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

[Stas Sergeev <stsp@list.ru>]

02.09.2015 08:12, Andy Lutomirski пишет:
> On Wed, Aug 19, 2015 at 9:30 AM, Stas Sergeev <stsp@list.ru> wrote:
>> 19.08.2015 18:46, Andy Lutomirski пишет:
>>> On Wed, Aug 19, 2015 at 2:35 AM, Stas Sergeev <stsp@list.ru> wrote:
>>>>> Incidentally, I tried implementing the sigaction flag approach.  I
>>>>> think it's no good.  When we return from a signal, there's no concept
>>>>> of sigaction -- it's just sigreturn.  Sigreturn can't look up the
>>>>> sigaction flags -- what if the signal handler calls sigaction itself.
>>>> How about the SA_hyz flag that does the following:
>>>> - Saves SS into sigcontext
>>>> - Forces SS to USER_DS on signal delivery
>>>> - Sets the uc_flags flag for sigreturn() to take care of the rest.
>>>> You'll have both the control on every bit of action, and a simple
>>>> detection logic: if SA_hyz didn't set the uc flag - it didn't work.
>>>> You can even employ your lar heuristic here for the case when the
>>>> aforementioned SA_hyz is not set. But please, please not when it is
>>>> set! In fact, I wonder if you had in mind exactly that: using the
>>>> lar heuristic only if the SA_hyz is not set. If so - I misunderstood.
>>>> Just please don't add it when it is set.
>>> Hmm, interesting.  Maybe that would work for everything.  How's this
>>> to make it concrete?
>>>
>>> Add a sigaction flag SA_RESTORE_SS.
>>>
>>> On signal delivery, always save SS into sigcontext->ss. if
>>> SA_RESTORE_SS is set, then unconditionally switch HW SS to __USER_DS
>>> and set UC_RESTORE_SS.  If SA_RESTORE_SS is clear, then leave HW SS
>>> alone (i.e. preserve the old behavior).
>> Either that, or employ the lar heuristic for the "not set" case
>> (I think its not needed).
>>
>>> On signal return, if UC_RESTORE_SS is set, then restore
>>> sigcontext->ss.  If not, then set SS to __USER_DS (as old kernels
>>> did).
>>>
>>> This should change nothing at all (except the initial value of
>>> sigcontext->ss / __pad0) on old kernels.
>> Agreed.
>>
> Let me throw out one more possibility, just for completeness:
>
> We don't add any SA_xyz flags.  On signal delivery, we use the LAR
> heuristic.  We always fill in sigcontext->ss, and we set a new
> UC_SIGCONTEXT_SS flag to indicate that we support the new behavior.
>
> On sigreturn, we honor the sigcontext's ss, *unless* CS is 64 bit and
> SS is invalid.  In the latter case, we replace the saved ss with
> __USER_DS.
But this is not a new proposal, see here:
https://lkml.org/lkml/2015/8/13/436
The very last sentence says exactly the same.
I thought this is in the past. :)

> This should work for old DOSEMU.  It's a bit gross, but it has the
> nice benefit that everyone (even things that aren't DOSEMU) gain the
> ability to catch signals thrown from bogus SS contexts, which probably
> improves debugability.  It's also nice to not have the SA flag.
Pros:
- No new SA flag
- May improve debugability in some unknown scenario where people
do not want to just use the new flag to get their things improved

Cons:
- Does not allow to cleanly use siglongjmp(), as then there is a risk
to jump to 64bit code with bad SS
- Async signals can silently "validate" SS behind your back
- No way to extend that solution to later fixing the TLS problem
- Many ugly checks in the code, that are not always even obvious
(eg you wanted to try verw instead, and there was a gotcha with
NP bit)

Is the new SA flag such a big deal here to even bother?