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
The 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 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 ptrace() system call 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 or a corresponding thread ID. The 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
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_I, PT_READ_D
These requests read a single int of data from the traced
process’s 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
FreeBSD implementation, these two requests are completely
identical. The 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 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
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 addr
argument specifies a pointer to a 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 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 addr argument
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. The 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
addr argument should be passed (caddr_t)1. The 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, 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 “struct reg” (defined in pointed to by addr.
PT_SETREGS This request is the converse of PT_GETREGS; it loads the
traced process’s machine registers from the “struct reg”
(defined in pointed to by addr.
PT_GETFPREGS This request reads the traced process’s floating-point
registers into the “struct fpreg” (defined in pointed to by
addr.
PT_SETFPREGS This request is the converse of PT_GETFPREGS; it loads the
traced process’s floating-point registers from the “struct
fpreg” (defined in pointed to by addr.
PT_GETDBREGS This request reads the traced process’s debug registers
into the “struct dbreg” (defined in pointed to by addr.
PT_SETDBREGS This request is the converse of PT_GETDBREGS; it loads the
traced process’s debug registers from the “struct dbreg”
(defined in pointed to by 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 addr argument
specifies a pointer to a struct ptrace_lwpinfo, which is
defined as follows:
struct ptrace_lwpinfo {
lwpid_t pl_lwpid; /* LWP described. */
int pl_event; /* Event received. */
};
The 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.
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 lwpid_t should be passed in
addr, with the array size specified by data. The return
value from 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.
Additionally, machine-specific requests can exist.
RETURN VALUES
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
afterwards.
ERRORS
The ptrace() system call may fail if:
[ESRCH]
· No process having the specified process ID exists.
[EINVAL]
· A process attempted to use PT_ATTACH on itself.
· The request argument was not one of the legal
requests.
· The signal number (in 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.)
[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.
[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.
SEE ALSO
execve(2), sigaction(2), wait(2), execv(3), i386_clr_watch(3),
i386_set_watch(3)
HISTORY
The ptrace() function appeared in Version 7 AT&T UNIX.