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Date: Tue, 24 Feb 2015 22:16:39 +0100
From: Corinna Vinschen <corinna-cygwin@cygwin.com>
To: cygwin@cygwin.com
Subject: Re: [PATCH] Teach gdb how to unwind cygwin _sigbe frames
Message-ID: <20150224211639.GP437@calimero.vinschen.de>
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References: <1424715301-13516-1-git-send-email-jon.turney@dronecode.org.uk> <20150223212427.GB437@calimero.vinschen.de> <54EC98B4.9050003@dronecode.org.uk>
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On Feb 24 15:28, Jon TURNEY wrote:
> On 23/02/2015 21:24, Corinna Vinschen wrote:
> >On Feb 23 18:15, Jon TURNEY wrote:
> >>I really wanted to do this by adding some DWARF CFI to the generated si=
gfe.s
> >>file, but there doesn't seem to currently be a way to correctly describe
> >>_sigbe's frame using that.
> >
> >I was wondering about this already.  It seems weird if the CFI stuff
> >doesn't allow for a situation like that
>=20
> My understanding is limited, but it seems to me that it only describes
> register unwinding, not unwinding the state of an arbitrary memory locati=
on.
>=20
> After writing my patch, which only unwinds one sigstack entry correctly, I
> went back and had another go at writing the equivalent CFI, which I think
> should be possible, but I still couldn't get it to work.
>=20
> I'll take another look.

Maybe the binutils folks can help?

> >>So instead, write a custom unwinder for _sigbe frames, which gets the r=
eturn
> >>address from the sigstack.
> >>
> >>Implemented for i386 and amd64.
> >
> >Cool.  Really.  But, shouldn't that go to the gdb-patches mailing list?
> >
> >>4. This unfortunately ends up hardcoding into gdb the offset of sigstac=
kptr in
> >>the cygwin TLS area from the top of stack, and so will break if that ch=
anges.
> >>Hopefully that doesn't happen to often.
> >
> >It happens too often to be ignored.  Wouldn't it make sense to add
> >a cygwin_internal call to return tls offsets or addresses, the later
> >if the argument is a pointer to the stack or cygtls area?
>=20
> That's not going to give the right answer when remote debugging.

Uh, good point.

> But the offset is, of course, encoded into the _sigbe routine, so the bet=
ter
> approach is probably to extract it from there.

You mean scanning the code and extracting the offset right from the
opcodes?  That's an... interesting... idea.  I would probably object to
it, but can't be bothered, considering Cygwin is doing the same with
ntdll.dll ;)


Corinna

--=20
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Maintainer                 cygwin AT cygwin DOT com
Red Hat

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