READ_D These requests read a single .Li 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 NetBSD 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 . t 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. t Dv PT_READ_U This request reads an .Li int from the traced process' user structure. The .Fa addr argument specifies the location of the int relative to the base of the user structure; it will usually be an integer value cast to .Li caddr_t either explicitly or via the presence of a prototype for .Eo \& .Fn ptrace .Ec . Unlike .Dv PT_READ_I and .Dv PT_READ_D , .Fa addr must be aligned on an .Li int boundary. The value read is returned as the return value from .Eo \& .Fn ptrace .Ec . t Dv PT_WRITE_U This request writes an .Li int into the traced process' user structure. .Fa addr specifies the offset, just as for .Dv PT_READ_U , and .Fa data specifies the value to be written, just as for .Dv PT_WRITE_I and .Dv PT_WRITE_D . t 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 (caddr_t)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. t Dv PT_KILL The traced process terminates, as if .Dv PT_CONTINUE had been used with .Dv SIGKILL given as the signal to be delivered. t 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.) The tracing process will see the newly-traced process stop and may then control it as if it had been traced all along. t Dv 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. .El
p Additionally, machine-specific requests can exist. On the SPARC, these are: l -tag -width 12n t Dv PT_GETREGS This request reads the traced process' machine registers into the .Dq Li "struct reg" (defined in .Aq Pa machine/reg.h ) pointed to by .Fa addr . t Dv PT_SETREGS This request is the converse of .Dv PT_GETREGS ; it loads the traced process' machine registers from the .Dq Li "struct reg" (defined in .Aq Pa machine/reg.h ) pointed to by .Fa addr . t Dv PT_GETFPREGS This request reads the traced process' floating-point registers into the .Dq Li "struct fpreg" (defined in .Aq Pa machine/reg.h ) pointed to by .Fa addr . t Dv PT_SETFPREGS This request is the converse of .Dv PT_GETFPREGS ; it loads the traced process' floating-point registers from the .Dq Li "struct fpreg" (defined in .Aq Pa machine/reg.h ) pointed to by .Fa addr . t Dv PT_SYSCALL This request is like .Dv PT_CONTINUE except that the process will stop next time it executes any system call. Information about the system call can be examined with .Dv PT_READ_U and potentially modified with .Dv PT_WRITE_U through the .Li u_kproc.kp_proc.p_md element of the user structure (see below). If the process is continued with another .Dv PT_SYSCALL request, it will stop again on exit from the syscall, at which point the return values can be examined and potentially changed. The .Li u_kproc.kp_proc.p_md element is of type .Dq Li "struct mdproc" , which should be declared by including .Aq Pa sys/param.h , .Aq Pa sys/user.h , and .Aq Pa machine/proc.h , and contains the following fields (among others): l -item -compact -offset indent t .Li syscall_num t .Li syscall_nargs t .Li syscall_args[8] t .Li syscall_err t .Li syscall_rv[2] .El When a process stops on entry to a syscall, .Li syscall_num holds the number of the syscall, .Li syscall_nargs holds the number of arguments it expects, and .Li syscall_args holds the arguments themselves. (Only the first .Li syscall_nargs elements of .Li syscall_args are guaranteed to be useful.) When a process stops on exit from a syscall, .Li syscall_num is .Eo \& .Li -1 .Ec , .Li syscall_err holds the error number
o see .Xr errno 2
c , or 0 if no error occurred, and .Li syscall_rv holds the return values. (If the syscall returns only one value, only .Li syscall_rv[0] is useful.) The tracing process can modify any of these with .Dv PT_WRITE_U ; only some modifications are useful.
p On entry to a syscall, .Li syscall_num can be changed, and the syscall actually performed will correspond to the new number (it is the responsibility of the tracing process to fill in .Li syscall_args appropriately for the new call, but there is no need to modify .Eo \& .Li syscall_nargs .Ec ). If the new syscall number is 0, no syscall is actually performed; instead, .Li syscall_err and .Li syscall_rv are passed back to the traced process directly (and therefore should be filled in). If the syscall number is otherwise out of range, a dummy syscall which simply produces an .Er ENOSYS error is effectively performed.
p On exit from a syscall, only .Li syscall_err and .Li syscall_rv can usefully be changed; they are set to the values returned by the syscall and will be passed back to the traced process by the normal syscall return mechanism. .El .Sh ERRORS Some requests can cause .Fn ptrace to return .Li -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: l -tag -width 4n t Bq Er ESRCH No process having the specified process ID exists. t Bq Er EINVAL l -bullet -compact t A process attempted to use .Dv PT_ATTACH on itself. t The .Fa request was not one of the legal requests. t The .Fa addr to .Dv PT_READ_U or .Dv PT_WRITE_U was not .Li int Ns -aligned. t The signal number (in .Fa data ) to .Dv PT_CONTINUE or .Dv PT_SYSCALL was neither 0 nor a legal signal number. t .Dv PT_GETREGS , .Dv PT_SETREGS , .Dv PT_GETFPREGS , or .Dv PT_SETFPREGS was attempted on a process with no valid register set. (This is normally true only of system processes.) .El t Bq Er EBUSY l -bullet -compact t .Dv PT_ATTACH was attempted on a process that was already being traced. t A request attempted to manipulate a process that was being traced by some process other than the one making the request. t A request (other than .Dv PT_ATTACH ) specified a process that wasn't stopped. .El t Bq Er EPERM l -bullet -compact t A request (other than .Dv PT_ATTACH ) attempted to manipulate a process that wasn't being traced at all. t An attempt was made to use .Dv PT_ATTACH on a process in violation of the requirements listed under .Dv PT_ATTACH above. .El .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 (caddr_t)1 to .Eo \& .Fn ptrace .Ec , should be able to sidestep this.
p Single-stepping is not available.
p When using .Dv PT_SYSCALL , there is no easy way to tell whether the traced process stopped because it made a syscall or because a signal was sent at a moment that it just happened to have valid-looking garbage in its .Dq Li "struct mdproc" .