Re: sigaltstack breaks swapcontext()

[Stas Sergeev]

09.01.2016 02:24, Andy Lutomirski ÐÐÑÐÑ:
> On Fri, Jan 8, 2016 at 5:49 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > 06.01.2016 21:05, Andy Lutomirski ÐÐÑÐÑ:
> > > On Wed, Jan 6, 2016 at 7:45 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > >
> > > > Hello.
> > > >
> > > > swapcontext() can be used with signal handlers,
> > > > it swaps the signal masks together with the other
> > > > parts of the context.
> > > > Unfortunately, linux implements the sigaltstack()
> > > > in a way that makes it impossible to use with
> > > > swapcontext().
> > > > Per the man page, sigaltstack is allowed to return
> > > > EPERM if the process is altering its sigaltstack while
> > > > running on sigaltstack. This is likely needed to
> > > > consistently return oss->ss_flags, that indicates
> > > > whether the process is being on sigaltstack or not.
> > > > Unfortunately, linux takes that permission to return
> > > > EPERM too literally: it returns EPERM even if you
> > > > don't want to change to another sigaltstack, but
> > > > only want to disable sigaltstack with SS_DISABLE.
> > > > To my reading of a man page, this is not a desired
> > > > behaviour. Moreover, you can't use swapcontext()
> > > > without disabling sigaltstack first, or the stack will
> > > > be re-used and overwritten by a subsequent signal.
> > > The EPERM thing is probably also to preserve the behavior that nested
> > > SA_ONSTACK signals are supposed to work.  (Of course, the kernel gets
> > > this a bit wrong because it forgets to check ss in addition to sp.
> > > That would be relatively straightforward to fix.)
> > I don't think it needs a fix: in 64bit mode SS doesn't matter, and
> > in 32bit mode the SS is properly restored in a sighandler, so no
> > one can run sigaltstack() with non-flat SS (unless the DOS code
> > itself does this, which it does not).
> It's not sigaltstack that I'm thinking about.  It's signal delivery.
> If you end up in DOS mode with SP coincidentally pointing to the
> sigaltstack (but with different SS so it's not really the
> sigaltstack), then the signal delivery will malfunction.
Ah, sounds like a real bug then!
Though if bitness differ (64bit mode and signal comes from
32bit code), there is probably no need to check anything and
just switch the stack.