From: Jon Turney <jon.turney@dronecode.org.uk>
Cc: cygwin-patches@cygwin.com
Subject: Re: [PATCH 1/5] Cygwin: Make 'ulimit -c' control writing a coredump
Date: Sat, 27 Jan 2024 15:12:57 +0000 [thread overview]
Message-ID: <d5fc9479-0882-417e-a047-752997eae0ad@dronecode.org.uk> (raw)
In-Reply-To: <ZbOc7C_3qSfjlT6x@calimero.vinschen.de>
On 26/01/2024 11:52, Corinna Vinschen wrote:
> On Jan 26 12:12, Corinna Vinschen wrote:
>> On Jan 25 20:03, Jon Turney wrote:
>>> On 25/01/2024 18:21, Corinna Vinschen wrote:
>>>> On Jan 25 14:50, Jon Turney wrote:
>>>>> On 24/01/2024 14:39, Corinna Vinschen wrote:
>>>>>> On Jan 24 13:28, Jon Turney wrote:
>>>>>>> On 23/01/2024 14:29, Corinna Vinschen wrote:
>>>>>>>> On Jan 23 14:20, Jon Turney wrote:
>>> [...]
>>>>> So this situation with a JIT debugger is even stranger than my emendations
>>>>> to the documentation say.
>>>>>
>>>>> Because we're hitting try_to_debug() in exception::handle(), which has some
>>>>> code to replay the exception via ExceptionContinueExecution (which I guess
>>>>> the debugger will catch as a first-chance) (and goes into a mode where it
>>>>> ignores the next half-million exceptions... no idea what that's about!)
>>>>>
>>>>> That's not the same as signal_exit() with a coredumping signal (haven't
>>>>> checked if those are all generated from exceptions, but seems probable, so
>>>>> the try_to_debug() there maybe isn't doing anything?), where we're going to
>>>>> exit thereafter.
>>>>
>>>> try_to_debug() is only calling IsDebuggerPresent() as test, and that's
>>>> nothing but a flag in the PEB which is set by the OS after a debugger
>>>> attached to the process. So the test is by definition extremely
>>>> flaky, if the debugger is connecting and disconnecting, as you
>>>> already pointed out.
>>>>
>>>> I'm wondering if we can't define our own way to attach to a process,
>>>> allowing to "WaitForDebugger" as long as the debugger is a Cygwin
>>>> debugger. If we define a matching function (along the lines of
>>>> prctl(2) on Linux), we could change our debuggers, core dumpers
>>>> and stracers to call this attach function.
>>>>
>>>> The idea would be to define some shared mutex object, the inferior
>>>> waits for and the debugger releases after having attached.
>>>>
>>>> Is there really any need to support non-Cygwin debuggers?
>>>
>>> idk
>>>
>>> I think something like that used to exist a long time ago, see commit
>>> 8abeff1ead5f3824c70111bc0ff74ff835dafa55
>>
>> Yeah, just, as was the default at the time, without any trace of a
>> *rational* why it has been removed. Also, it was too simple anyway.
>>
>> First, if we want to support WIndows debugger, the inferior has to check
>> if the debugger is a Cygwin or native debugger. If a native debugger,
>> just stick to what we have today. Otherwise:
>>
>> - Create a named mutex with a reproducible name (no need to use
>> the name as parameter) and immediately grab it.
>> - Call CreateProcess to start the debugger with CREATE_SUSPENDED
>> flag.
>> - Create a HANDLE array with the mutex and the process HANDLE.
>
> On second thought, it might be a good idea to make this
> interruptible as well, but given this is called from the
> exception handler this may have weird results...
>
>> - Call ResumeThread on the primary debugger thread.
>> - Call WFMO with timeout.
>>
>> Later on, the debugger either fails and exits or it calls
>> ReleaseMutex after having attached to the process.
>>
>> - WFMO returns
>> - If the mutex has triggered, we're being debugged (but check
>> IsDebuggerPresent() just to be sure)
>> - If the process has triggered, the debugger exited
>> - If the timeout triggers... oh well.
This seems like quite a lot of work, for very marginal benefit.
And doing lots of complex work inside the process when we're in the
middle of handling a SEGV seems like asking for trouble.
I think I'll leave this alone for the moment, and we can see what (if
any) problems surface.
next prev parent reply other threads:[~2024-01-27 15:12 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-01-12 14:09 [PATCH 0/5] Coredump under 'ulimit -c' control (v2) Jon Turney
2024-01-12 14:09 ` [PATCH 1/5] Cygwin: Make 'ulimit -c' control writing a coredump Jon Turney
2024-01-13 14:20 ` Jon Turney
2024-01-15 9:46 ` Corinna Vinschen
2024-01-15 13:27 ` Jon Turney
2024-01-15 14:28 ` Corinna Vinschen
2024-01-16 13:52 ` Jon Turney
2024-01-16 13:54 ` Corinna Vinschen
2024-01-23 14:20 ` Jon Turney
2024-01-23 14:29 ` Corinna Vinschen
2024-01-24 13:28 ` Jon Turney
2024-01-24 14:39 ` Corinna Vinschen
2024-01-25 14:50 ` Jon Turney
2024-01-25 18:21 ` Corinna Vinschen
2024-01-25 20:03 ` Jon Turney
2024-01-26 11:12 ` Corinna Vinschen
2024-01-26 11:52 ` Corinna Vinschen
2024-01-27 15:12 ` Jon Turney [this message]
2024-01-29 11:16 ` Corinna Vinschen
2024-01-12 14:09 ` [PATCH 2/5] Cygwin: Disable writing core dumps by default Jon Turney
2024-01-12 14:09 ` [PATCH 3/5] Cygwin: Define and use __WCOREFLAG Jon Turney
2024-01-12 14:09 ` [PATCH 4/5] Cygwin: Treat api_fatal() similarly to a core-dumping signal Jon Turney
2024-01-12 14:09 ` [PATCH 5/5] Cygwin: Update documentation for cygwin_stackdump Jon Turney
2024-01-12 18:44 ` Corinna Vinschen
2024-01-13 13:40 ` Jon Turney
2024-01-12 18:41 ` [PATCH 0/5] Coredump under 'ulimit -c' control (v2) Corinna Vinschen
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