bpf: Replace deprecated strncpy with strscpy

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

bpf sym names get looked up and compared/cleaned with various string
apis. This suggests they need to be NUL-terminated (strncpy() suggests
this but does not guarantee it).

|	static int compare_symbol_name(const char *name, char *namebuf)
|	{
|		cleanup_symbol_name(namebuf);
|		return strcmp(name, namebuf);
|	}

|	static void cleanup_symbol_name(char *s)
|	{
|		...
|		res = strstr(s, ".llvm.");
|		...
|	}

Use strscpy() as this method guarantees NUL-termination on the
destination buffer.

This patch also replaces two uses of strncpy() used in log.c. These are
simple replacements as postfix has been zero-initialized on the stack
and has source arguments with a size less than the destination's size.

Note that this patch uses the new 2-argument version of strscpy
introduced in commit e6584c39 ("string: Allow 2-argument strscpy()").
Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: https://github.com/KSPP/linux/issues/90
Link: https://lore.kernel.org/bpf/20240402-strncpy-kernel-bpf-core-c-v1-1-7cb07a426e78@google.com

kernel/bpf/core.c

@@ -747,7 +747,7 @@ const char *__bpf_address_lookup(unsigned long addr, unsigned long *size,
 		unsigned long symbol_start = ksym->start;
 		unsigned long symbol_end = ksym->end;
 
-		strncpy(sym, ksym->name, KSYM_NAME_LEN);
+		strscpy(sym, ksym->name, KSYM_NAME_LEN);
 
 		ret = sym;
 		if (size)
@@ -813,7 +813,7 @@ int bpf_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
 		if (it++ != symnum)
 			continue;
 
-		strncpy(sym, ksym->name, KSYM_NAME_LEN);
+		strscpy(sym, ksym->name, KSYM_NAME_LEN);
 
 		*value = ksym->start;
 		*type  = BPF_SYM_ELF_TYPE;

kernel/bpf/log.c

@@ -467,9 +467,9 @@ const char *reg_type_str(struct bpf_verifier_env *env, enum bpf_reg_type type)
 
 	if (type & PTR_MAYBE_NULL) {
 		if (base_type(type) == PTR_TO_BTF_ID)
-			strncpy(postfix, "or_null_", 16);
+			strscpy(postfix, "or_null_");
 		else
-			strncpy(postfix, "_or_null", 16);
+			strscpy(postfix, "_or_null");
 	}
 
 	snprintf(prefix, sizeof(prefix), "%s%s%s%s%s%s%s",