[PATCH 2/3] Provide ucontext to signal handlers
Corinna Vinschen
corinna-cygwin@cygwin.com
Sat Apr 4 08:40:00 GMT 2015
On Apr 3 23:09, Jon TURNEY wrote:
> On 01/04/2015 15:22, Corinna Vinschen wrote:
> >On Apr 1 14:19, Jon TURNEY wrote:
> >>Add ucontext.h header, defining ucontext_t and mcontext_t types.
> >>
> >>Provide sigaction sighandlers with a ucontext_t parameter, containing stack and
> >>context information.
> >>
> >> * include/sys/ucontext.h : New header.
> >> * include/ucontext.h : Ditto.
> >> * exceptions.cc (call_signal_handler): Provide ucontext_t
> >> parameter to signal handler function.
> >
> >Patch is ok with a single change: Please add a "FIXME?" comment to:
> >
> > else
> > RtlCaptureContext();
> >
> >On second thought, calling RtlCaptureContext here is probably wrong.
>
> Wrong and also dangerous.
>
> This causes random crashes on x86.
>
> It seems that RtlCaptureContext requires the framepointer of the calling
> function in ebp, which it uses to report the rip and rsp of it's caller.
>
> It also seems that gcc can decide to optimize the setting of the
> framepointer away, irrespective of the fact that -fomit-frame-pointer is not
> used when building exceptions.cc
>
> If _cygtls::call_signal_handler() happens to get called with ebp pointing to
> an invalid memory address, as seems to happen occasionally, we will fault in
> RtlCaptureContext. (in all cases, the eip and ebp in the returned context
> are incorrect)
>
> I wrote the attached patch, which fakes a callframe for RtlCaptureContext to
> avoid these possible crashes, but this needs more work to correctly report
> eip and ebp
Maybe it's simpler than that? Looking into the GCC info pages, I found
this:
Starting with GCC version 4.6, the default setting (when not
optimizing for size) for 32-bit GNU/Linux x86 and 32-bit Darwin x86
targets has been changed to '-fomit-frame-pointer'. The default
can be reverted to '-fno-omit-frame-pointer' by configuring GCC
with the '--enable-frame-pointer' configure option.
Enabled at levels '-O', '-O2', '-O3', '-Os'.
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
So it seems adding -fomit-frame-pointer file by file in Makefile.in
(when building with -O2) is moot and only has an effect when building
unoptimized, otherwise all files are built with -fomit-frame-pointer
anyway.
So, what if we drop all the -fomit-frame-pointer from Makefile.in and
add an
exceptions_CFLAGS:=-fno-omit-frame-pointer
Does that help?
Corinna
--
Corinna Vinschen Please, send mails regarding Cygwin to
Cygwin Maintainer cygwin AT cygwin DOT com
Red Hat
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