[NET]: Zerocopy sequential reading of skb data

Implements sequential reading for both linear and non-linear
skb data at zerocopy cost. The data is returned in chunks of
arbitary length, therefore random access is not possible.

Usage:
	from	 := 0
	to	 := 128
	state	 := undef
	data	 := undef
	len	 := undef
	consumed := 0

	skb_prepare_seq_read(skb, from, to, &state)
	while (len = skb_seq_read(consumed, &data, &state)) != 0 do
		/* do something with 'data' of length 'len' */
		if abort then
			/* abort read if we don't wait for
			 * skb_seq_read() to return 0 */
			skb_abort_seq_read(&state)
			return
		endif
		/* not necessary to consume all of 'len' */
		consumed += len
	done

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 135 insertions, 0 deletions

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index d7c839a21842..171a37dff83a 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -321,6 +321,24 @@ extern void	      skb_over_panic(struct sk_buff *skb, int len,
 extern void           skb_under_panic(struct sk_buff *skb, int len,
                                       void *here);
 
+struct skb_seq_state
+{
+        __u32           lower_offset;
+        __u32           upper_offset;
+        __u32           frag_idx;
+        __u32           stepped_offset;
+        struct sk_buff  *root_skb;
+        struct sk_buff  *cur_skb;
+        __u8            *frag_data;
+};
+
+extern void           skb_prepare_seq_read(struct sk_buff *skb,
+                                           unsigned int from, unsigned int to,
+                                           struct skb_seq_state *st);
+extern unsigned int   skb_seq_read(unsigned int consumed, const u8 **data,
+                                   struct skb_seq_state *st);
+extern void           skb_abort_seq_read(struct skb_seq_state *st);
+
 /* Internal */
 #define skb_shinfo(SKB)         ((struct skb_shared_info *)((SKB)->end))
 
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 6d68c03bc051..d285f2f7e812 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1500,6 +1500,120 @@ void skb_split(struct sk_buff *skb, struct sk_buff *skb1, const u32 len)
                 skb_split_no_header(skb, skb1, len, pos);
 }
 
+/**
+ * skb_prepare_seq_read - Prepare a sequential read of skb data
+ * @skb: the buffer to read
+ * @from: lower offset of data to be read
+ * @to: upper offset of data to be read
+ * @st: state variable
+ *
+ * Initializes the specified state variable. Must be called before
+ * invoking skb_seq_read() for the first time.
+ */
+void skb_prepare_seq_read(struct sk_buff *skb, unsigned int from,
+                          unsigned int to, struct skb_seq_state *st)
+{
+        st->lower_offset = from;
+        st->upper_offset = to;
+        st->root_skb = st->cur_skb = skb;
+        st->frag_idx = st->stepped_offset = 0;
+        st->frag_data = NULL;
+}
+
+/**
+ * skb_seq_read - Sequentially read skb data
+ * @consumed: number of bytes consumed by the caller so far
+ * @data: destination pointer for data to be returned
+ * @st: state variable
+ *
+ * Reads a block of skb data at &consumed relative to the
+ * lower offset specified to skb_prepare_seq_read(). Assigns
+ * the head of the data block to &data and returns the length
+ * of the block or 0 if the end of the skb data or the upper
+ * offset has been reached.
+ *
+ * The caller is not required to consume all of the data
+ * returned, i.e. &consumed is typically set to the number
+ * of bytes already consumed and the next call to
+ * skb_seq_read() will return the remaining part of the block.
+ *
+ * Note: The size of each block of data returned can be arbitary,
+ *       this limitation is the cost for zerocopy seqeuental
+ *       reads of potentially non linear data.
+ *
+ * Note: Fragment lists within fragments are not implemented
+ *       at the moment, state->root_skb could be replaced with
+ *       a stack for this purpose.
+ */
+unsigned int skb_seq_read(unsigned int consumed, const u8 **data,
+                          struct skb_seq_state *st)
+{
+        unsigned int block_limit, abs_offset = consumed + st->lower_offset;
+        skb_frag_t *frag;
+
+        if (unlikely(abs_offset >= st->upper_offset))
+                return 0;
+
+next_skb:
+        block_limit = skb_headlen(st->cur_skb);
+
+        if (abs_offset < block_limit) {
+                *data = st->cur_skb->data + abs_offset;
+                return block_limit - abs_offset;
+        }
+
+        if (st->frag_idx == 0 && !st->frag_data)
+                st->stepped_offset += skb_headlen(st->cur_skb);
+
+        while (st->frag_idx < skb_shinfo(st->cur_skb)->nr_frags) {
+                frag = &skb_shinfo(st->cur_skb)->frags[st->frag_idx];
+                block_limit = frag->size + st->stepped_offset;
+
+                if (abs_offset < block_limit) {
+                        if (!st->frag_data)
+                                st->frag_data = kmap_skb_frag(frag);
+
+                        *data = (u8 *) st->frag_data + frag->page_offset +
+                                (abs_offset - st->stepped_offset);
+
+                        return block_limit - abs_offset;
+                }
+
+                if (st->frag_data) {
+                        kunmap_skb_frag(st->frag_data);
+                        st->frag_data = NULL;
+                }
+
+                st->frag_idx++;
+                st->stepped_offset += frag->size;
+        }
+
+        if (st->cur_skb->next) {
+                st->cur_skb = st->cur_skb->next;
+                st->frag_idx = 0;
+                goto next_skb;
+        } else if (st->root_skb == st->cur_skb &&
+                   skb_shinfo(st->root_skb)->frag_list) {
+                st->cur_skb = skb_shinfo(st->root_skb)->frag_list;
+                goto next_skb;
+        }
+
+        return 0;
+}
+
+/**
+ * skb_abort_seq_read - Abort a sequential read of skb data
+ * @st: state variable
+ *
+ * Must be called if skb_seq_read() was not called until it
+ * returned 0.
+ */
+void skb_abort_seq_read(struct skb_seq_state *st)
+{
+        if (st->frag_data)
+                kunmap_skb_frag(st->frag_data);
+}
+
 void __init skb_init(void)
 {
         skbuff_head_cache = kmem_cache_create("skbuff_head_cache",
@@ -1538,3 +1652,6 @@ EXPORT_SYMBOL(skb_queue_tail);
 EXPORT_SYMBOL(skb_unlink);
 EXPORT_SYMBOL(skb_append);
 EXPORT_SYMBOL(skb_split);
+EXPORT_SYMBOL(skb_prepare_seq_read);
+EXPORT_SYMBOL(skb_seq_read);
+EXPORT_SYMBOL(skb_abort_seq_read);