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 [off-list ref] wrote:

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On Tue, Oct 06, 2020 at 05:12:29AM -0700, H.J. Lu wrote:

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On Tue, Oct 6, 2020 at 2:25 AM Dave Martin [off-list ref] wrote:

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On Mon, Oct 05, 2020 at 10:17:06PM +0100, H.J. Lu wrote:

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On Mon, Oct 5, 2020 at 6:45 AM Dave Martin [off-list ref] wrote:

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On Tue, Sep 29, 2020 at 01:57:42PM -0700, Chang S. Bae wrote:

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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superseded by someone (i.e., other than me) who actually knows what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

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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.

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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.

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Or are people reporting real stack overruns on x86 today?

I hope so.

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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?

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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.

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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.

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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.

Cheers
---Dave