On Thu, Aug 13, 2015 at 9:38 AM, Stas Sergeev <stsp@xxxxxxx> wrote:But that's too late to affect the signal _delivery_ anyhow, no?
13.08.2015 19:24, Andy Lutomirski ÐÐÑÐÑ:User code could rewrite it in the signal handler to request something.
On Thu, Aug 13, 2015 at 9:20 AM, Stas Sergeev <stsp@xxxxxxx> wrote:Isn't uc_flags read-only for the user?
13.08.2015 19:09, Andy Lutomirski ÐÐÑÐÑ:Maybe. I think I'm more nervous about adding new flags in sigaction
On Thu, Aug 13, 2015 at 9:03 AM, Stas Sergeev <stsp@xxxxxxx> wrote:Not quite.
13.08.2015 18:38, Andy Lutomirski ÐÐÑÐÑ:I'm not sure what the proposal is exactly.
Andy, why do you constantly ignore the proposal to make
So... what do we do about it? We could revert the whole mess. We
could tell everyone to fix their DOSEMU, which violates policy and is
especially annoying given how much effort we've put into keeping
16-bit mode fully functional lately. We could add yet more heuristics
and teach sigreturn to ignore the saved SS value in sigcontext if the
saved CS is 64-bit and the saved SS is unusable.
new behaviour explicitly controlable? You don't have to agree
with it, but you could at least comment on that possibility
and/or mention it with the ones you listed above.
We could add a new uc_flags flag. If set, it means that
sigcontext->ss is valid and should be used by sigreturn. If clear,
then we ignore sigcontext->ss and just restore __USER_DS.
The problem is that, by itself, this won't fix old DOSEMU. We somehow
need to either detect that something funny is going on or just leave
the flag clear by default.
We could do this: always save SS to sigcontext->ss, but only restore
sigcontext->ss if userspace explicitly sets the flag before sigreturn.
If we do that, we'd need to also add my patch to preserve the actual
HW SS selector if possible so that old DOSEMU knows what SS to program
into its trampoline.
This at least lets *new* DOSEMU set the flag and get the improved
behavior. I still don't know what effect it'll have on Wine and CRIU.
Stas, is that what you were thinking, or were you thinking of something
I mean the flag that will control not only sigreturn, but
the signal delivery as well. This may probably be a sigaction()
flag or some other. If not set - ss is ignored by both signal
delivery and sigreturn(). If set - ss is saved/restored (and in
the future - also fs/gs).
Is such a flag possible?
than I am in uc_flags.
I look into setup_rt_frame
<http://lxr.free-electrons.com/ident?v=2.4.37;i=setup_rt_frame>() and see
/* Create the ucontext. */
err |= __put_user(0, &frame->uc.uc_flags);
so it doesn't look like the flag that user can use to _request_
something from the kernel. And I am talking about exactly
the flag to request the new behaviour, as only that can remove
the regression completely without patching dosemu.