linux-kernel - Re: sigaltstack breaks swapcontext()

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Date:	Fri, 8 Jan 2016 16:49:04 +0300
From:	Stas Sergeev <stsp@...t.ru>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linux kernel <linux-kernel@...r.kernel.org>
Subject: Re: sigaltstack breaks swapcontext()

06.01.2016 21:05, Andy Lutomirski пишет:
> On Wed, Jan 6, 2016 at 7:45 AM, Stas Sergeev <stsp@...t.ru> 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.  (Of course, the kernel gets
> this a bit wrong because it forgets to check ss in addition to sp.
> That would be relatively straightforward to fix.)
I don't think it needs a fix: in 64bit mode SS doesn't matter, and
in 32bit mode the SS is properly restored in a sighandler, so no
one can run sigaltstack() with non-flat SS (unless the DOS code
itself does this, which it does not).