Re: [patch/cygwin] Remove dependency on __COPY_CONTEXT_SIZE

[Corinna Vinschen <vinschen@redhat.com>]

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On Apr  1 11:44, Pedro Alves wrote:
> On 03/31/2015 07:32 PM, Corinna Vinschen wrote:
> > On Mar 31 17:30, Pedro Alves wrote:
> >> I think it's bad to hard code
> >> the size of the CONTEXT structure, but won't argue further.  Patch
> >> is OK if you'd really like to apply it as is.
> > 
> > Thanks.  But incidentally I retract the patch.  It seems we made a
> > mistake both, on 32 and 64 bit Cygwin as far as the definition of
> > __COPY_CONTEXT_SIZE is concerned.  Changing that to sizeof(CONTEXT) now
> > would potentially break backward compatibility with all Cygwin versions
> > up to today.  Oh well.
> 
> Not sure I understand what you mean, but OK.

I was a bit unclear, sorry.  What I was trying to say is this.

Only yesterday in a discussion on IRC it turned out that the definition
of __COPY_CONTEXT_SIZE was never identical to sizeof(CONTEXT).  The
definition of __COPY_CONTEXT_SIZE was based on an underlying datatype,
almost, but not quite identical to CONTEXT (i.e. a bug).

As a result, __COPY_CONTEXT_SIZE < sizeof(CONTEXT).  Worse, Cygwin
itself up to the current version 1.7.35 (fixed in the git repo) only
stored the leading __COPY_CONTEXT_SIZE bytes of the signal CONTEXT to 
internal storage for GDB's digestion.

Assuming we change GDB now to copy sizeof(CONTEXT), and assuming we're
running under a Cygwin <= 1.7.35.

GDB would copy random data following the leading __COPY_CONTEXT_SIZE
bytes of a CONTEXT to its own CONTEXT.  Thus, when later calling
SetThreadContext with this data, it would copy random data into the
OSes thread context.  Which sounds like a really, really bad idea to
me.

Therefore, for backward compat reasons we should keep this up for a
while, until we decide not to support Cygwin versions <= 1.7.35 in
GDB any longer.  Given that only the leading part of the signal
context (actually, basically only the content of Eip/Rip) is really
important here, nothing much is lost.

I hope I could clarify the situation.  If you have another idea
how we could handle this a bit..., well, more correct or something,
please do tell.

> > Sorry for the longish discussion for nothing :(
> 
> Sounds like it wasn't for nothing then.

Indeed :)


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer
Red Hat

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