Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 6 Jan 2016 10:05:00 -0800 | Subject | Re: sigaltstack breaks swapcontext() |
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On Wed, Jan 6, 2016 at 7:45 AM, Stas Sergeev <stsp@list.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 see anything terribly wrong with allowing SS_DISABLE even if you're on the alt stack. You could also add a new flag SS_FORCE that just overrides the check.
> 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.
> 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?
--Andy
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