bpf: allow option for setting bpf_l4_csum_replace from scratch

When programs need to calculate the csum from scratch for small UDP
packets and use bpf_l4_csum_replace() to feed the result from helpers
like bpf_csum_diff(), then we need a flag besides BPF_F_MARK_MANGLED_0
that would ignore the case of current csum being 0, and which would
still allow for the helper to set the csum and transform when needed
to CSUM_MANGLED_0.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/bpf.h

@@ -522,6 +522,7 @@ enum bpf_func_id {
 /* BPF_FUNC_l4_csum_replace flags. */
 #define BPF_F_PSEUDO_HDR		(1ULL << 4)
 #define BPF_F_MARK_MANGLED_0		(1ULL << 5)
+#define BPF_F_MARK_ENFORCE		(1ULL << 6)
 
 /* BPF_FUNC_clone_redirect and BPF_FUNC_redirect flags. */
 #define BPF_F_INGRESS			(1ULL << 0)

net/core/filter.c

@@ -1522,10 +1522,11 @@ BPF_CALL_5(bpf_l4_csum_replace, struct sk_buff *, skb, u32, offset,
 {
 	bool is_pseudo = flags & BPF_F_PSEUDO_HDR;
 	bool is_mmzero = flags & BPF_F_MARK_MANGLED_0;
+	bool do_mforce = flags & BPF_F_MARK_ENFORCE;
 	__sum16 *ptr;
 
-	if (unlikely(flags & ~(BPF_F_MARK_MANGLED_0 | BPF_F_PSEUDO_HDR |
-			       BPF_F_HDR_FIELD_MASK)))
+	if (unlikely(flags & ~(BPF_F_MARK_MANGLED_0 | BPF_F_MARK_ENFORCE |
+			       BPF_F_PSEUDO_HDR | BPF_F_HDR_FIELD_MASK)))
 		return -EINVAL;
 	if (unlikely(offset > 0xffff || offset & 1))
 		return -EFAULT;
@@ -1533,7 +1534,7 @@ BPF_CALL_5(bpf_l4_csum_replace, struct sk_buff *, skb, u32, offset,
 		return -EFAULT;
 
 	ptr = (__sum16 *)(skb->data + offset);
-	if (is_mmzero && !*ptr)
+	if (is_mmzero && !do_mforce && !*ptr)
 		return 0;
 
 	switch (flags & BPF_F_HDR_FIELD_MASK) {