BPF indexed byte load can sign extend

classic Classic list List threaded Threaded
1 message Options
Reply | Threaded
Open this post in threaded view
|

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;
                                int len;

                                if (buflen != 0)
                                        return 0;
                                m = (struct mbuf *)p;
                                MINDEX(len, m, k);
                                A = mtod(m, u_char *)[k];
                                continue;
#else
                                return 0;
#endif
                        }
                        A = p[k];
                        continue;

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 (k >= buflen) {
#ifdef _KERNEL
                                struct mbuf *m;
                                int len;

                                if (buflen != 0)
                                        return 0;
                                m = (struct mbuf *)p;
                                MINDEX(len, m, k);
                                A = mtod(m, char *)[k];
                                continue;
#else
                                return 0;
#endif
                        }
                        A = p[k];
                        continue;

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 NetBSD, it's "u_char" in both "mtod()" calls, fixing that.

A patch to fix this is attached.
Index: bpf_filter.c
===================================================================
RCS file: /cvs/src/sys/net/bpf_filter.c,v
retrieving revision 1.15
diff -c -r1.15 bpf_filter.c
*** bpf_filter.c 28 Sep 2005 20:53:56 -0000 1.15
--- bpf_filter.c 30 Nov 2005 07:58:29 -0000
***************
*** 301,307 ****
  return 0;
  m = (struct mbuf *)p;
  MINDEX(len, m, k);
! A = mtod(m, char *)[k];
  continue;
  #else
  return 0;
--- 301,307 ----
  return 0;
  m = (struct mbuf *)p;
  MINDEX(len, m, k);
! A = mtod(m, u_char *)[k];
  continue;
  #else
  return 0;