Re: [regression] x86/signal/64: Fix SS handling for signals delivered to 64-bit programs breaks dosemu

[Stas Sergeev]

12.08.2015 23:47, Andy Lutomirski ÐÐÑÐÑ:
> On Wed, Aug 12, 2015 at 1:45 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > 12.08.2015 23:28, Andy Lutomirski ÐÐÑÐÑ:
> >
> > > On Wed, Aug 12, 2015 at 1:14 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > > > 12.08.2015 23:01, Andy Lutomirski ÐÐÑÐÑ:
> > > > > On Wed, Aug 12, 2015 at 12:55 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > > > > > 12.08.2015 22:20, Andy Lutomirski ÐÐÑÐÑ:
> > > > > > > current kernels, it stays switched.  If we change this, it won't stay
> > > > > > > switched.  Even ignoring old ABI, it's not really clear to me what the
> > > > > > > right thing to do is.
> > > > > > There can be the following cases:
> > > > > > - switch_userspace_thread() switches fs to non-zero selector
> > > > > > - switch_userspace_thread() switches the fs base via syscall
> > > > > > - switch_userspace_thread() switches fs in sigcontext
> > > > > > - switch_userspace_thread() switches fs_base in sigcontext (???)
> > > > > > What exactly case do you have in mind?
> > > > > > I'd say, the way x86_32 is doing things - is good, but the
> > > > > > bases... perhaps, in ideal world, they should be a part of
> > > > > > the sigcontext as well?
> > > > > Any of the above.  What do you want the kernel to do, and how does the
> > > > > kernel know you want to do that?  The kernel has to pick *some*
> > > > > semantics here.
> > > > Assuming the bases are made the part of a sigcontext,
> > > > I'd say there would be no ambiguities remained at all:
> > > > whatever you change in a sigcontext, will be "applied" by
> > > > the sigreturn(). Whatever you put in the registers
> > > > (either segregs or MSRs), is valid until sigreturn(), then
> > > > forgotten forever.
> > > > The mess only comes in when some things are part of
> > > > sigcontext and some are not. But if you have _all_ things
> > > > accessable in sigcontext, then the user has a way of expressing
> > > > his needs very clearly: he'll either touch sigcontext or direct
> > > > values, depending on what he need.
> > > >
> > > > Is this right?
> > > Maybe, except that doing this might break existing code (Wine and Java
> > > come to mind).  I'm not really sure.
> > Yes, but that's why I was talking about some new
> > flag. Maybe a new sigaction() flag? Or something else that
> > will allow the user to request explicitly the new handling
> > where the things are all switched by the kernel. Then
> > the old programs that don't use that flag, will remain
> > unaffected. I realize this may be a lot of work... But please
> > note that there will be no more a chance like this one,
> > when things are already badly broken. :)
> I think that, with my patch, we get the best of both worlds.  We keep
> the old behavior in cases where it would work, and we switch to the
> new behavior in cases where the old behavior would result in killing
> the task.
But I mean also fs/TLS.
There is a chance now to fix things for good, all at once. :)
With such an ss patch applied to stable, there will be no more
such a chance ever. What's your opinion on the possibility of
fixing the TLS problem?
Also I am not sure about the sigreturn()'s detection: is it
a subject of the subsequent patch, or you dropped an idea?
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