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Re: Thread safety of coroutine-sigaltstack

From: Laszlo Ersek
Subject: Re: Thread safety of coroutine-sigaltstack
Date: Fri, 22 Jan 2021 22:34:14 +0100

On 01/22/21 21:38, Laszlo Ersek wrote:
> On 01/21/21 18:24, Paolo Bonzini wrote:
>> On 21/01/21 17:44, Peter Maydell wrote:
>>> On Thu, 21 Jan 2021 at 16:10, Daniel P. Berrangé <berrange@redhat.com>
>>> wrote:
>>>> FWIW The libucontext impl is all ASM based and has coverage for all the
>>>> arches we care about:
>>>>     https://github.com/kaniini/libucontext
>>>> so doesn't seem like there's a need for  coroutine-asm if we can rely
>>>> on libucontext for portability where neede.
>>> The README for that notes a couple of pretty major omissions:
>>>   * it doesn't handle floating point registers
>>>   * it doesn't do anything about the signal mask
>>> I'm pretty sure that not handling the fp regs could cause breakage
>>> for Arm at least (depending on what the compiler chooses to do
>>> in the functions that work with the ucontext functions). The
>>> signal mask stuff might be OK for us because of the carefully
>>> limited use we make of the ucontext functions, but it would be
>>> a bit of a pain to have to analyse that code for both sets of semantics.
>> The lack of signal mask handling is an improvement for us. :)  We want
>> the signal mask to be per thread, not per coroutine.
> I didn't quite get this when I first read it, but now that I'm digging
> through the code, I have a follow-up comment.
> According to POSIX, passing savemask=0 to sigsetjmp() may or may not
> save the current signal mask, into "env". A nonzero savemask is required
> to save the signal mask, but a zero savemask is not forbidden to -- it
> is only not required to:
> https://pubs.opengroup.org/onlinepubs/9699919799/functions/sigsetjmp.html#tag_16_554_07
>     Note that since this function is defined in terms of setjmp(), if
>     savemask is zero, it is unspecified whether the signal mask is
>     saved.
> And I feel that's a bit of a problem, because when we first exit the
> trampoline -- executed as a signal handler -- via sigsetjmp(), *all
> signals* are masked, and sigsetjmp might actually stash that mask in
> "tr_reenter", because savemask=0 does not suffice for forbidding that.
> When we reenter the trampoline via siglongjmp(tr_reenter), and
> subsequently call coroutine_bootstrap(), it's possible (per POSIX, see
> above) that all signals are masked again. And then that could further be
> remembered in "self->env", in coroutine_bootstrap(). Which would be
> wrong IMO; co-routines in general should receive synchronous signals if
> they mess up somewhere (terminating the process).
> IOW, just before the call to coroutine_bootstrap(),
> coroutine_trampoline() should explicitly restore the signal mask that
> was in effect when qemu_coroutine_new() was entered.
> Has this been a problem in practice, or should we ignore it?
> IOW, should we assume "savemask=0" for *never* saving the signal mask?
> The behavior of "savemask=0" is a platform trait that platforms are not
> required to document (the behavior is unspecified, not
> implementation-defined), so it really boils down to where this code
> actually runs...
> NB Linux is more specific:
> https://man7.org/linux/man-pages/man3/setjmp.3.html
>    sigsetjmp() and siglongjmp()
>        sigsetjmp() and siglongjmp() also perform nonlocal gotos, but
>        provide predictable handling of the process signal mask.
>        If, and only if, the savesigs argument provided to sigsetjmp() is
>        nonzero, the process's current signal mask is saved in env and
>        will be restored if a siglongjmp() is later performed with this
>        env.
> Cue "and only if".

... I notice commit 6ab7e5465a4d ("Replace all setjmp()/longjmp() with
sigsetjmp()/siglongjmp()", 2013-02-23) chose the Linux definition, not
the POSIX one.


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