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

[Florian Weimer]

* 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.

> 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).

Thanks,
Florian
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