Make sure filter_pattern can fit in test_filter

[ihilt]

Discovered an issue while compiling with cmake -DCMAKE_BUILD_TYPE=Release,

In function ‘strncpy’,
    inlined from ‘parse_args’ at /rktest/include/rktest/rktest.h:929:4,
    inlined from ‘initialize’ at /rktest/include/rktest/rktest.h:981:27,
/rktest/include/rktest/rktest.h:1188:27:
/usr/include/x86_64-linux-gnu/bits/string_fortified.h:95:10: error: ‘__builtin___strncpy_chk’ specified bound depends on the length of the source argument [-Werror=stringop-truncation]
   95 |   return __builtin___strncpy_chk (__dest, __src, __len,
      |          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   96 |                                   __glibc_objsize (__dest));
      |                                   ~~~~~~~~~~~~~~~~~~~~~~~~~

The third parameter was being determined by the length on the source, which is what this error is complaining about. Since strncpy will copy up to the first NULL byte in the source, copying to the destination size is safer, and I think expected by strncpy.

This also means we should only copy up to one less than the destination size since strncpy won't NULL-terminate the destination for us if the source doesn't have one. I handle this by adding a local variable that is one less than RKTEST_MAX_FILTER_LENGTH.

[Warwolt]

Thanks for the pull request! I think we can probably keep it pretty succinct.

[Warwolt]

Suggested change

	const size_t max_filter_len = RKTEST_MAX_FILTER_LENGTH - 1;
	if (filter_len > max_filter_len) {
	fprintf(stderr, "Error: filter pattern too long. Max length is (%lu)", max_filter_len);
	if (strlen(filter_pattern) > RKTEST_MAX_FILTER_LENGTH - 1) {
	fprintf(stderr, "Error: filter pattern too long. Max length is (%lu)", RKTEST_MAX_FILTER_LENGTH - 1));

[Warwolt]

Suggested change

	// strncpy will copy all bytes up to a NULL in the source or until it
	// reaches the size in the third param.
	//
	// This means if the source string is equal to or greater than the length,
	// it will not add a NULL to the end of the destination.
	//
	// Examples from the strncpy man page are useful,
	//
	// strncpy(buf, "1", 5); // { '1', 0, 0, 0, 0 }
	// strncpy(buf, "1234", 5); // { '1', '2', '3', '4', 0 }
	// strncpy(buf, "12345", 5); // { '1', '2', '3', '4', '5' }
	// strncpy(buf, "123456", 5); // { '1', '2', '3', '4', '5' }
	//
	// So if test_filter is a char[256], then max_filter_len will be 255 and if
	// filter_pattern is also 255, then it will have one character left which
	// strncpy will make a NULL.
	strncpy(config.test_filter, filter_pattern, max_filter_len);
	strncpy(config.test_filter, filter_pattern, RKTEST_MAX_FILTER_LENGTH - 1);