lib/55829: memmem_oob test case sometimes fails

[gson%gson.org@localhost (Andreas Gustafsson)]

>Number:         55829
>Category:       lib
>Synopsis:       memmem_oob test case sometimes fails
>Confidential:   no
>Severity:       serious
>Priority:       high
>Responsible:    lib-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Fri Nov 27 13:40:00 +0000 2020
>Originator:     Andreas Gustafsson
>Release:        NetBSD-current
>Organization:
>Environment:
System: NetBSD
Architecture: x86_64
Machine: amd64
>Description:

The memmem_oob test case of the lib/libc/string/t_memmem test
sometimes fails with a SIGSEGV.  The failures are rare, but I'm
calling this serious because the test is supposedly testing for
out-of-bounds accesses, and if that is in fact what's happening,
it should be treated as a potential security issue.

There has been two failures so far this year on my testbed, on
real amd64 hardware:

  https://www.gson.org/netbsd/bugs/build/amd64-baremetal/2020/2020.04.24.02.27.59/test.html#lib_libc_string_t_memmem_memmem_oob
  https://www.gson.org/netbsd/bugs/build/amd64-baremetal/2020/2020.11.18.20.00.15/test.html#lib_libc_string_t_memmem_memmem_oob

and one on the TNF testbed, on qemu/nvmm:

  http://releng.netbsd.org/b5reports/amd64/2020/2020.11.20.13.33.07/test.html#lib_libc_string_t_memmem_memmem_oob

>How-To-Repeat:

>Fix: