bpf: Avoid using ARRAY_SIZE on an uninitialized pointer

The cppcheck static code analysis reported the following error:

    if (WARN_ON_ONCE(nest_level > ARRAY_SIZE(bufs->tmp_bufs))) {
                                             ^
ARRAY_SIZE is a macro that expands to sizeofs, so bufs is not actually
dereferenced at runtime, and the code is actually safe. But to keep
things tidy, this patch removes the need for a call to ARRAY_SIZE by
extracting the size of the array into a macro. Cppcheck should no longer
be confused and the code ends up being a bit cleaner.

Fixes: e2d5b2bb ("bpf: Fix nested bpf_bprintf_prepare with more per-cpu buffers")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Florent Revest <revest@chromium.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/bpf/20210517092830.1026418-2-revest@chromium.org

kernel/bpf/helpers.c

@@ -698,8 +698,9 @@ static int bpf_trace_copy_string(char *buf, void *unsafe_ptr, char fmt_ptype,
 #define MAX_BPRINTF_BUF_LEN	512
 
 /* Support executing three nested bprintf helper calls on a given CPU */
+#define MAX_BPRINTF_NEST_LEVEL	3
 struct bpf_bprintf_buffers {
-	char tmp_bufs[3][MAX_BPRINTF_BUF_LEN];
+	char tmp_bufs[MAX_BPRINTF_NEST_LEVEL][MAX_BPRINTF_BUF_LEN];
 };
 static DEFINE_PER_CPU(struct bpf_bprintf_buffers, bpf_bprintf_bufs);
 static DEFINE_PER_CPU(int, bpf_bprintf_nest_level);
@@ -711,7 +712,7 @@ static int try_get_fmt_tmp_buf(char **tmp_buf)
 
 	preempt_disable();
 	nest_level = this_cpu_inc_return(bpf_bprintf_nest_level);
-	if (WARN_ON_ONCE(nest_level > ARRAY_SIZE(bufs->tmp_bufs))) {
+	if (WARN_ON_ONCE(nest_level > MAX_BPRINTF_NEST_LEVEL)) {
 		this_cpu_dec(bpf_bprintf_nest_level);
 		preempt_enable();
 		return -EBUSY;