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

From: Andy Lutomirski
Date: Thu Aug 13 2015 - 21:38:24 EST

On Thu, Aug 13, 2015 at 6:32 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
> 14.08.2015 04:21, Andy Lutomirski ÐÐÑÐÑ:
>> On Thu, Aug 13, 2015 at 5:50 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>> 14.08.2015 03:27, Linus Torvalds ÐÐÑÐÑ:
>>>> On Thu, Aug 13, 2015 at 5:17 PM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>>>> For example because you can as well do:
>>>>> prctl(ARCH_SET_SIGNAL_SS, 0)
>>>>> which will mean "restore ss in sighandler to its current value",
>>>> I really think a prctl() is the wrong thing to do.
>>>> If you want a signal handler to save/restore segments, I think it
>>>> should be a SA_xyz flag to sigaction() (the way we have SA_RESTART
>>> Yes, I was proposing the new sigaction() flag in this thread
>>> already too. But at the end, prctl() looks better to me because
>>> it allows to pass the TLS value to use when restoring FS.
>>> The thing is that I am trying to find the similar treatment for
>>> both the SS and FS problems. If you don't think they need a
>>> similar treatment, then perhaps the Andy's patch is enough.
>>>> etc). And off by default because of the obvious compatibility issues.
>>> Of course.
>>> So, what we have right now (in the latest Andy's patch) is:
>>> 1. lar heuristics
>>> 2. new uc_flags flag
>>> What it solves: dosemu's regression.
>>> What prctl() can give:
>>> - fix to dosemu's regression
>>> - fix to the TLS problem in the future
>>> - no hack and heuristics
>>> With SA_xyz you can only solve the SS problem, so it is
>>> probably not any better than the uc_flags things coded
>>> up by Andy.
>> I'm leaning slightly toward LAR heuristic + SA_SAVE_SS.
> Stop right here, doesn't the SA_xyz allow to avoid the
> lar heuristic? Why would you still need the lar heuristic then?
> Just call it SA_RESTORE_SS instead of SA_SAVE_SS, and
> the lar heuristic is gone.

The LAR heuristic is about five lines of code, and it makes signal
delivery more reliable. Sure, we could gate the "regs->ss =
__USER_DS" line on a flag, but why?

>> Unfortunately, I don't think we were clever enough to allow this to be
>> probed easily -- we silently ignore unrecognized sa_flags bits.
> Big deal, check the kversion. :)

Not so good. For example, if you made your DOSEMU patch to use the
saved SS check the version, then the backported revert would break

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