policycoreutils/secon: fix discarded-qualifiers warning with glibc 2.43

[dustinkirkland]

Problem

In my_getXcon_raw(), ptr is declared as const char * but is
assigned from fgets(), which returns char *. With glibc 2.43,
strchr(const char *, int) now correctly returns const char *
(matching the constness of its input), so the subsequent assignment
to char *tmp triggers a build failure:

secon.c: In function 'my_getXcon_raw':
secon.c:365:18: error: initialization discards 'const' qualifier from
pointer target type [-Werror=discarded-qualifiers]
  365 |         char *tmp = strchr(ptr, '\n');
      |                     ^~~~~~

This breaks builds on any system with glibc 2.43+ and gcc with
-Werror (which the SELinux Makefile uses).

Fix

The const on ptr was always incorrect:

• fgets() returns char * (a mutable pointer into buf, which is
  char[4096])
• The result of strchr() through ptr is immediately used mutably:
  *tmp = 0

Remove the erroneous const qualifier from ptr.

Relation to issue #506

This is the same class of bug reported in #506
(libselinux/src/selinux_config.c:284). That issue covers
strrchr() in selinux_set_policy_root(); this patch covers
strchr() in my_getXcon_raw() in policycoreutils/secon/secon.c.

Both instances have the same root cause: a local variable was
unnecessarily const-qualified, which glibc 2.43's strengthened
strchr/strrchr const-propagation now correctly rejects.

Testing

Verified by building policycoreutils version 3.10 against glibc 2.43
on a Wolfi-based system (Chainguard) where this was a build failure.

Signed-off-by: Dustin Kirkland [email protected]