Various NULL pointer warnings with '%s' format in libaccfg.c when compiling with clearlinux build options

[ColinIanKing]

Hi,

When building idxd-config on ClearLinux I noticed some warnings when using gcc-13

Tool chain: gcc 13.2.1
The build flags that trigger this are: CC=gcc-13 CFLAGS="-Wformat-security -O3" ./configure

seems that the -O3 flag triggers more code analysis and triggers the warnings

libaccfg.c: In function 'accfg_group_set_traffic_class_a':
libaccfg.c:1736:48: warning: '%s' directive argument is null [-Wformat-overflow=]
 1736 |                 rc = sprintf(group->group_buf, "%s/%s", \
      |                                                ^~~~~~~
libaccfg.c:1762:1: note: in expansion of macro 'accfg_group_set_field'
 1762 | accfg_group_set_field(group, val, traffic_class_a)
      | ^~~~~~~~~~~~~~~~~~~~~
libaccfg.c:1736:52: note: format string is defined here
 1736 |                 rc = sprintf(group->group_buf, "%s/%s", \
      |                                                    ^~
libaccfg.c: In function 'accfg_group_set_traffic_class_b':
libaccfg.c:1736:48: warning: '%s' directive argument is null [-Wformat-overflow=]
 1736 |                 rc = sprintf(group->group_buf, "%s/%s", \
      |                                                ^~~~~~~
libaccfg.c:1763:1: note: in expansion of macro 'accfg_group_set_field'
 1763 | accfg_group_set_field(group, val, traffic_class_b)
      | ^~~~~~~~~~~~~~~~~~~~~
libaccfg.c:1736:52: note: format string is defined here
 1736 |                 rc = sprintf(group->group_buf, "%s/%s", \
      |                                                    ^~
libaccfg.c: In function 'accfg_group_set_desc_progress_limit':
libaccfg.c:1736:48: warning: '%s' directive argument is null [-Wformat-overflow=]
 1736 |                 rc = sprintf(group->group_buf, "%s/%s", \
      |                                                ^~~~~~~
libaccfg.c:1764:1: note: in expansion of macro 'accfg_group_set_field'
 1764 | accfg_group_set_field(group, val, desc_progress_limit)
      | ^~~~~~~~~~~~~~~~~~~~~
libaccfg.c:1736:52: note: format string is defined here
 1736 |                 rc = sprintf(group->group_buf, "%s/%s", \
      |                                                    ^~
libaccfg.c: In function 'accfg_group_set_batch_progress_limit':
libaccfg.c:1736:48: warning: '%s' directive argument is null [-Wformat-overflow=]
 1736 |                 rc = sprintf(group->group_buf, "%s/%s", \
      |                                                ^~~~~~~
libaccfg.c:1765:1: note: in expansion of macro 'accfg_group_set_field'
 1765 | accfg_group_set_field(group, val, batch_progress_limit)
      | ^~~~~~~~~~~~~~~~~~~~~
libaccfg.c:1736:52: note: format string is defined here
 1736 |                 rc = sprintf(group->group_buf, "%s/%s", \

The reason for this is that deprecated_attr() can potentially return NULL (which it probably doesn't but the compiler believes it's a possibility):

static const char *deprecated_attr(char *attr)
{
        int i;

        for (i = 0; i < (int) ARRAY_SIZE(attr_dict); i++)
                if (!strcmp(attr, attr_dict[i].key))
                        return attr_dict[i].val;

        return NULL;
}

I guess NULL could be replaced with a known invalid string instead, or "".