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

[Dave Martin]

On Tue, Oct 06, 2020 at 08:21:00PM +0200, Florian Weimer wrote:
> * Dave Martin via Libc-alpha:
> 
> > On Tue, Oct 06, 2020 at 08:33:47AM -0700, Dave Hansen wrote:
> >> On 10/6/20 8:25 AM, Dave Martin wrote:
> >> > Or are people reporting real stack overruns on x86 today?
> >> 
> >> We have real overruns.  We have ~2800 bytes of XSAVE (regisiter) state
> >> mostly from AVX-512, and a 2048 byte MINSIGSTKSZ.
> >
> > Right.  Out of interest, do you believe that's a direct consequence of
> > the larger kernel-generated signal frame, or does the expansion of
> > userspace stack frames play a role too?
> 
> I must say that I do not quite understand this question.
> 
> 32 64-*byte* registers simply need 2048 bytes of storage space worst
> case, there is really no way around that.

If the architecture grows more or bigger registers, and if those
registers are used in general-purpose code, then all stack frames will
tend to grow, not just the signal frame.

So a stack overflow might be caused by the larger signal frame by
itself; or it might be caused by the growth of the stack of 20 function
frames created by someone's signal handler.

In the latter case, this is just a "normal" stack overflow, and nothing
really to do with signals or SIGSTKSZ.  Rebuilding with different
compiler flags could also grow the stack usage and cause just the same
problem.

I also strongly suspect that people often don't think about signal
nesting when allocating signal stacks.  So, there might be a pre-
existing potential overflow that just becomes more likely when the
signal frame grows.  That's not really SIGSTKSZ's fault.


Of course, AVX-512 might never be used in general-purpose code.  On
AArch64, SVE can be used in general-purpose code, but it's too early to
say what its prevalence will be in signal handlers.  Probably low.


> > In practice software just assumes SIGSTKSZ and then ignores the problem
> > until / unless an actual stack overflow is seen.
> >
> > There's probably a lot of software out there whose stack is
> > theoretically too small even without AVX-512 etc. in the mix, especially
> > when considering the possibility of nested signals...
> 
> That is certainly true.  We have seen problems with ntpd, which
> requested a 16 KiB stack, at a time when there were various deductions
> from the stack size, and since the glibc dynamic loader also uses XSAVE,
> ntpd exceeded the remaining stack space.  But in this case, we just
> fudged the stack size computation in pthread_create and made it less
> likely that the dynamic loader was activated, which largely worked
> around this particular problem.  For MINSIGSTKSZ, we just don't have
> this option because it's simply too small in the first place.
> 
> I don't immediately recall a bug due to SIGSTKSZ being too small.  The
> test cases I wrote for this were all artificial, to raise awareness of
> this issue (applications treating these as recommended values, rather
> than minimum value to avoid immediately sigaltstack/phtread_create
> failures, same issue with PTHREAD_STACK_MIN).

Ack, I think if SIGSTKSZ was too small significantly often, there would
be more awareness of the issue.

Cheers
---Dave