[PATCH bpf-next 04/13] bpf: Support bitfield read access in btf_struct_access

[Martin KaFai Lau <kafai@fb.com>]

This patch allows bitfield access as a scalar.  It currently limits
the access to sizeof(u64) and upto the end of the struct.  It is needed
in a later bpf-tcp-cc example that reads bitfield from
inet_connection_sock and tcp_sock.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
---
 kernel/bpf/btf.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 6e652643849b..011194831499 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3744,10 +3744,6 @@ int btf_struct_access(struct bpf_verifier_log *log,
 	}
 
 	for_each_member(i, t, member) {
-		if (btf_member_bitfield_size(t, member))
-			/* bitfields are not supported yet */
-			continue;
-
 		/* offset of the field in bytes */
 		moff = btf_member_bit_offset(t, member) / 8;
 		if (off + size <= moff)
@@ -3757,6 +3753,15 @@ int btf_struct_access(struct bpf_verifier_log *log,
 		if (off < moff)
 			continue;
 
+		if (btf_member_bitfield_size(t, member)) {
+			if (off == moff &&
+			    !(btf_member_bit_offset(t, member) % 8) &&
+			    size <= sizeof(u64) &&
+			    off + size <= t->size)
+				return SCALAR_VALUE;
+			continue;
+		}
+
 		/* type of the field */
 		mtype = btf_type_by_id(btf_vmlinux, member->type);
 		mname = __btf_name_by_offset(btf_vmlinux, member->name_off);
-- 
2.17.1