Re: [RFC bpf-next 2/7] bpf: extend bpf_pcap support to tracing programs

[Yonghong Song <yhs@fb.com>]

On 9/7/19 2:40 PM, Alan Maguire wrote:
> packet capture is especially valuable in tracing contexts, so
> extend bpf_pcap helper to take a tracing-derived skb pointer
> as an argument.
> 
> In the case of tracing programs, the starting protocol
> (corresponding to libpcap DLT_* values; 1 for Ethernet, 12 for
> IP, etc) needs to be specified and should reflect the protocol
> type which is pointed to by the skb->data pointer; i.e. the
> start of the packet.  This can derived in a limited set of cases,
> but should be specified where possible.  For skb and xdp programs
> this protocol will nearly always be 1 (BPF_PCAP_TYPE_ETH).
> 
> Example usage for a tracing program, where we use a
> struct bpf_pcap_hdr array map to pass in preferences for
> protocol and max len:
> 
> struct bpf_map_def SEC("maps") pcap_conf_map = {
> 	.type = BPF_MAP_TYPE_ARRAY,
> 	.key_size = sizeof(int),
> 	.value_size = sizeof(struct bpf_pcap_hdr),
> 	.max_entries = 1,
> };
> 
> struct bpf_map_def SEC("maps") pcap_map = {
> 	.type = BPF_MAP_TYPE_PERF_EVENT_ARRAY,
> 	.key_size = sizeof(int),
> 	.value_size = sizeof(int),
> 	.max_entries = 1024,
> };
> 
> SEC("kprobe/kfree_skb")
> int probe_kfree_skb(struct pt_regs *ctx)
> {
> 	struct bpf_pcap_hdr *conf;
> 	int key = 0;
> 
> 	conf = bpf_map_lookup_elem(&pcap_conf_map, &key);
> 	if (!conf)
> 		return 0;
> 
> 	bpf_pcap((void *)PT_REGS_PARM1(ctx), conf->cap_len, &pcap_map,
> 		 conf->protocol, BPF_F_CURRENT_CPU);
> 	return 0;
> }
> 
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> ---
[...]
> @@ -2977,6 +2992,8 @@ enum bpf_func_id {
>   
>   /* BPF_FUNC_pcap flags */
>   #define	BPF_F_PCAP_ID_MASK		0xffffffffffff
> +#define BPF_F_PCAP_ID_IIFINDEX		(1ULL << 48)
> +#define BPF_F_PCAP_STRICT_TYPE         (1ULL << 56)
>   
>   /* Mode for BPF_FUNC_skb_adjust_room helper. */
>   enum bpf_adj_room_mode {
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index ca1255d..311883b 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -13,6 +13,8 @@
>   #include <linux/kprobes.h>
>   #include <linux/syscalls.h>
>   #include <linux/error-injection.h>
> +#include <linux/skbuff.h>
> +#include <linux/ip.h>
>   
>   #include <asm/tlb.h>
>   
> @@ -530,6 +532,216 @@ u64 bpf_event_output(struct bpf_map *map, u64 flags, void *meta, u64 meta_size,
>   	return __bpf_perf_event_output(regs, map, flags, sd);
>   }
>   
> +/* Essentially just skb_copy_bits() using probe_kernel_read() where needed. */
> +static unsigned long bpf_trace_skb_copy(void *tobuf, const void *from,
> +					unsigned long offset,
> +					unsigned long len)
> +{
> +	const struct sk_buff *frag_iterp, *skb = from;
> +	struct skb_shared_info *shinfop, shinfo;
> +	struct sk_buff frag_iter;
> +	unsigned long copy, start;
> +	void *to = tobuf;
> +	int i, ret;
> +
> +	start = skb_headlen(skb);
> +
> +	copy = start - offset;
> +	if (copy > 0) {
> +		if (copy > len)
> +			copy = len;
> +		ret = probe_kernel_read(to, skb->data, copy);
> +		if (unlikely(ret < 0))
> +			goto out;
> +		len -= copy;
> +		if (len == 0)
> +			return 0;
> +		offset += copy;
> +		to += copy;
> +	}
> +
> +	if (skb->data_len == 0)
> +		goto out;
> +
> +	shinfop = skb_shinfo(skb);
> +
> +	ret = probe_kernel_read(&shinfo, shinfop, sizeof(shinfo));
> +	if (unlikely(ret < 0))
> +		goto out;
> +
> +	if (shinfo.nr_frags > MAX_SKB_FRAGS) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +	for (i = 0; i < shinfo.nr_frags; i++) {
> +		skb_frag_t *f = &shinfo.frags[i];
> +		int end;
> +
> +		if (start > offset + len) {
> +			ret = -E2BIG;
> +			goto out;
> +		}
> +
> +		end = start + skb_frag_size(f);
> +		copy = end - offset;
> +		if (copy > 0) {
> +			u32 poff, p_len, copied;
> +			struct page *p;
> +			u8 *vaddr;
> +
> +			if (copy > len)
> +				copy = len;
> +
> +			skb_frag_foreach_page(f,
> +					      skb_frag_off(f) + offset - start,
> +					      copy, p, poff, p_len, copied) {
> +
> +				vaddr = kmap_atomic(p);
> +				ret = probe_kernel_read(to + copied,
> +							vaddr + poff, p_len);
> +				kunmap_atomic(vaddr);
> +
> +				if (unlikely(ret < 0))
> +					goto out;
> +			}
> +			len -= copy;
> +			if (len == 0)
> +				return 0;
> +			offset += copy;
> +			to += copy;
> +		}
> +		start = end;
> +	}
> +
> +	for (frag_iterp = shinfo.frag_list; frag_iterp;
> +	     frag_iterp = frag_iter.next) {
> +		int end;
> +
> +		if (start > offset + len) {
> +			ret = -E2BIG;
> +			goto out;
> +		}
> +		ret = probe_kernel_read(&frag_iter, frag_iterp,
> +					sizeof(frag_iter));
> +		if (ret)
> +			goto out;
> +
> +		end = start + frag_iter.len;
> +		copy = end - offset;
> +		if (copy > 0) {
> +			if (copy > len)
> +				copy = len;
> +			ret = bpf_trace_skb_copy(to, &frag_iter,
> +						offset - start,
> +						copy);
> +			if (ret)
> +				goto out;
> +
> +			len -= copy;
> +			if (len == 0)
> +				return 0;
> +			offset += copy;
> +			to += copy;
> +		}
> +		start = end;
> +	}
> +out:
> +	if (ret)
> +		memset(tobuf, 0, len);
> +
> +	return ret;
> +}

For net side bpf_perf_event_output, we have
static unsigned long bpf_skb_copy(void *dst_buff, const void *skb,
                                   unsigned long off, unsigned long len)
{
         void *ptr = skb_header_pointer(skb, off, len, dst_buff);

         if (unlikely(!ptr))
                 return len;
         if (ptr != dst_buff)
                 memcpy(dst_buff, ptr, len);

         return 0;
}

BPF_CALL_5(bpf_skb_event_output, struct sk_buff *, skb, struct bpf_map 
*, map,
            u64, flags, void *, meta, u64, meta_size)
{
         u64 skb_size = (flags & BPF_F_CTXLEN_MASK) >> 32;

         if (unlikely(flags & ~(BPF_F_CTXLEN_MASK | BPF_F_INDEX_MASK)))
                 return -EINVAL;
         if (unlikely(skb_size > skb->len))
                 return -EFAULT;

         return bpf_event_output(map, flags, meta, meta_size, skb, skb_size,
                                 bpf_skb_copy);
}

It does not really consider output all the frags.
I understand that to get truly all packet data, frags should be
considered, but seems we did not do it before? I am wondering
whether we need to do here.

If we indeed do not need to handle frags here, I think maybe
bpf_probe_read() in existing bpf kprobe function should be
enough, we do not need this helper?

> +
> +/* Derive protocol for some of the easier cases.  For tracing, a probe point
> + * may be dealing with packets in various states. Common cases are IP
> + * packets prior to adding MAC header (_PCAP_TYPE_IP) and a full packet
> + * (_PCAP_TYPE_ETH).  For other cases the caller must specify the
> + * protocol they expect.  Other heuristics for packet identification
> + * should be added here as needed, since determining the packet type
> + * ensures we do not capture packets that fail to match the desired
> + * pcap type in BPF_F_PCAP_STRICT_TYPE mode.
> + */
> +static inline int bpf_skb_protocol_get(struct sk_buff *skb)
> +{
> +	switch (htons(skb->protocol)) {
> +	case ETH_P_IP:
> +	case ETH_P_IPV6:
> +		if (skb_network_header(skb) == skb->data)
> +			return BPF_PCAP_TYPE_IP;
> +		else
> +			return BPF_PCAP_TYPE_ETH;
> +	default:
> +		return BPF_PCAP_TYPE_UNSET;
> +	}
> +}
> +
> +BPF_CALL_5(bpf_trace_pcap, void *, data, u32, size, struct bpf_map *, map,
> +	   int, protocol_wanted, u64, flags)

Up to now, for helpers, verifier has a way to verifier it is used 
properly regarding to the context. For example, for xdp version
perf_event_output, the help prototype,
   BPF_CALL_5(bpf_xdp_event_output, struct xdp_buff *, xdp, struct 
bpf_map *, map,
            u64, flags, void *, meta, u64, meta_size)
the verifier is able to guarantee that the first parameter
has correct type xdp_buff, not something from type cast.
   .arg1_type      = ARG_PTR_TO_CTX,

This helper, in the below we have
   .arg1_type	= ARG_ANYTHING,

So it is not really enforced. Bringing BTF can help, but type
name matching typically bad.


> +{
> +	struct bpf_pcap_hdr pcap;
> +	struct sk_buff skb;
> +	int protocol;
> +	int ret;
> +
> +	if (unlikely(flags & ~(BPF_F_PCAP_ID_IIFINDEX | BPF_F_PCAP_ID_MASK |
> +			       BPF_F_PCAP_STRICT_TYPE)))
> +		return -EINVAL;
> +
> +	ret = probe_kernel_read(&skb, data, sizeof(skb));
> +	if (unlikely(ret < 0))
> +		return ret;
> +
> +	/* Sanity check skb len in case we get bogus data. */
> +	if (unlikely(!skb.len))
> +		return -ENOENT;
> +	if (unlikely(skb.len > GSO_MAX_SIZE || skb.data_len > skb.len))
> +		return -E2BIG;
> +
> +	protocol = bpf_skb_protocol_get(&skb);
> +
> +	if (protocol_wanted == BPF_PCAP_TYPE_UNSET) {
> +		/* If we cannot determine protocol type, bail. */
> +		if (protocol == BPF_PCAP_TYPE_UNSET)
> +			return -EPROTO;
> +	} else {
> +		/* if we determine protocol type, and it's not what we asked
> +		 * for _and_ we are in strict mode, bail.  Otherwise we assume
> +		 * the packet is the requested protocol type and drive on.
> +		 */
> +		if (flags & BPF_F_PCAP_STRICT_TYPE &&
> +		    protocol != BPF_PCAP_TYPE_UNSET &&
> +		    protocol != protocol_wanted)
> +			return -EPROTO;
> +		protocol = protocol_wanted;
> +	}
> +
> +	/* If we specified a matching incoming ifindex, bail if not a match. */
> +	if (flags & BPF_F_PCAP_ID_IIFINDEX) {
> +		int iif = flags & BPF_F_PCAP_ID_MASK;
> +
> +		if (iif && skb.skb_iif != iif)
> +			return -ENOENT;
> +	}
> +
> +	ret = bpf_pcap_prepare(protocol, size, skb.len, flags, &pcap);
> +	if (ret)
> +		return ret;
> +
> +	return bpf_event_output(map, BPF_F_CURRENT_CPU, &pcap, sizeof(pcap),
> +				&skb, pcap.cap_len, bpf_trace_skb_copy);
> +}
> +
> +static const struct bpf_func_proto bpf_trace_pcap_proto = {
> +	.func		= bpf_trace_pcap,
> +	.gpl_only	= true,
> +	.ret_type	= RET_INTEGER,
> +	.arg1_type	= ARG_ANYTHING,
> +	.arg2_type	= ARG_ANYTHING,
> +	.arg3_type	= ARG_CONST_MAP_PTR,
> +	.arg4_type	= ARG_ANYTHING,
> +	.arg5_type	= ARG_ANYTHING,
> +};
> +
>   BPF_CALL_0(bpf_get_current_task)
>   {
>   	return (long) current;
> @@ -709,6 +921,8 @@ static void do_bpf_send_signal(struct irq_work *entry)
>   #endif
>   	case BPF_FUNC_send_signal:
>   		return &bpf_send_signal_proto;
> +	case BPF_FUNC_pcap:
> +		return &bpf_trace_pcap_proto;
>   	default:
>   		return NULL;
>   	}
>