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

[H.J. Lu]

On Tue, Oct 6, 2020 at 2:25 AM Dave Martin <Dave.Martin@xxxxxxx> wrote:
>
> On Mon, Oct 05, 2020 at 10:17:06PM +0100, H.J. Lu wrote:
> > 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
>
> Thanks for the link.
>
> Are there patches yet?  I already had some hacks in the works, but I can
> drop them if there's something already out there.

I am working on it.

>
> > 1. Define SIGSTKSZ and MINSIGSTKSZ to 64KB.
>
> Can we do this?  IIUC, this is an ABI break and carries the risk of
> buffer overruns.
>
> The reason for not simply increasing the kernel's MINSIGSTKSZ #define
> (apart from the fact that it is rarely used, due to glibc's shadowing
> definitions) was that userspace binaries will have baked in the old
> value of the constant and may be making assumptions about it.
>
> For example, the type (char [MINSIGSTKSZ]) changes if this #define
> changes.  This could be a problem if an newly built library tries to
> memcpy() or dump such an object defined by and old binary.
> Bounds-checking and the stack sizes passed to things like sigaltstack()
> and makecontext() could similarly go wrong.

With my original proposal:

https://sourceware.org/pipermail/libc-alpha/2020-September/118028.html

char [MINSIGSTKSZ] won't compile.  The feedback is to increase the
constants:

https://sourceware.org/pipermail/libc-alpha/2020-September/118092.html

>
> > 2. Add _SC_RSVD_SIG_STACK_SIZE for signal stack size reserved by the kernel.
>
> How about "_SC_MINSIGSTKSZ"?  This was my initial choice since only the
> discovery method is changing.  The meaning of the value is exactly the
> same as before.
>
> If we are going to rename it though, it could make sense to go for
> something more directly descriptive, say, "_SC_SIGNAL_FRAME_SIZE".
>
> The trouble with including "STKSZ" is that is sounds like a
> recommendation for your stack size.  While the signal frame size is
> relevant to picking a stack size, it's not the only thing to
> consider.

The problem is that AT_MINSIGSTKSZ is the signal frame size used by
kernel.   The minimum stack size for a signal handler is more likely
AT_MINSIGSTKSZ + 1.5KB unless AT_MINSIGSTKSZ returns the signal
frame size used by kernel + 6KB for user application.

>
> Also, do we need a _SC_SIGSTKSZ constant, or should the entire concept
> of a "recommended stack size" be abandoned?  glibc can at least make a
> slightly more informed guess about suitable stack sizes than the kernel
> (and glibc already has to guess anyway, in order to determine the
> default thread stack size).

Glibc should try to deduct signal frame size if AT_MINSIGSTKSZ isn't
available.

>
> > 3. Deprecate SIGSTKSZ and MINSIGSTKSZ if _SC_RSVD_SIG_STACK_SIZE
> > is in use.
>
> Great if we can do it.  I was concerned that this might be
> controversial.
>
> Would this just be a recommendation, or can we enforce it somehow?

It is just an idea.  We need to move away from constant SIGSTKSZ and
MINSIGSTKSZ.

-- 
H.J.