[Bug 2001975] Re: overflow bug in wcsncmp_avx2

[Simon Chopin]

Verified in a fresh container:

root at focal-glibc:~# ./test_wcsncmp
root at focal-glibc:~# echo $?
0
root at focal-glibc:~# dpkg -l libc6
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name           Version          Architecture Description
+++-==============-================-============-=================================
ii  libc6:amd64    2.31-0ubuntu9.12 amd64        GNU C Library: Shared libraries

** Tags removed: verification-needed verification-needed-focal
** Tags added: verification-done verification-done-focal

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https://bugs.launchpad.net/bugs/2001975

Title:
  overflow bug in wcsncmp_avx2

Status in GLibC:
  Fix Released
Status in glibc package in Ubuntu:
  Fix Released
Status in glibc source package in Focal:
  Fix Committed

Bug description:
  [Impact]

  See https://sourceware.org/bugzilla/show_bug.cgi?id=28755

  Note that we're only impacted by the avx2 issue, the evex-optimized
  version isn't present in the 2.31 branch.

  [Test case]

  > test_wcsncmp.c cat <<EOF
  #include <wchar.h>
  #include <assert.h>

  int
  main(int argc, char ** argv) {
      assert(__wcsncmp_avx2(L"abc", L"abd", (1UL << 62)) != 0);
  }
  EOF
  gcc -static -o test_wcsncmp test_wcsncmp.c
  ./test_scsncmp

  [Regression potential]

  The patch is contained within the AVX-2 optimized routine, but it
  could still cause introduce a new bug there. In addition, we could see
  performance regressions.

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