Re: [RFC PATCH 0/4] x86: Improve Minimum Alternate Stack Size

[H.J. Lu]

On Mon, Oct 5, 2020 at 6:45 AM Dave Martin <Dave.Martin@xxxxxxx> wrote:
>
> On Tue, Sep 29, 2020 at 01:57:42PM -0700, Chang S. Bae wrote:
> > During signal entry, the kernel pushes data onto the normal userspace
> > stack. On x86, the data pushed onto the user stack includes XSAVE state,
> > which has grown over time as new features and larger registers have been
> > added to the architecture.
> >
> > MINSIGSTKSZ is a constant provided in the kernel signal.h headers and
> > typically distributed in lib-dev(el) packages, e.g. [1]. Its value is
> > compiled into programs and is part of the user/kernel ABI. The MINSIGSTKSZ
> > constant indicates to userspace how much data the kernel expects to push on
> > the user stack, [2][3].
> >
> > However, this constant is much too small and does not reflect recent
> > additions to the architecture. For instance, when AVX-512 states are in
> > use, the signal frame size can be 3.5KB while MINSIGSTKSZ remains 2KB.
> >
> > The bug report [4] explains this as an ABI issue. The small MINSIGSTKSZ can
> > cause user stack overflow when delivering a signal.
> >
> > In this series, we suggest a couple of things:
> > 1. Provide a variable minimum stack size to userspace, as a similar
> >    approach to [5]
> > 2. Avoid using a too-small alternate stack
>
> I can't comment on the x86 specifics, but the approach followed in this
> series does seem consistent with the way arm64 populates
> AT_MINSIGSTKSZ.
>
> I need to dig up my glibc hacks for providing a sysconf interface to
> this...

Here is my proposal for glibc:

https://sourceware.org/pipermail/libc-alpha/2020-September/118098.html

1. Define SIGSTKSZ and MINSIGSTKSZ to 64KB.
2. Add _SC_RSVD_SIG_STACK_SIZE for signal stack size reserved by the kernel.
3. Deprecate SIGSTKSZ and MINSIGSTKSZ if _SC_RSVD_SIG_STACK_SIZE
is in use.


-- 
H.J.