[Bug 2001932] Re: segfault in strncmp for avx2 at page boundaries

[Simon Chopin]

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

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Title:
  segfault in strncmp for avx2 at page boundaries

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]

  Depending on size and location of the compared buffers in memory,
  particularly at the end of their respective pages, the AVX-2
  specialized code for strncmp has an off-by-one bug that can cause a
  segfault.

  See https://sourceware.org/bugzilla/show_bug.cgi?id=25933

  [Test case]

  > test_strncmp.c cat <<EOF
  #include <sys/mman.h>
  #include <string.h>
  #include <stdio.h>

  #define PAGE_SIZE 4096
  #define VEC_SIZE 32

  int main()
  {
  	int ret;
  	char *s1 = (char *)mmap(0, PAGE_SIZE*2, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
  	char *s2 = (char *)mmap(0, PAGE_SIZE*2, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0);
  	mprotect(s1+PAGE_SIZE, PAGE_SIZE, PROT_NONE);
  	mprotect(s2+PAGE_SIZE, PAGE_SIZE, PROT_NONE);
  	memset(s1, 'a', PAGE_SIZE);
  	memset(s2, 'a', PAGE_SIZE);
  	s1[PAGE_SIZE-1] = 0;
  	ret = strncmp(
              s1+PAGE_SIZE-VEC_SIZE*4-1,
              s2+PAGE_SIZE-VEC_SIZE*4,
              VEC_SIZE*4);
  	printf("strncmp returned %d\n", ret);
  	return ret;
  }
  EOF
  gcc -o test_strncmp test_strncmp.c
  ./test_strncmp
  # On buggy systems (e.g. mine), that last call segfaults

  [Regression potential]

  The fix could introduce another bug in the routine, and/or a
  performance regression.

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