bpf, verifier: Track when data_meta pointer is loaded

Introduce PA_F_DATA_META_LOAD flag to track when a BPF program loads the
skb->data_meta pointer.

This information will be used by gen_prologue() to handle cases where there
is a gap between metadata end and skb->data, requiring metadata to be
realigned.

Signed-off-by: Jakub Sitnicki <[email protected]>

Author: jsitnicki

include/linux/bpf_verifier.h

@@ -639,6 +639,7 @@ enum priv_stack_mode {
 
 enum packet_access_flags {
 	PA_F_DIRECT_WRITE = BIT(0),
+	PA_F_DATA_META_LOAD = BIT(1),
 };
 
 struct bpf_subprog_info {

kernel/bpf/verifier.c

@@ -6160,6 +6160,10 @@ static int check_ctx_access(struct bpf_verifier_env *env, int insn_idx, int off,
 		} else {
 			env->insn_aux_data[insn_idx].ctx_field_size = info->ctx_field_size;
 		}
+
+		if (base_type(info->reg_type) == PTR_TO_PACKET_META)
+			env->seen_packet_access |= PA_F_DATA_META_LOAD;
+
 		/* remember the offset of last byte accessed in ctx */
 		if (env->prog->aux->max_ctx_offset < off + size)
 			env->prog->aux->max_ctx_offset = off + size;