+static struct slist *
+gen_load_prism_llprefixlen()
+{
+ struct slist *s1, *s2;
+ struct slist *sjeq_avs_cookie;
+ struct slist *sjcommon;
+
+ /*
+ * This code is not compatible with the optimizer, as
+ * we are generating jmp instructions within a normal
+ * slist of instructions
+ */
+ no_optimize = 1;
+
+ /*
+ * Generate code to load the length of the radio header into
+ * the register assigned to hold that length, if one has been
+ * assigned. (If one hasn't been assigned, no code we've
+ * generated uses that prefix, so we don't need to generate any
+ * code to load it.)
+ *
+ * Some Linux drivers use ARPHRD_IEEE80211_PRISM but sometimes
+ * or always use the AVS header rather than the Prism header.
+ * We load a 4-byte big-endian value at the beginning of the
+ * raw packet data, and see whether, when masked with 0xFFFFF000,
+ * it's equal to 0x80211000. If so, that indicates that it's
+ * an AVS header (the masked-out bits are the version number).
+ * Otherwise, it's a Prism header.
+ *
+ * XXX - the Prism header is also, in theory, variable-length,
+ * but no known software generates headers that aren't 144
+ * bytes long.
+ */
+ if (reg_off_ll != -1) {
+ /*
+ * Load the cookie.
+ */
+ s1 = new_stmt(BPF_LD|BPF_W|BPF_ABS);
+ s1->s.k = 0;
+
+ /*
+ * AND it with 0xFFFFF000.
+ */
+ s2 = new_stmt(BPF_ALU|BPF_AND|BPF_K);
+ s2->s.k = 0xFFFFF000;
+ sappend(s1, s2);
+
+ /*
+ * Compare with 0x80211000.
+ */
+ sjeq_avs_cookie = new_stmt(JMP(BPF_JEQ));
+ sjeq_avs_cookie->s.k = 0x80211000;
+ sappend(s1, sjeq_avs_cookie);
+
+ /*
+ * If it's AVS:
+ *
+ * The 4 bytes at an offset of 4 from the beginning of
+ * the AVS header are the length of the AVS header.
+ * That field is big-endian.
+ */
+ s2 = new_stmt(BPF_LD|BPF_W|BPF_ABS);
+ s2->s.k = 4;
+ sappend(s1, s2);
+ sjeq_avs_cookie->s.jt = s2;
+
+ /*
+ * Now jump to the code to allocate a register
+ * into which to save the header length and
+ * store the length there. (The "jump always"
+ * instruction needs to have the k field set;
+ * it's added to the PC, so, as we're jumping
+ * over a single instruction, it should be 1.)
+ */
+ sjcommon = new_stmt(JMP(BPF_JA));
+ sjcommon->s.k = 1;
+ sappend(s1, sjcommon);
+
+ /*
+ * Now for the code that handles the Prism header.
+ * Just load the length of the Prism header (144)
+ * into the A register. Have the test for an AVS
+ * header branch here if we don't have an AVS header.
+ */
+ s2 = new_stmt(BPF_LD|BPF_W|BPF_IMM);
+ s2->s.k = 144;
+ sappend(s1, s2);
+ sjeq_avs_cookie->s.jf = s2;
+
+ /*
+ * Now allocate a register to hold that value and store
+ * it. The code for the AVS header will jump here after
+ * loading the length of the AVS header.
+ */
+ s2 = new_stmt(BPF_ST);
+ s2->s.k = reg_off_ll;
+ sappend(s1, s2);
+ sjcommon->s.jf = s2;
+
+ /*
+ * Now move it into the X register.
+ */
+ s2 = new_stmt(BPF_MISC|BPF_TAX);
+ sappend(s1, s2);
+
+ return (s1);
+ } else
+ return (NULL);
+}
+
+static struct slist *
+gen_load_avs_llprefixlen()
+{
+ struct slist *s1, *s2;
+
+ /*
+ * Generate code to load the length of the AVS header into
+ * the register assigned to hold that length, if one has been
+ * assigned. (If one hasn't been assigned, no code we've
+ * generated uses that prefix, so we don't need to generate any
+ * code to load it.)
+ */
+ if (reg_off_ll != -1) {
+ /*
+ * The 4 bytes at an offset of 4 from the beginning of
+ * the AVS header are the length of the AVS header.
+ * That field is big-endian.
+ */
+ s1 = new_stmt(BPF_LD|BPF_W|BPF_ABS);
+ s1->s.k = 4;
+
+ /*
+ * Now allocate a register to hold that value and store
+ * it.
+ */
+ s2 = new_stmt(BPF_ST);
+ s2->s.k = reg_off_ll;
+ sappend(s1, s2);
+
+ /*
+ * Now move it into the X register.
+ */
+ s2 = new_stmt(BPF_MISC|BPF_TAX);
+ sappend(s1, s2);
+
+ return (s1);
+ } else
+ return (NULL);
+}
+
projects / libpcap / blobdiff