Re: sigaltstack breaks swapcontext()

[Stas Sergeev]

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).