Re: [RFC 3/4] x86/signal/64: Re-add support for SS in the 64-bit signal context

[Stas Sergeev <stsp@list.ru>]

14.10.2015 21:06, Andy Lutomirski пишет:
>> Also it doesn't seem to be saying what happens if CS is 32-bit
>> and SS is invalid (the flag is not set).
> 
> A new signal will be delivered.  sigreturn doesn't modify its behavior
> in this case -- it does the default thing, which is to honor the SS in
> the saved context.
Hmm, no, it didn't do this in the past for sure.
It simply ignored SS, no matter to what mode it returns.

>  So it will actually try to use that saved SS
> value, which will fail, causing SIGSEGV.
So it seems this logic assumes that when dosemu returns to 32bit,
the previous SS is always still valid, am I right with the understanding?
I.e. the one that kernel have saved on a signal delivery (because
old dosemu does not overwrite it).
If it is so, I'd say this assumption is very risky and will likely
not hold. But maybe I am missing the point.

>>>> - with siglongjmp()
>>>
>>> siglongjmp is a glibc thing.  It should work the same way it always
>>> did.  If it internally does a syscall (sigprocmask or whatever), that
>>> will override SS.
>> IMHO this side-effect needs to be documented somewhere.
>> I was scared about using it because I thought SS could be left bad.
>> Why I think it IS the kernel's problem is because in an ideal world
>> the sighandler should not run with LDT SS at all, so there will be no
>> fear about a bad SS after siglongjmp().
> 
> I agree, but that ship sailed quite a few years ago :(
Please, once again you are claiming there were no solutions proposed
in that area. :(
If you didn't repeatedly ignore the SA_hyz solution, there will be the
chance to do exactly that. But whatever.