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Message-ID: <53ADC8D7.90203@mit.edu>
Date: Fri, 27 Jun 2014 12:41:11 -0700
From: Andy Lutomirski <luto@...capital.net>
To: musl@...ts.openwall.com
Subject: Re: getcontext etc was Re: Re: go support (was: Best place
to discuss other lightweight libraries?)
On 06/23/2014 03:32 PM, Rich Felker wrote:
> On Mon, Jun 23, 2014 at 10:09:29PM +0100, Justin Cormack wrote:
>> On Wed, Apr 24, 2013 at 2:42 PM, Rich Felker <dalias@...ifal.cx> wrote:
>>>> i tried to build gcc 4.7.2 with go support (--enable-languages=c,c++,go)
>>>> and that fails due to a lack of set/getcontext().
>>>> (see pkg/gcc472 in sabotage)
>>>>
>>>> according to rich, adding that to musl requires a non-trivial amount
>>>> of arch specific asm.
>>>
>>> Yes, but it is a wanted feature, so I wouldn't mind it getting done.
>>> It was even part of the standard prior to POSIX 2008, and the reason
>>> for removing it was stupid. (The reason was that the makecontext
>>> function's calling convention is bogus and impossible to support
>>> properly, but they could have fixed this by deprecating the use of the
>>> variadic arguments in any way except passing a single void* argument,
>>> rather than deprecating the whole set of interfaces.)
>>
>> Just to revive this post 1.0, it would be very nice indeed to have
>> set,get,swapcontext.
>
> One approach to setcontext would be using rt_sigreturn, but I've heard
> there are issues using it for setcontext involving the alternate
> signal stack. It's the only way to do resuming _async_ (i.e. the
> ucontext received by a signal handler for the state it interrupted)
> contexts though, so maybe there's some way to make it work. This issue
> was discussed recently on libc-alpha (the glibc list).
What's the async issue?
I think that using rt_sigreturn for this directly (rather than sending
yourself a signal, adjusting the context, and returning) is a bad idea:
there are patches floating around to mitigate SROP (sigreturn-oriented
programming) by adding signal frame canaries, and you'll go boom if that
happens.
--Andy
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