[Bug 217001] New: Undocumented behavior of ptrace(2) when a seized process gets SIGCONT

[bugzilla-daemon@kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=217001

            Bug ID: 217001
           Summary: Undocumented behavior of ptrace(2) when a seized
                    process gets SIGCONT
           Product: Documentation
           Version: unspecified
          Hardware: All
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: man-pages
          Assignee: documentation_man-pages@kernel-bugs.osdl.org
          Reporter: takahashi.shuhei@gmail.com
        Regression: No

When we use ptrace(PTRACE_SEIZE) to attach to a process, sending SIGCONT to the
tracee causes the tracer to get notified with PTRACE_EVENT_STOP with SIGTRAP,
but this behavior is not documented in the ptrace(2) man page.

The man page says:

> PTRACE_EVENT_STOP
> Stop induced by PTRACE_INTERRUPT command, or group-stop,
> or initial ptrace-stop when a new child is attached (only
> if attached using PTRACE_SEIZE).

Here's code to reproduce the behavior (in Rust):

https://github.com/nix-rust/nix/pull/1996/files#diff-65efa0db9bf3f9cbccfee8c987cb6c98ecf77576e0b46a87cab7ec4cfcbff172R324

I saw this behavior is explained in the following stackoverflow entry.
https://stackoverflow.com/a/49468347

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