Range checking on r1 in function reg_set_seen in arch/s390/net/bpf_jit_comp.c

[Colin Ian King <colin.king@xxxxxxxxxxxxx>]

Hi

Static analysis with cppcheck picked up an interesting issue with the
following inline helper function in arch/s390/net/bpf_jit_comp.c :

static inline void reg_set_seen(struct bpf_jit *jit, u32 b1)
{
        u32 r1 = reg2hex[b1];

        if (!jit->seen_reg[r1] && r1 >= 6 && r1 <= 15)
                jit->seen_reg[r1] = 1;
}

Although I believe r1 is always within range, the range check on r1 is
being performed before the more cache/memory expensive lookup on
jit->seen_reg[r1].  I can't see why the range change is being performed
after the access of jit->seen_reg[r1]. The following seems more correct:

	if (r1 >= 6 && r1 <= 15 && !jit->seen_reg[r1])
                jit->seen_reg[r1] = 1;

..since the check on r1 are less expensive than !jit->seen_reg[r1] and
also the range check ensures the array access is not out of bounds. I
was just wondering if I'm missing something deeper to why the order is
the way it is.

Colin