BPF indexed byte load can sign extend

[Guy Harris]

The code to handle BPF non-indexed loads in bpf_filter() in bpf_filter.c is:

		case BPF_LD|BPF_B|BPF_ABS:
			k = pc->k;
			if (k > buflen) {
#ifdef _KERNEL
				struct mbuf *m;
				if (buflen != 0)
					return 0;
				m = (struct mbuf *)p;
				MINDEX(m, k);
				A = mtod(m, u_char *)[k];
				continue;
#else
				return 0;
			}
which will load the byte at the offset in the instruction into the 
accumulator, zero-extending it.

The code to handle indexed loads is

		case BPF_LD|BPF_B|BPF_IND:
			k = X + pc->k;
			if (pc->k >= buflen || X >= buflen - pc->k) {
#ifdef _KERNEL
				struct mbuf *m;
				if (buflen != 0)
					return 0;
				m = (struct mbuf *)p;
				MINDEX(m, k);
				A = mtod(m, char *)[k];
				continue;
#else
				return 0;
			}
which will load the byte at the offset that's the sum of the offset in 
the instruction and the contents of the index register; if buflen is 0, 
as is the case when this is called from bpf_mtap(), it will sign-extend 
it on most if not all platforms, as the mbuf data pointer is cast to 
"char *" rather than "u_char *". Otherwise, as is the case when this is 
called from bpf_tap(), it'll zero-extend it, as "p" is a "u_char *".

Presumably those semantics are not intended, as the same instruction 
behaves differently depending on how the tapping is being done, and on 
whether the load is indexed or not. (In libpcap's user-mode filter, it's 
never sign-extended.)

In current NetBSD, OpenBSD, and FreeBSD, it's "u_char" in both "mtod()" 
calls, fixing that.

Here's a patch:

*** bpf_filter.c	Tue Jan  1 18:46:19 2008
--- bpf_filter.c.UNSIGNED	Tue Jan  1 18:44:03 2008
***************
*** 326,332 ****
  					return 0;
  				m = (struct mbuf *)p;
  				MINDEX(m, k);
! 				A = mtod(m, char *)[k];
  				continue;
  #else
  				return 0;
--- 326,332 ----
  					return 0;
  				m = (struct mbuf *)p;
  				MINDEX(m, k);
! 				A = mtod(m, u_char *)[k];
  				continue;
  #else
  				return 0;