Re: [PATCH] Only construct ucontext for SA_SIGINFO signal handlers

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Apr  3 13:51, Jon TURNEY wrote:
> On 03/04/2015 13:17, Corinna Vinschen wrote:
> >On Apr  3 13:18, Corinna Vinschen wrote:
> >>On Apr  2 20:30, Jon TURNEY wrote:
> >>
> >>>        sigset_t this_oldmask = set_process_mask_delta ();
> >>>-      thiscontext.uc_sigmask = this_oldmask;
> >>>+      context.uc_sigmask = this_oldmask;
> >>          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> >>
> >>This call to set_process_mask_delta() should occur before constructing
> >>the context, so that filling in uc_sigmask can be moved into the above
> >>`'if' branch.
> 
> Ok, I will move it.
> 
> >>On second thought, isn't this slightly wrong anyway?  Shouldn't that be
> >>
> >>          context.uc_sigmask = _my_tls.sigmask;
> >>	 context.uc_mcontext.oldmask = this_oldmask;
> 
> As I wrote elsewhere:  You'll have to help me understand what the difference
> in meaning between ucontext_t.uc_sigmask and ucontext_t.uc_mcontext.oldmask
> is.
> 
> I don't see how the value of _my_tls.sigmask has any meaning at that point
> in the code.

Ok, I had a look into the Linux source and searched the web, and here's
the problem.

One is that sigset_t on Linux is not just a 32 or 64 bit bitmask anymore,
but an array of ulong's used as a rather big sigmask.

OTOH, mcontext_t::oldmask is only the size of "unsigned long".  In fact,
as it turns out by inspecting the Linux kernel, oldmask is nothing else
than the first bits of uc_sigmask which fit into an unsigned long.  And
in the net I found that oldmask is just the old representation of
sigset_t, before the Linux kernel allowed more signals than fit into
a bitmask of unsigned long size.  In fact, it's only for backward compat,
but unused these days.

Given that, setting context.uc_sigmask to this_oldmask is apparently
the right thing to do.  For emulating backward compat (which we don't
need, but it also doesn't hurt), we could set oldmask to the same
value:

  context.uc_sigmask = context.uc_mcontext.oldmask = this_oldmask;

> >Oh, btw., what about cr2?  Right now, with the above code, it contains
> >a random value.  It should at least be zero'ed out.  Alternatively:
> >
> >   context.uc_mcontext.cr2 = (thissi.si_signo == SIGSEGV
> >			     || thissi.si_signo == SIGBUS)
> >			    ? (uintptr_t) thissi.si_addr : 0;
> >
> 
> Sure, but can we deal with that as a separate patch?

Yes, but you can just apply it as well.  cr2 is the address of a page
fault, so that's equivalent to the value in ExceptionInformation[1]
which, in turn, is stored in si_addr in exception::handle.


Thanks,
Corinna

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

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