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Re: Consensus on MT-, AS- and AC-Safety docs.

On Fri, Nov 29, 2013 at 12:21:02AM -0500, Carlos O'Donell wrote:
> On 11/27/2013 08:46 AM, Florian Weimer wrote:
> > On 11/27/2013 02:34 PM, Joseph S. Myers wrote:
> >> On Wed, 27 Nov 2013, Florian Weimer wrote:
> >>> I think we should do that in the code that wraps the signal
> >>> handler call, to address this class of errors once and for all.
> >>> Is this feasible?
> >> 
> >> On many architectures the signal trampoline is provided by the
> >> kernel and so has no access to libc implementation details like
> >> errno.  As far as I understand, even when provided by libc all it
> >> does is call the sigreturn syscall and provide appropriate unwind
> >> information for the registers the kernel saved on the stack - that
> >> is, there is no userspace code called before the signal handler
> >> that would have a chance to save errno.
> > 
> > The number of signals is fixed and small, so it should be feasible to
> > install our own handler in the kernel and store the user-supplied
> > handler in a lookup table that gets called from our handler.  There
> > might be a slight performance impact, though.
> > 
> > But if we think this errno issue is serious, rather than patching
> > almost any signal handler out there right now, we should address it
> > in glibc.
> All we need to do is extend the kernel infrastructure to create
> a signal entry trampoline like it does for signal return. This isn't
> impossible, but requires work.

Kernel help is not required for this. You just set set the signal
handler address to your wrapper in libc when making the rt_sigaction
syscall, and then have the wrapper call the real signal handler
function, whose address is stored in userspace. There are some nasty
atomicity/synchronization issues to cover with the race window between
the rt_sigaction syscall (which might change flags affecting how the
signal handler runs even though the new handler address will always be
the same) and replacing the pointer in userspace, and keeping this
entire operation AS-safe, but I'm pretty sure it's possible.


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