Use countof() where appropriate, and do it implicitly where possible

[alejandro-colomar]

This improves bounds safety by using countof(3) where appropriate, and doing it implicitly where possible.

Cc: @kees

---

Revisions:

v1b

• Update lib/string/README.

$ git rd 
1:  91814a70 = 1:  91814a70 lib/, src/, tests/: Use countof() or sizeof_a() where appropriate
2:  35962749 = 2:  35962749 lib/salt.c: crypt_make_salt(): Use countof() instead of a hardcoded value
3:  b53f7325 = 3:  b53f7325 lib/sizeof.h: endof(): Add macro
4:  f355a1b4 = 4:  f355a1b4 lib/fields.[ch]: change_field_a(): Add macro
5:  cd6d3c42 = 5:  cd6d3c42 src/: Use change_field_a() instead of its pattern
6:  39e963ed ! 6:  b5e41a06 lib/string/memset/: bzero_a(): Add macro
    @@ lib/Makefile.am: libshadow_la_SOURCES = \
        string/memset/memzero.h \
        string/sprintf/aprintf.c \
     
    + ## lib/string/README ##
    +@@ lib/string/README: ctype/ - Character classification and conversion functions
    + 
    + memset/ - Memory zeroing
    + 
    ++    bzero_a()
    ++  Like bzero(3), but takes an array.
    ++
    +     memzero()
    +   Synonym of explicit_bzero(3).
    +     memzero_a()
    +
      ## lib/string/memset/bzero.c (new) ##
     @@
     +// SPDX-FileCopyrightText: 2024, Alejandro Colomar <alx@kernel.org>
7:  b02d91a9 = 7:  210f91ef lib/: Use bzero_a() instead of its pattern
8:  077542bd = 8:  dd2262a8 lib/audit_help.c: Use countof() instead of sizeof()

v2

• Consistently use sizeof_a() with read(2).

$ git rd 
1:  91814a70 ! 1:  580ccf69 lib/, src/, tests/: Use countof() or sizeof_a() where appropriate
    @@ src/newgidmap.c: static void write_setgroups(int proc_dir_fd, bool allow_setgrou
         * fail.
         */
     -  if (read(setgroups_fd, policy_buffer, sizeof(policy_buffer)) < 0) {
    -+  if (read(setgroups_fd, policy_buffer, countof(policy_buffer)) < 0) {
    ++  if (read(setgroups_fd, policy_buffer, sizeof_a(policy_buffer)) < 0) {
                fprintf(stderr, _("%s: failed to read setgroups: %s\n"),
                        Prog, strerrno());
                exit(EXIT_FAILURE);
2:  35962749 = 2:  3805154d lib/salt.c: crypt_make_salt(): Use countof() instead of a hardcoded value
3:  b53f7325 = 3:  aa6a3dcd lib/sizeof.h: endof(): Add macro
4:  f355a1b4 = 4:  d29e1cac lib/fields.[ch]: change_field_a(): Add macro
5:  cd6d3c42 = 5:  524beee5 src/: Use change_field_a() instead of its pattern
6:  b5e41a06 = 6:  2d4f61d4 lib/string/memset/: bzero_a(): Add macro
7:  210f91ef = 7:  68da424c lib/: Use bzero_a() instead of its pattern
8:  dd2262a8 = 8:  8ae2c113 lib/audit_help.c: Use countof() instead of sizeof()