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1902283053
considered running, so WaitStop doesn't lock up and breakpoints get installed and uninstalled. We also don't try to detach from them, since that will fail. R=rsc APPROVED=rsc DELTA=35 (26 added, 2 deleted, 7 changed) OCL=31683 CL=31731
133 lines
6.2 KiB
Plaintext
133 lines
6.2 KiB
Plaintext
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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ptrace and NTPL, the missing manpage
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== Signals ==
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A signal sent to a ptrace'd process or thread causes only the thread
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that receives it to stop and report to the attached process.
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Use tgkill to target a signal (for example, SIGSTOP) at a particular
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thread. If you use kill, the signal could be delivered to another
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thread in the same process.
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Note that SIGSTOP differs from its usual behavior when a process is
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being traced. Usually, a SIGSTOP sent to any thread in a thread group
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will stop all threads in the thread group. When a thread is traced,
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however, a SIGSTOP affects only the receiving thread (and any other
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threads in the thread group that are not traced).
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SIGKILL behaves like it does for non-traced processes. It affects all
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threads in the process and terminates them without the WSTOPSIG event
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generated by other signals. However, if PTRACE_O_TRACEEXIT is set,
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the attached process will still receive PTRACE_EVENT_EXIT events
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before receiving WIFSIGNALED events.
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See "Following thread death" for a caveat regarding signal delivery to
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zombie threads.
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== Waiting on threads ==
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Cloned threads in ptrace'd processes are treated similarly to cloned
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threads in your own process. Thus, you must use the __WALL option in
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order to receive notifications from threads created by the child
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process. Similarly, the __WCLONE option will wait only on
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notifications from threads created by the child process and *not* on
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notifications from the initial child thread.
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Even when waiting on a specific thread's PID using waitpid or similar,
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__WALL or __WCLONE is necessary or waitpid will return ECHILD.
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== Attaching to existing threads ==
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libthread_db (which gdb uses), attaches to existing threads by pulling
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the pthread data structures out of the traced process. The much
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easier way is to traverse the /proc/PID/task directory, though it's
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unclear how the semantics of these two approaches differ.
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Unfortunately, if the main thread has exited (but the overall process
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has not), it sticks around as a zombie process. This zombie will
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appear in the /proc/PID/task directory, but trying to attach to it
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will yield EPERM. In this case, the third field of the
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/proc/PID/task/PID/stat file will be "Z". Attempting to open the stat
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file is also a convenient way to detect races between listing the task
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directory and the thread exiting. Coincidentally, gdb will simply
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fail to attach to a process whose main thread is a zombie.
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Because new threads may be created while the debugger is in the
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process of attaching to existing threads, the debugger must repeatedly
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re-list the task directory until it has attached to (and thus stopped)
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every thread listed.
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In order to follow new threads created by existing threads,
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PTRACE_O_TRACECLONE must be set on each thread attached to.
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== Following new threads ==
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With the child process stopped, use PTRACE_SETOPTIONS to set the
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PTRACE_O_TRACECLONE option. This option is per-thread, and thus must
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be set on each existing thread individually. When an existing thread
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with PTRACE_O_TRACECLONE set spawns a new thread, the existing thread
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will stop with (SIGTRAP | PTRACE_EVENT_CLONE << 8) and the PID of the
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new thread can be retrieved with PTRACE_GETEVENTMSG on the creating
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thread. At this time, the new thread will exist, but will initially
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be stopped with a SIGSTOP. The new thread will automatically be
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traced and will inherit the PTRACE_O_TRACECLONE option from its
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parent. The attached process should wait on the new thread to receive
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the SIGSTOP notification.
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When using waitpid(-1, ...), don't rely on the parent thread reporting
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a SIGTRAP before receiving the SIGSTOP from the new child thread.
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Without PTRACE_O_TRACECLONE, newly cloned threads will not be
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ptrace'd. As a result, signals received by new threads will be
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handled in the usual way, which may affect the parent and in turn
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appear to the attached process, but attributed to the parent (possibly
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in unexpected ways).
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== Following thread death ==
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If any thread with the PTRACE_O_TRACEEXIT option set exits (either by
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returning or pthread_exit'ing), the tracing process will receive an
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immediate PTRACE_EVENT_EXIT. At this point, the thread will still
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exist. The exit status, encoded as for wait, can be queried using
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PTRACE_GETEVENTMSG on the exiting thread's PID. The thread should be
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continued so it can actually exit, after which its wait behavior is
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the same as for a thread without the PTRACE_O_TRACEEXIT option.
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If a non-main thread exits (either by returning or pthread_exit'ing),
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its corresponding process will also exit, producing a WIFEXITED event
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(after the process is continued from a possible PTRACE_EVENT_EXIT
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event). It is *not* necessary for another thread to ptrace_join for
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this to happen.
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If the main thread exits by returning, then all threads will exit,
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first generating a PTRACE_EVENT_EXIT event for each thread if
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appropriate, then producing a WIFEXITED event for each thread.
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If the main thread exits using pthread_exit, then it enters a
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non-waitable zombie state. It will still produce an immediate
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PTRACE_O_TRACEEXIT event, but the WIFEXITED event will be delayed
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until the entire process exits. This state exists so that shells
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don't think the process is done until all of the threads have exited.
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Unfortunately, signals cannot be delivered to non-waitable zombies.
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Most notably, SIGSTOP cannot be delivered; as a result, when you
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broadcast SIGSTOP to all of the threads, you must not wait for
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non-waitable zombies to stop. Furthermore, any ptrace command on a
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non-waitable zombie, including PTRACE_DETACH, will return ESRCH.
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== Multi-threaded debuggers ==
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If the debugger itself is multi-threaded, ptrace calls must come from
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the same thread that originally attached to the remote thread. The
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kernel simply compares the PID of the caller of ptrace against the
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tracer PID of the process passed to ptrace. Because each debugger
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thread has a different PID, calling ptrace from a different thread
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might as well be calling it from a different process and the kernel
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will return ESRCH.
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wait, on the other hand, does not have this restriction. Any debugger
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thread can wait on any thread in the attached process.
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