Fix BPF verifier rejection in push__bytebuf on older kernels

Widen len_to_read to unsigned long and mask with & 0xFFFF before
bpf_probe_read calls. Older verifiers lose uint16_t bounds through
32-bit register moves. Reverts per-file snaplen masks as they
addressed the wrong level.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Stringy

driver/modern_bpf/helpers/base/push_data.h

@@ -264,15 +264,22 @@ static __always_inline uint16_t push__bytebuf(uint8_t *data,
                                               unsigned long bytebuf_pointer,
                                               uint16_t len_to_read,
                                               enum read_memory mem) {
+	/* Older BPF verifiers (RHEL 8 4.18, COS 6.6) lose uint16_t bounds
+	 * through 32-bit register moves (w2 = w8), causing "R2 unbounded
+	 * memory access". Widen to unsigned long and mask to force clang
+	 * to emit an AND instruction the verifier can track.
+	 */
+	unsigned long safe_len = len_to_read;
+	safe_len &= 0xFFFF;
 	if(mem == KERNEL) {
 		if(bpf_probe_read_kernel(&data[SAFE_ACCESS(*payload_pos)],
-		                         len_to_read,
+		                         safe_len,
 		                         (void *)bytebuf_pointer) != 0) {
 			return 0;
 		}
 	} else {
 		if(bpf_probe_read_user(&data[SAFE_ACCESS(*payload_pos)],
-		                       len_to_read,
+		                       safe_len,
 		                       (void *)bytebuf_pointer) != 0) {
 			return 0;
 		}

driver/modern_bpf/programs/tail_called/events/syscall_dispatched_events/recvfrom.bpf.c

@@ -42,7 +42,7 @@ int BPF_PROG(recvfrom_x, struct pt_regs *regs, long ret) {
 		uint16_t snaplen = maps__get_snaplen();
 		apply_dynamic_snaplen(regs, &snaplen, &snaplen_args);
 		if(snaplen > ret) {
-			snaplen = ret & 0xFFFF;
+			snaplen = ret;
 		}
 
 		/* Parameter 2: data (type: PT_BYTEBUF) */

driver/modern_bpf/programs/tail_called/events/syscall_dispatched_events/recvmsg.bpf.c

@@ -46,7 +46,7 @@ int BPF_PROG(recvmsg_x, struct pt_regs *regs, long ret) {
 		uint16_t snaplen = maps__get_snaplen();
 		apply_dynamic_snaplen(regs, &snaplen, &snaplen_args);
 		if(snaplen > ret) {
-			snaplen = ret & 0xFFFF;
+			snaplen = ret;
 		}
 
 		/* Parameter 3: data (type: PT_BYTEBUF) */

driver/modern_bpf/programs/tail_called/events/syscall_dispatched_events/sendmsg.bpf.c

@@ -59,7 +59,7 @@ int BPF_PROG(sendmsg_x, struct pt_regs *regs, long ret) {
 		};
 		apply_dynamic_snaplen(regs, &snaplen, &snaplen_args);
 		if(ret > 0 && snaplen > ret) {
-			snaplen = ret & 0xFFFF;
+			snaplen = ret;
 		}
 
 		unsigned long iov_pointer = (unsigned long)msghdr.msg_iov;