Re: sigaltstack breaks swapcontext()

[Andy Lutomirski]

On Wed, Jan 6, 2016 at 11:31 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> 06.01.2016 22:14, Andy Lutomirski ÐÐÑÐÑ:
>
>> On Wed, Jan 6, 2016 at 10:42 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>>
>>> 06.01.2016 21:05, Andy Lutomirski ÐÐÑÐÑ:
>>>>
>>>> On Wed, Jan 6, 2016 at 7:45 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
>>>>>
>>>>> Hello.
>>>>>
>>>>> swapcontext() can be used with signal handlers,
>>>>> it swaps the signal masks together with the other
>>>>> parts of the context.
>>>>> Unfortunately, linux implements the sigaltstack()
>>>>> in a way that makes it impossible to use with
>>>>> swapcontext().
>>>>> Per the man page, sigaltstack is allowed to return
>>>>> EPERM if the process is altering its sigaltstack while
>>>>> running on sigaltstack. This is likely needed to
>>>>> consistently return oss->ss_flags, that indicates
>>>>> whether the process is being on sigaltstack or not.
>>>>> Unfortunately, linux takes that permission to return
>>>>> EPERM too literally: it returns EPERM even if you
>>>>> don't want to change to another sigaltstack, but
>>>>> only want to disable sigaltstack with SS_DISABLE.
>>>>> To my reading of a man page, this is not a desired
>>>>> behaviour. Moreover, you can't use swapcontext()
>>>>> without disabling sigaltstack first, or the stack will
>>>>> be re-used and overwritten by a subsequent signal.
>>>>>
>>>> The EPERM thing is probably also to preserve the behavior that nested
>>>> SA_ONSTACK signals are supposed to work.
>>>
>>> Could you please clarify?
>>> If I set up another stack inside the sighandler, the
>>> nested SA_ONSTACK signal can just use that new stack,
>>> which seems safe and sane. So I don't think EPERM helps
>>> the nested signals, or could you explain the possible breakage
>>> scenario?
>>
>> It's probably safe in most cases, but the current behavior explicitly
>> checks whether you're on the alt stack during signal delivery,
>
> ... to not re-use it occasionally.
> But if you set up a new sigaltstack, then perhaps the
> kernel can just check for overlaps with the current one to stay
> safe (but I'd rather not check anything).
>
>>>>> The work-around from this, is not even trivial: I have
>>>>> to use the shm tricks to duplicate the sigaltstack in
>>>>> the VA space, and move the stack pointer to another
>>>>> mirror before calling sigaltstack. Then I use longjmp()
>>>>> to restore the stack pointer. Then I can finally use
>>>>> swapcontext(). This is an unpleasant work-around.
>>>>>
>>>>> The fix on a kernel side looks simple: kernel should
>>>>> just use ss_flags to determine whether the sigaltstack
>>>>> is active. I can make a patch for that, but the problem
>>>>> is that the arch-specific code is not using any helper
>>>>> function to check for sigaltstack; instead it just uses
>>>>> "if (ss_size)" checks.
>>>>
>>>> Huh?  I'm not sure I understand what you're talking about.  It seems
>>>> reasonable to have the invariant that ss_size != 0 if and only if an
>>>> alt stack is enabled, and do_sigaltstack seems to enforce that
>>>> invariant.
>>>
>>> But we have that (IMO quite silly) requirement that the
>>> returned oss->ss_flags is consistent.
>>> So if inside the signal handler I use SS_DISABLE and
>>> the kernel translates this into "ss_size = 0", the next
>>> call to sigaltstack() will return 0 in oss->ss_flags.
>>
>> It should returns SS_DISABLE, right?
>
> Who knows?
> Man page says:
>        SS_ONSTACK
>               The process is  currently  executing  on  the alternate
> signal
>               stack.   (Note  that  it is not possible to change the
> alternate
>               signal stack if the process is currently executing on it.)
>
>        SS_DISABLE
>               The alternate signal stack is currently disabled.
>
> Both applies, and, judging from the description of EPERM, I
> can assume that SS_ONSTACK should be returned.
> If we remove EPERM completely, then obviously SS_DISABLE
> should be returned.
>
>>    And it won't set SS_ONSTACK
>> because you're not in the alt stack because there is no alt stack.
>>
>> Of course, there *was* an alt stack when the signal was delivered, and
>> you're on that stack.
>
> Exactly.
> Do you think this can be ignored?
> A man page should then be corrected with EPERM and the
> above note removed, right?
>

I think it can be ignored.  I'd go the SS_FORCE route, though, to
maintain POSIX compliance.

>>>>> So the patch will need to update
>>>>> all arches... I wonder if maybe someone can fix that
>>>>> problem and update the arch-specific code. If not,
>>>>> I'll probably need to update only the x86-specific code
>>>>> and add an arch-specific define, which is a bit nasty.
>>>>
>>>> Just change do_sigaltstack?
>>>
>>> But if its that easy and we do not even need a consistent
>>> oss->ss_flags - why not to remove the EPERM check entirely,
>>> rather than only for SS_DISABLE? Note that if it is removed
>>> only for SS_DISABLE and yet SS_DISABLE is translated to
>>> "ss_size=0", then by the next sigaltstack() call you can do
>>> whatever you want: the EPERM check will be entirely bypassed.
>>> So if you are fine with even this, I can send the patch to
>>> completely remove the check. Much easier for me. :)
>>> I think the semantic of oss->ss_size is quite bad, but it is
>>> already documented, so I am not sure.
>>
>> I would send a patch to remove the check or a patch to add a new
>> SS_FORCE that disables the check.  It should be just a couple of lines
>> of code.  A selftests patch along with it would help.  Cc linux-abi on
>> all of it.
>
> Hmm, OKey. But this can potentially contradict the man page,
> and I fear it can be rejected.
> So don't be surprised if I add your Acked-by, warning-warning. :)
>

You can add my Suggested-by.  No Acked-by until I actually ack it,
which involves seeing the code :)

>> BTW, the sigcontext SS stuff is queued for -next.  I doubt it'll make
>> 4.5 since I think that all the relevant maintainers are just
>> recovering from vacations, and I already have a decent backlog of
>> stuff that hasn't landed in -tip yet.
>
> Thanks for taking care of that!



-- 
Andy Lutomirski
AMA Capital Management, LLC
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