On 10/8/20 4:09 PM, Jesper Dangaard Brouer wrote:
Multiple BPF-helpers that can manipulate/increase the size of the SKB uses __bpf_skb_max_len() as the max-length. This function limit size against the current net_device MTU (skb->dev->mtu).When a BPF-prog grow the packet size, then it should not be limited to the MTU. The MTU is a transmit limitation, and software receiving this packet should be allowed to increase the size. Further more, current MTU check in __bpf_skb_max_len uses the MTU from ingress/current net_device, which in case of redirects uses the wrong net_device. Keep a sanity max limit of IP6_MAX_MTU (under CONFIG_IPV6) which is 64KiB plus 40 bytes IPv6 header size. If compiled without IPv6 use IP_MAX_MTU. V3: replace __bpf_skb_max_len() with define and use IPv6 max MTU size. Signed-off-by: Jesper Dangaard Brouer <bro...@redhat.com> --- net/core/filter.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) diff --git a/net/core/filter.c b/net/core/filter.c index 05df73780dd3..ddc1f9ba89d1 100644 --- a/net/core/filter.c +++ b/net/core/filter.c @@ -3474,11 +3474,11 @@ static int bpf_skb_net_shrink(struct sk_buff *skb, u32 off, u32 len_diff, return 0; }-static u32 __bpf_skb_max_len(const struct sk_buff *skb)-{ - return skb->dev ? skb->dev->mtu + skb->dev->hard_header_len : - SKB_MAX_ALLOC; -} +#ifdef IP6_MAX_MTU /* Depend on CONFIG_IPV6 */ +#define BPF_SKB_MAX_LEN IP6_MAX_MTU +#else +#define BPF_SKB_MAX_LEN IP_MAX_MTU +#endif
Shouldn't that check on skb->protocol? The way I understand it is that a number of devices including virtual ones use ETH_MAX_MTU as their dev->max_mtu, so the mtu must be in the range of dev->min_mtu(=ETH_MIN_MTU), dev->max_mtu(=ETH_MAX_MTU). __dev_set_mtu() then sets the user value to dev->mtu in the core if within this range. That means in your case skb->dev->hard_header_len for example is left out, meaning if we go for some constant, that would need to be higher.
BPF_CALL_4(sk_skb_adjust_room, struct sk_buff *, skb, s32, len_diff, u32, mode, u64, flags) @@ -3527,7 +3527,7 @@ BPF_CALL_4(bpf_skb_adjust_room, struct sk_buff *, skb, s32, len_diff, { u32 len_cur, len_diff_abs = abs(len_diff); u32 len_min = bpf_skb_net_base_len(skb); - u32 len_max = __bpf_skb_max_len(skb); + u32 len_max = BPF_SKB_MAX_LEN; __be16 proto = skb->protocol; bool shrink = len_diff < 0; u32 off; @@ -3610,7 +3610,7 @@ static int bpf_skb_trim_rcsum(struct sk_buff *skb, unsigned int new_len) static inline int __bpf_skb_change_tail(struct sk_buff *skb, u32 new_len, u64 flags) { - u32 max_len = __bpf_skb_max_len(skb); + u32 max_len = BPF_SKB_MAX_LEN; u32 min_len = __bpf_skb_min_len(skb); int ret;@@ -3686,7 +3686,7 @@ static const struct bpf_func_proto sk_skb_change_tail_proto = {static inline int __bpf_skb_change_head(struct sk_buff *skb, u32 head_room, u64 flags) { - u32 max_len = __bpf_skb_max_len(skb); + u32 max_len = BPF_SKB_MAX_LEN; u32 new_len = skb->len + head_room; int ret;
