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    SubjectPtrace documentation, draft #1
    Hi Tejun,

    On Monday 09 May 2011 12:09, Tejun Heo wrote:
    > Hmmm... I was thinking about writing a proper ptrace API doc with
    > example programs under Documentation/. It's userland visible API so
    > it shouldn't change all that much and the amount of necessary
    > documentation would be too much for comments.

    Since I am not helping you much in kernel ptrace hacking,
    I can at least help you with the docs.

    Ptrace discussions repeatedly display a higher than average amount
    of misunderstanding and confusion. New ptrace users and even people
    who already worked with it are repeatedly confused by details
    which are not documented anywhere and knowledge about which exists
    mostly in the brains of strace/gdb/other_such_tools developers.

    This document is meant as a brain dump of this knowledge.
    It assumes that the reader has basic understanding what ptrace is.

    Tejun, Oleg, Jan, please comment. If some paragraphs are unclear,
    post your version of the whole paragraph(s).

    I am especially interested in adding info on ptrace quirks which
    I forgot or don't know yet - Jan, you must know quite a bit
    of those from your gdb work.

    Tejun, Oleg - "2. Linux kernel implementation" section is for you
    to fill with a brain dump of kernel side of the story.


    Ptrace API (ab)uses standard Unix parent/child signaling over waitpid.
    An unfortunate effect of it is that resulting API is complex and has
    subtle quirks. This document aims to describe these quirks.

    It is split into two parts. First part focuses exclusively on
    userspace-visible API and behavior. Second section describes kernel
    internals of ptrace.

    1. Userspace API.

    (Note to editors: in this section, do not use kernel concepts and terms
    which are not observable through userspace API and user-visible
    behavior. Use section 2 for that.)

    Debugged processes (tracees) first need to be attached to the debugging
    process (tracer). Attachment and subsequent commands are per-thread: in
    multi-threaded process, every thread can be individually attached to a
    (potentially different) tracer, or left not attached and thus not
    debugged. Therefore, "tracee" always means "(one) thread", never "a
    (possibly multi-threaded) process". Ptrace commands are always sent to
    a specific tracee using ptrace(PTRACE_foo, pid, ...), where pid is a
    TID of the corresponding Linux thread.

    After attachment, each tracee can be in two states: running or stopped.

    There are many kinds of states when tracee is stopped, and in ptrace
    discussions they are often conflated. Therefore, it is important to use
    precise terms.

    In this document, any stopped state in which tracee is ready to accept
    ptrace commands from the tracer is called ptrace-stop. Ptrace-stops can
    be further subdivided into signal-delivery-stop, group-stop,
    syscall-stop and so on. They are described in detail later.

    1.x Death under ptrace.

    When a (possibly multi-threaded) process receives a killing signal (a
    signal set to SIG_DFL and whose default action is to kill the process),
    all threads exit. Tracees report their death to the tracer(s). This is
    not a ptrace-stop (because tracer can't query tracee status such as
    register contents, cannot restart tracee etc) but the notification
    about this event is delivered through waitpid API similarly to

    Note that killing signal will first cause signal-delivery-stop (on one
    tracee only), and only after it is injected by tracer (or after it was
    dispatched to a thread which isn't traced), death from signal will
    happen on ALL tracees within multi-threaded process.

    SIGKILL operates similarly, with exceptions. No signal-delivery-stop is
    generated for SIGKILL and therefore tracer can't suppress it. SIGKILL
    kills even within syscalls (syscall-exit-stop is not generated prior to
    death by SIGKILL). The net effect is that SIGKILL always kills the
    process (all its threads), even if some threads of the process are

    Tracer can kill a tracee with ptrace(PTRACE_KILL, pid, 0, 0).

    ??? Does it kill only the tracee or the whole process? What is exit
    status? Will tracer see the death? Other tracers?

    When tracee executes exit syscall, it reports its death to its tracer.
    Other threads are not affected.

    When any thread executes exit_group syscall, every tracee reports its
    death to its tracer.

    ??? Is it true that *every* thread reports death?
    IIRC it isn't... one does (leader? what if there is no leader?)
    and the rest simply disappear.

    Tracer cannot assume that ptrace-stopped tracee exists. There are many
    scenarios when tracee may die while stopped (such as SIGKILL).
    Therefore, tracer must always be prepared to handle ESRCH error on any
    ptrace operation. Unfortunately, the same error is returned if tracee
    exists but is not ptrace-stopped (for commands which require stopped
    tracee). Tracer needs to keep track of stopped/running state, and
    interpret ESRCH as "tracee died unexpectedly" only if it knows that
    tracee has been observed to enter ptrace-stop.

    ??? is there a recommended usage of waitpid(WNOHANG) to check whether
    tracee is dead or alive?

    1.x Stopped states.

    When running tracee enters ptrace-stop, it notifies its tracer using
    waitpid API. Tracer should use waitpid family of syscalls to wait for
    tracee to stop. Most of this document assumes that tracer waits with:

    pid = waitpid(pid_or_minus_1, &status, __WALL);

    Ptrace-stopped tracees are reported as returns with pid > 0 and
    WIFSTOPPED(status) == true.

    ??? any pitfalls with WNOHANG (I remember that there are bugs in this
    area)? effects of WSTOPPED, WEXITED, WCONTINUED bits? Are they ok?
    waitid usage? WNOWAIT?

    1.x.x Signal-delivery-stop

    When (possibly multi-threaded) process receives any signal except
    SIGKILL, kernel selects a thread which handles the signal (if signal is
    generated with tgkill, thread selection is done by user). If selected
    thread is traced, it enters signal-delivery-stop. By this point, signal
    is not yet delivered to the process, and can be suppressed by tracer.
    If tracer doesn't suppress the signal, it passes signal to tracee in
    the next ptrace request. This is called "signal injection" and will be
    described later. Note that if signal is blocked, signal-delivery-stop
    doesn't happen until signal is unblocked, with the usual exception that
    SIGSTOP can't be blocked.

    Signal-delivery-stop is observed by tracer as waitpid returning with
    WIFSTOPPED(status) == true, WSTOPSIG(status) == signal. If
    WSTOPSIG(status) == SIGTRAP, this may be a different kind of
    ptrace-stop - see syscall-stop section below for details. If
    WSTOPSIG(status) == stopping signal, this may be a group-stop - see

    Kernel delivers an extra SIGTRAP to tracee after execve syscall
    returns. This is an ordinary signal (similar to one generated by kill
    -TRAP), not a special kind of ptrace-stop. If PTRACE_O_TRACEEXEC option
    is in effect, a PTRACE_EVENT_EXEC-stop is generated instead.

    ??? can this SIGTRAP be distinguished from "real" user-generated SIGTRAP
    by looking at its siginfo?

    Usually, tracer (for example, strace) would not want to show this extra
    post-execve SIGTRAP signal to the user, and would suppress its delivery
    to the tracee (if SIGTRAP is set to SIG_DFL, it is a killing signal).

    1.x.x Signal injection and suppression.

    After signal-delivery-stop is observed by tracer, PTRACE_GETSIGINFO can
    be used to retrieve corresponding siginfo_t structure.
    PTRACE_SETSIGINFO may be used to modify it.

    Tracer should restart tracee with
    ptrace(PTRACE_rest, pid, 0, sig)
    call, where PTRACE_rest is one of the restarting ptrace ops. If sig is
    0, then signal is not delivered and has no effect. Otherwise, signal
    sig is delivered. This operation is called "signal injection", to
    distinguish it from signal delivery which causes signal-delivery-stop.

    Note that sig value may be different from WSTOPSIG(status) value -
    tracer can cause a different signal to be injected. If before
    restarting PTRACE_SETSIGINFO command was used to alter injected
    signal's siginfo_t, si_signo field and sig parameter in restarting
    command must match.

    ??? Are syscalls interrupted by signals which are suppressed by tracer?
    If yes, document it here

    Note that restarting ptrace commands issued in ptrace-stops other than
    signal-delivery-stop do NOT inject a signal, even if sig is nonzero. No
    error is reported either. This is a cause of confusion among ptrace
    users. One typical scenario is that tracer observes group-stop,
    mistakes it for signal-delivery-stop, restarts tracee with
    ptrace(PTRACE_rest, pid, 0, stopsig) with the intention of injecting
    stopsig, but stopsig gets ignored and tracee continues to run.

    ??? TODO: there *are* some other ptrace-stops which can take sig.
    Document them.

    SIGCONT signal has a side effect of waking up (all threads of)
    group-stopped process. This side effect happens before
    signal-delivery-stop. Tracer can't suppress this side-effect (it can
    only suppress signal injection, which only causes SIGCONT handler to
    not be executed in the tracee, if such handler is installed). In fact,
    waking up from group-stop may be followed by signal-delivery-stop for
    signal(s) *other than* SIGCONT, if they were pending when SIGCONT was
    delivered. IOW: SIGCONT may be not the first signal observed by the
    tracee after it was sent.

    Stopping signals cause (all threads of) process to enter group-stop.
    This side effect happens after signal injection, and therefore can be
    suppressed by tracer.

    1.x.x Group-stop

    When a (possibly multi-threaded) process receives a stopping signal,
    all threads stop. If some threads are traced, they enter a group-stop.
    Note that stopping signal will first cause signal-delivery-stop (on one
    tracee only), and only after it is injected by tracer (or after it was
    dispatched to a thread which isn't thraced), group-stop will be
    initiated on ALL tracees within multi-threaded process. As usual, every
    tracee reports its group-stop to corresponding tracer.

    Group-stop is observed by tracer as waitpid returning with the
    following result: WIFSTOPPED(status) is true, WSTOPSIG(status) is the
    delivered signal. The same result is returned by some other classes of
    ptrace-stops, therefore the recommended practice is to perform
    ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo)
    call. The call can be avoided if signal number is not SIGSTOP, SIGTSTP,
    SIGTTIN or SIGTTOU - only these four signals are stopping signals. If
    tracer sees something else, it can't be group-stop. Otherwise, tracer
    needs to call PTRACE_GETSIGINFO. If PTRACE_GETSIGINFO fails, then it is
    a group-stop. If it succeeds, it's a signal-delivery-stop.

    As of kernel 2.6.38, after tracer sees tracee ptrace-stop and until it
    restarts or kills it, tracee will not run, and will not send
    notifications (except SIGKILL death) to tracer, even if tracer enters
    into another waitpid call.

    Currently, it causes problem with transparent handling of stopping
    signals: if tracer restarts tracee after group-stop, SIGSTOP is
    effectively ignored: tracee doesn't remain stopped, it runs. If tracer
    doesn't restart tracee before entering into next waitpid, future
    SIGCONT will not be reported to the tracer. Which would make SIGCONT to
    have no effect.

    ??? least no effect on this tracee - how will other threads
    be affected?

    1.x.x Syscall-stops

    If tracee was restarted by PTRACE_SYSCALL, tracee enters
    syscall-enter-stop just prior to entering any syscall. If tracer
    restarts it with PTRACE_SYSCALL, tracee enters syscall-exit-stop when
    syscall is finished, or if it is interrupted by a signal. (That is,
    signal-delivery-stop never happens between syscall-enter-stop and
    syscall-exit-stop, it happens after syscall-exit-stop). Other
    possibilities are that tracee may exit (if it entered exit or
    exit_group syscall), be killed by SIGKILL, or die when other threads'
    actions terminate all threads of the process (such as execve syscall).

    ??? how such death-because-of-other-thread is reported?

    Syscall-enter-stop and syscall-exit-stop are observed by tracer as
    waitpid returning with the following result: WIFSTOPPED(status) is
    true, WSTOPSIG(status) == SIGTRAP. If PTRACE_O_TRACESYSGOOD option was
    set by tracer, then WSTOPSIG(status) == (SIGTRAP | 0x80). Otherwise, it
    is impossible to distinguish them from signal-delivery-stop with

    Syscall-enter-stop and syscall-exit-stop are indistinguishable by
    tracer. Tracer needs to keep track of the sequence of ptrace-stops in
    order to not misinterpret syscall-enter-stop as syscall-exit-stop or
    vice versa. The rule is that syscall-enter-stop is always followed by
    syscall-exit-stop or tracee's death - no other kinds of ptrace-stop can
    occur in between.

    ??? What will happen if trace uses *NOT* PTRACE_SYSCALL to restart
    tracee after syscall-enter-stop?

    ??? what PTRACE_GETSIGINFO returns on syscall stops?



    1.x.x PTRACE_EVENT-stops

    If tracer sets TRACE_O_TRACEfoo options, tracee will enter ptrace-stops
    called PTRACE_EVENT-stops.

    PTRACE_EVENT-stops are observed by tracer as waitpid returning with the
    following result: WIFSTOPPED(status) is true, WSTOPSIG(status) ==
    SIGTRAP. Additional bit is set in a higher byte of status word: value
    ((status >> 8) & 0xffff) will be (SIGTRAP | PTRACE_EVENT_foo << 8).

    PTRACE_EVENT_VFORK - stop after return from vfork/clone+CLONE_VFORK ???
    continuing it will make it wait for child to exit/exec, right?
    PTRACE_EVENT_FORK - stop after return from fork/clone+SIGCHLD
    PTRACE_EVENT_CLONE - stop after return from clone
    PTRACE_EVENT_VFORK_DONE - stop after vfork child unblocks this tracee
    For all four: stop occurs in parent, not in new thread;
    PTRACE_GETEVENTMSG can be used to retrieve new thread's tid.
    PTRACE_EVENT_EXEC - stop after return from exec PTRACE_EVENT_EXIT -
    stop before exit
    PTRACE_GETEVENTMSG returns exit status.
    Registers can be examined (unlike when "real" exit happens).
    ??? needs to be PTRACE_CONTed to finish exit, or not?

    ??? what PTRACE_GETSIGINFO returns on PTRACE_EVENT-stops?

    1.x Informational and restarting ptrace commands.

    Most ptrace commands (all except ATTACH, TRACEME, KILL, SETOPTIONS)
    require tracee to be in ptrace-stop, otherwise they fail with ESRCH.

    When tracee is in ptrace-stop, tracer can read and write data to tracee
    using informational commands. They leave tracee in ptrace-stopped state.

    addr, 0);
    addr, long_val);
    ptrace(PTRACE_GETREGS/PTRACE_GETFPREGS, pid, 0, &struct);
    ptrace(PTRACE_SETREGS/PTRACE_SETFPREGS, pid, 0, &struct);
    ptrace(PTRACE_GETSIGINFO, pid, 0, &siginfo);
    ptrace(PTRACE_SETSIGINFO, pid, 0, &siginfo);
    ptrace(PTRACE_GETEVENTMSG, pid, 0, &long_var);

    Note that some errors are not reported. For example, setting siginfo
    may have no effect in some ptrace-stops, yet the call will succeed
    (return 0 and not set errno).

    Another group of ptrace commands makes ptrace-stopped tracee run. They
    all have the form:
    ptrace(PTRACE_cmd, pid, 0, sig);
    SYSEMU_SINGLESTEP. If tracee is in signal-delivery-stop, sig is the
    signal to be injected. Otherwise, sig is ignored.

    1.x Attaching and detaching

    A thread can be attached to using ptrace(PTRACE_ATTACH, pid, 0, 0)
    call. This also sends SIGSTOP to this thread. If tracer wants this
    SIGSTOP to have no effect, it needs to suppress it. Note that if other
    signals are concurrently sent to this thread during attach, tracer may
    see tracee enter signal-delivery-stop with other signal(s) first! The
    usual practice is to reinject these signals until SIGSTOP is seen, then
    suppress SIGSTOP injection. The design bug here is that attach and
    concurrent SIGSTOP race and SIGSTOP may be lost.

    ??? Is there a bug/misfeature that attaching interrupts some syscalls,
    such as nanosleep? Document it. (I guess even suppressed SIGSTOP
    causes syscall to return prematurely).

    ptrace(PTRACE_TRACEME, 0, 0, 0) request turns current thread into a
    tracee. It continues to run (doesn't enter ptrace-stop). A common
    practice is follow ptrace(PTRACE_TRACEME) with raise(SIGSTOP) and allow
    parent (which is our tracer now) to observe our signal-delivery-stop.

    If PTRACE_O_TRACE[V]FORK or PTRACE_O_TRACECLONE options are in effect,
    then children created by (vfork or clone(CLONE_VFORK)), (fork or
    clone(SIGCHLD)) and (other kinds of clone) respectively are
    automatically attached to the same tracer which traced their parent.
    SIGSTOP is delivered to them, causing them to enter
    signal-delivery-stop as they exit syscall which created them.

    Detaching of tracee is performed by ptrace(PTRACE_DETACH, pid, 0, sig).
    PTRACE_DETACH is a restarting operation, therefore it requires tracee
    to be in ptrace-stop. If tracee is in signal-delivery-stop, signal can
    be injected. Othervice, sig parameter is ignored.

    If tracee is running when tracer wants to detach it, the usual solution
    is to send SIGSTOP (using tgkill, to mkae sure it goes to the correct
    thread), wait for tracee to stop in signal-delivery-stop for SIGSTOP
    and then detach it (suppressing SIGSTOP injection). Design bug is that
    this can race with concurrent SIGSTOPs. Another complication is that
    tracee may enter other ptrace-stops and needs to be restarted and
    waited for again, until SIGSTOP is seen. Yet another complication is to
    be sure that tracee is not already group-stopped, because no signal
    delivery happens while it is - not even SIGSTOP.

    ??? is above accurate?

    If tracer dies, all tracees are automatically detached.

    ??? are they restarted if they were in some ptrace-stop?
    Even those which were in group-stop?
    Is signal injected if they were in signal-delivery-stop?

    1.x Real parent

    Ptrace API (ab)uses standard Unix parent/child signaling over waitpid.
    This used to cause real parent of the process to stop receiving several
    kinds of waitpid notifications when child process is traced by some
    other process.

    Many of these bugs have been fixed, but as of 2.6.38 several still

    As of 2.6.38, the following is believed to work correctly:

    - exit/death by signal is reported both to tracer and to real parent.
    If they are the same process, the report is sent only once.

    - ??? add more docs

    Following bugs still exist:

    - group-stop notifications are sent to tracer, but not to real parent.

    - ??? add more known bugs here

    2. Linux kernel implementation


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      Last update: 2011-05-15 22:37    [W:0.046 / U:50.448 seconds]
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