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Message-ID: <15add7ba-0a22-4dc7-ab78-8d2759f05c64@dronecode.org.uk>
Date: Fri, 16 May 2025 22:37:59 +0100
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Subject: Re: strace: infinite exception c0000005 loop on segmentation fault
To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>,
        The Cygwin Mailing List <cygwin AT cygwin DOT com>
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From: Jon Turney via Cygwin <cygwin AT cygwin DOT com>
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On 16/05/2025 16:43, Takashi Yano wrote:
> Hi Jon,
[...]
>>
>> I could reproduce that. And also found cygwin 3.4.10 does not have this
>> issue. I'll look into this.
> 
> I could bisect the issue and found the next commit triggered the issue.
> 
> commit 91457377d6c9f89a08b1b70e45cbae87ef467119
> Author: Jon Turney <jon DOT turney AT dronecode DOT org DOT uk>
> Date:   Thu Jan 11 20:00:14 2024 +0000
> 
[...]

Thanks for looking into this.

> I also found the following patch fixes the issue. I'm not sure
> this is the right thing, so could you please have a look?
> 
> diff --git a/winsup/cygwin/exceptions.cc b/winsup/cygwin/exceptions.cc
> index 9763a1b04..aea2821f6 100644
> --- a/winsup/cygwin/exceptions.cc
> +++ b/winsup/cygwin/exceptions.cc
> @@ -597,7 +597,7 @@ try_to_debug ()
>       {
>         extern void break_here ();
>         break_here ();
> -      return 1;
> +      return 0;
>       }
>   
>     /* Otherwise, invoke the JIT debugger, if set */

Hmm... at this remove in time, I'm not sure why I made that change.

Well, "consistently return non-zero from try_to_debug() if we debugged", 
but maybe that misunderstands the meaning of the return value...

(I'm not sure what the pure bafflegab at the end of exception::handle() 
does to make things work differently, we exit with 
ExceptionContinueExecution in both cases.)

I think this is similar to/related to the problem [1], which I 
unfortunately never got around to investigating properly.

I think perhaps we should probably also restore the behavior of not 
trying to break_here() (==DebugBreak()) if no JIT debugger is 
configured. (i.e. try_to_debug() should just immediately return 0, if 
debugger_command is NULL).

(This would seem to make try_to_debug() less useful, as it then does 
nothing if you're just run under gdb, but it's what the code used to do...)

I'll put together a patch tomorrow.


[1] https://cygwin.com/pipermail/cygwin/2024-May/256058.html


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