* 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 -- Red Hat GmbH, https://de.redhat.com/ , Registered seat: Grasbrunn, Commercial register: Amtsgericht Muenchen, HRB 153243, Managing Directors: Charles Cachera, Brian Klemm, Laurie Krebs, Michael O'Neill
