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

From: Dave Martin
Date: Wed Oct 07 2020 - 06:47:29 EST

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On Tue, Oct 06, 2020 at 10:44:14AM -0700, H.J. Lu wrote:
> On Tue, Oct 6, 2020 at 9:55 AM Dave Martin <Dave.Martin@xxxxxxx> wrote:
> >
> > On Tue, Oct 06, 2020 at 08:34:06AM -0700, H.J. Lu wrote:
> > > On Tue, Oct 6, 2020 at 8:25 AM Dave Martin <Dave.Martin@xxxxxxx> wrote:
> > > >
> > > > On Tue, Oct 06, 2020 at 05:12:29AM -0700, H.J. Lu wrote:
> > > > > 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.
> > > >
> > > > OK. I may post something for discussion, but I'm happy for it to be
> > > > superseded by someone (i.e., other than me) who actually knows what
> > > > they're doing...
> > >
> > > Please see my previous email for my glibc patch:
> > >
> > > https://gitlab.com/x86-glibc/glibc/-/commits/users/hjl/AT_MINSIGSTKSZ
> > >
> > > > > >
> > > > > > > 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
> > > >
> > > > Ah, I see. But both still API and ABI breaks; moreover, declaraing an
> > > > array with size based on (MIN)SIGSTKSZ is not just reasonable, but the
> > > > obvious thing to do with this constant in many simple cases. Such usage
> > > > is widespread, see:
> > > >
> > > > * https://codesearch.debian.net/search?q=%5BSIGSTKSZ%5D&literal=1
> > > >
> > > >
> > > > Your two approaches seem to trade off two different sources of buffer
> > > > overruns: undersized stacks versus ABI breaks across library boundaries.
> > >
> > > We can't get everything we want.
> > >
> > > > Since undersized stack is by far the more familiar problem and we at
> > > > least have guard regions to help detect overruns, I'd vote to keep
> > > > MINSIGSTKSZ and SIGSTKSZ as-is, at least for now.
> > >
> > > Agree.
> > >
> > > > Or are people reporting real stack overruns on x86 today?
> > >
> > > I hope so.
> > >
> > > >
> > > > For arm64, we made large vectors on SVE opt-in, so that oversized signal
> > > > frames are not seen by default. Would somethine similar be feasible on
> > > > x86?
> > > >
> > > >
> > > > > > > 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.
> > > >
> > > > Ack; to be correct, you also need to take into account which signals may
> > > > be unmasked while running on this stack, and the stack requirements of
> > > > all their handlers. Unfortunately, that's hard :(
> > > >
> > > > What's your view on my naming suggesions?
> > >
> > > I used _SC_MINSIGSTKSZ:
> > >
> > > https://gitlab.com/x86-glibc/glibc/-/commit/73ca53bfbc1c105bc579f55f15af011a07fcded9
> >
> > Apologies, I missed that.
> >
> > Otherwise, the changes look much as I would expect, except for the
> > "6K for user program" thing. This is strictly not included in the
> > legacy MINSIGSTKSZ.
> >
> > >
> > > >
> > > > > > 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.
> > > >
> > > > In my code, I generate _SC_SIGSTKSZ as the equivalent of
> > > >
> > > > max(sysconf(_SC_MINSIGSTKSZ) * 4, SIGSTKSZ)
> > > >
> > > > which is >= the legacy value, and broadly reperesentative of the
> > > > relationship between MINSIGSTKSZ and SIGSTKSZ on most arches.
> > > >
> > > >
> > > > What do you think?
> > >
> > > sysconf(_SC_MINSIGSTKSZ) should be usable ASIS for most cases.
> >
> > Why, though?
> >
> > MINSIGSTKSZ is not specified to be usable as-is for any case whatsoever.
> >
> >
> > Software that calculates its own needs to know the actual system values,
> > not estimates based on guesses about how much stack a typical program
> > might need if it were recompiled for x86.
> >
> > This doesn't mean we can't have a generic suggested value that's suitable
> > for common scenarios (like SIGSTKSZ), but if we do then I think it
> > should be a separate constant.
>
> I updated my glibc patch to add both _SC_MINSIGSTKSZ and _SC_SIGSTKSZ.
> _SC_MINSIGSTKSZ is the minimum signal stack size from AT_MINSIGSTKSZ,
> which is the signal frame size used by kernel, and _SC_SIGSTKSZ is the value
> of sysconf (_SC_MINSIGSTKSZ) + 6KB for user application.

Can I suggest sysconf (_SC_MINSIGSTKSZ) * 4 instead?

If the arch has more or bigger registers to save in the signal frame,
the chances are that they're going to get saved in some userspace stack
frames too. So I suspect that the user signal handler stack usage may
scale up to some extent rather than being a constant.

To help people migrate without unpleasant surprises, I also figured it
would be a good idea to make sure that sysconf (_SC_MINSIGSTKSZ) >=
legacy MINSIGSTKSZ, and sysconf (_SC_SIGSTKSZ) >= legacy SIGSTKSZ.
This should makes it safer to use sysconf (_SC_MINSIGSTKSZ) as a
drop-in replacement for MINSIGSTKSZ, etc.

(To explain: AT_MINSIGSTKSZ may actually be < MINSIGSTKSZ on AArch64.
My idea was that sysconf () should hide this surprise, but people who
really want to know the kernel value would tolerate some
nonportability and read AT_MINSIGSTKSZ directly.)

So then:

kernel_minsigstksz = getauxval(AT_MINSIGSTKSZ);
minsigstksz = LEGACY_MINSIGSTKSZ;
if (kernel_minsigstksz > minsigstksz)
minsistksz = kernel_minsigstksz;

sigstksz = LEGACY_SIGSTKSZ;
if (minsigstksz * 4 > sigstksz)
sigstksz = minsigstksz * 4;

(Or something like that, unless the architecture provides its own
definitions. ia64's MINSIGSTKSZ is enormous, so it probably doesn't
want this.)

Also: should all these values be rounded up to a multiple of the
architecture's preferred stack alignment?

Should the preferred stack alignment also be exposed through sysconf?
Portable code otherwise has no way to know this, though if the
preferred alignment is <= the minimum malloc()/alloca() alignment then
this is generally not an issue.)

> >
> > > > > > > 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.
> > > >
> > > > Totally agree with that.
> > > >
> > >
> > > With my glibc patch, -D_SC_MINSIGSTKSZ_SOURCE will fail to compile
> > > if the source assumes constant SIGSTKSZ or MINSIGSTKSZ.
> >
> > Ah yes, I see. That's a sensible precaution.
> >
> > Is it accepted in general that defining different feature test macros
> > can lead to ABI incompatibilities?
> >
> > I have thought that building a shared library with _GNU_SOURCE (say)
> > doesn't mean that a program that loads that library must also be built
> > with _GNU_SOURCE. For one thing, that's hard to police.
> >
> >
> > However, there are already combinations that could break, e.g., mixing
> > -D_FILE_OFFSET_BITS=64 with -D_FILE_OFFSET_BITS=32 would be broken if
> > this define changes off_t.
> >
> >
> > So, maybe having _SC_MINSIGSTKSZ_SOURCE break things in this way is an
> > acceptable compromise. Interfaces that depend on the value of
> > MINSIGSTKSZ or SIGSTKSZ are possible, but probably rare in practice --
> > I don't know of a specific example.
> >
>
> I changed it to _SC_SIGSTKSZ_SOURCE:
>
> https://gitlab.com/x86-glibc/glibc/-/commit/41d5e6b31025721590f12d5aa543eb0bc53ce263
>
> #ifdef __USE_SC_SIGSTKSZ
> # include <unistd.h>
> /* Minimum stack size for a signal handler: sysconf (SC_SIGSTKSZ). */
> # undef MINSIGSTKSZ
> # define MINSIGSTKSZ sysconf (_SC_SIGSTKSZ)
> /* System default stack size for a signal handler: MINSIGSTKSZ. */
> # undef SIGSTKSZ
> # define SIGSTKSZ MINSIGSTKSZ
> #endif
>
> Compilation will fail if the source assumes constant MINSIGSTKSZ or
> SIGSTKSZ.

I don't understand all the glibc-fu, bit it looks reasonable overall
(notwithstanding my comments above).

Cheers
---Dave

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