Subject: Re: [PATCH v2 4/5] bpf: allow new DECAP flags and add guard rails

From: Willem de Bruijn <[email-masked]>
Date: Thu, 19 Mar 2026 09:24:39 -0400

Hudson, Nick wrote:
> 
> 
> > On 18 Mar 2026, at 20:02, Willem de Bruijn <[email-masked]> wrote:
> > 
> > !-------------------------------------------------------------------|
> >  This Message Is From an External Sender
> >  This message came from outside your organization.
> > |-------------------------------------------------------------------!
> > 
> > Nick Hudson wrote:
> >> Add checks to require shrink-only decap, reject conflicting decap flag
> >> combinations, and verify removed length is sufficient for claimed header
> >> decapsulation.
> >> 
> >> Co-developed-by: Max Tottenham <[email-masked]>
> >> Signed-off-by: Max Tottenham <[email-masked]>
> >> Co-developed-by: Anna Glasgall <[email-masked]>
> >> Signed-off-by: Anna Glasgall <[email-masked]>
> >> Signed-off-by: Nick Hudson <[email-masked]>
> >> ---
> >> net/core/filter.c | 47 +++++++++++++++++++++++++++++++++++++----------
> >> 1 file changed, 37 insertions(+), 10 deletions(-)
> >> 
> >> diff --git a/net/core/filter.c b/net/core/filter.c
> >> index 7c2871b40fe4..47aec44a9cd3 100644
> >> --- a/net/core/filter.c
> >> +++ b/net/core/filter.c
> >> @@ -56,6 +56,7 @@
> >> #include <net/sock_reuseport.h>
> >> #include <net/busy_poll.h>
> >> #include <net/tcp.h>
> >> +#include <net/gre.h>
> >> #include <net/xfrm.h>
> >> #include <net/udp.h>
> >> #include <linux/bpf_trace.h>
> >> @@ -3496,7 +3497,9 @@ static u32 bpf_skb_net_base_len(const struct sk_buff *skb)
> >> BPF_F_ADJ_ROOM_ENCAP_L2( \
> >>  BPF_ADJ_ROOM_ENCAP_L2_MASK))
> >> 
> >> -#define BPF_F_ADJ_ROOM_DECAP_MASK (BPF_F_ADJ_ROOM_DECAP_L3_MASK)
> >> +#define BPF_F_ADJ_ROOM_DECAP_MASK (BPF_F_ADJ_ROOM_DECAP_L3_MASK | \
> >> + BPF_F_ADJ_ROOM_DECAP_L4_MASK | \
> >> + BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)
> >> 
> >> #define BPF_F_ADJ_ROOM_MASK (BPF_F_ADJ_ROOM_FIXED_GSO | \
> >> BPF_F_ADJ_ROOM_ENCAP_MASK | \
> >> @@ -3743,20 +3746,44 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff,
> >> return -ENOTSUPP;
> >> }
> >> 
> >> - if (flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) {
> >> + if (flags & BPF_F_ADJ_ROOM_DECAP_MASK) {
> >> + u32 len_decap_min = 0;
> >> +
> >> if (!shrink)
> >> return -EINVAL;
> >> 
> >> - switch (flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) {
> >> - case BPF_F_ADJ_ROOM_DECAP_L3_IPV4:
> >> + /* Reject mutually exclusive decap flag pairs. */
> >> + if ((flags & BPF_F_ADJ_ROOM_DECAP_L3_MASK) ==
> >> +    BPF_F_ADJ_ROOM_DECAP_L3_MASK)
> >> + return -EINVAL;
> >> +
> >> + if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) ==
> >> +    BPF_F_ADJ_ROOM_DECAP_L4_MASK)
> >> + return -EINVAL;
> >> +
> >> + if ((flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK) ==
> >> +    BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK)
> >> + return -EINVAL;
> >> +
> >> + /* Reject mutually exclusive decap tunnel type flags. */
> >> + if ((flags & BPF_F_ADJ_ROOM_DECAP_L4_MASK) &&
> >> +    (flags & BPF_F_ADJ_ROOM_DECAP_IPXIP_MASK))
> >> + return -EINVAL;
> >> +
> >> + if (flags & BPF_F_ADJ_ROOM_DECAP_L4_UDP)
> >> + len_decap_min += sizeof(struct udphdr);
> >> +
> >> + if (flags & BPF_F_ADJ_ROOM_DECAP_L4_GRE)
> >> + len_decap_min += sizeof(struct gre_base_hdr);
> >> +
> >> + if (len_diff_abs < len_decap_min)
> >> + return -EINVAL;
> > 
> > Should this test come after the below IP flags?
> 
> Should it?
> 
> Seems to me it can bail early without having to check the IP flags. len_decap_min vs len_min.
> 
> What am I missing?

I would think it common that UDP decap also includes an L3 decap, in
which case the len_decap_min should include both header lengths.