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

On Tue, Oct 06, 2020 at 11:30:42AM -0700, Dave Hansen wrote:

On 10/6/20 10:00 AM, Dave Martin wrote:

quoted

On Tue, Oct 06, 2020 at 08:33:47AM -0700, Dave Hansen wrote:

quoted

On 10/6/20 8:25 AM, Dave Martin wrote:

quoted

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?

The kernel-generated signal frame is entirely responsible for the ~2800
bytes that I'm talking about.

I'm sure there are some systems where userspace plays a role, but those
are much less of a worry at the moment, since the kernel-induced
overflows mean an instant crash that userspace has no recourse for.

Ack, that sounds pretty convincing.

Cheers
---Dave