|From:||Denys Vlasenko <email@example.com>|
|To:||Tejun Heo <firstname.lastname@example.org>, email@example.com, firstname.lastname@example.org|
|Subject:||Ptrace documentation, draft #1|
|Date:||Sun, 15 May 2011 22:35:32 +0200|
|Cc:||email@example.com, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org|
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 ptrace-stop. 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 ptraced. 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 below. 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. ??? ...at 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 SIGTRAP. 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 SINGLESTEP, SYSEMU, SYSEMU_SINGLESTEP ??? document PTRACE_SINGLESTEP, PTRACE_SYSEMU, PTRACE_SYSEMU_SINGLESTEP 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. longv = ptrace(PTRACE_PEEKTEXT/PTRACE_PEEKDATA/PTRACE_PEEKUSER, pid, addr, 0); ptrace(PTRACE_POKETEXT/PTRACE_POKEDATA/PTRACE_POKEUSER, pid, 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); where cmd is CONT, DETACH, SYSCALL, SINGLESTEP, SYSEMU, 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 exist. 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 TODO -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to email@example.com More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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