[PATCH v8 0/5] skb_to_sgvec hardening

[PATCH v8 0/5] skb_to_sgvec hardening

[Jason A. Donenfeld]

The recent bug with macsec and historical one with virtio have
indicated that letting skb_to_sgvec trounce all over an sglist
without checking the length is probably a bad idea. And it's not
necessary either: an sglist already explicitly marks its last
item, and the initialization functions are diligent in doing so.
Thus there's a clear way of avoiding future overflows.

So, this patchset, from a high level, makes skb_to_sgvec return
a potential error code, and then adjusts all callers to check
for the error code. There are two situations in which skb_to_sgvec
might return such an error:

   1) When the passed in sglist is too small; and
   2) When the passed in skbuff is too deeply nested.

So, the first patch in this series handles the issues with
skb_to_sgvec directly, and the remaining ones then handle the call
sites.

Changes v7->v8:
   - Added __must_check annotation.
   - Rebased against latest upstream ipsec changes.

Jason A. Donenfeld (5):
  skbuff: return -EMSGSIZE in skb_to_sgvec to prevent overflow
  ipsec: check return value of skb_to_sgvec always
  rxrpc: check return value of skb_to_sgvec always
  macsec: check return value of skb_to_sgvec always
  virtio_net: check return value of skb_to_sgvec always

 drivers/net/macsec.c     | 13 ++++++++--
 drivers/net/virtio_net.c |  9 +++++--
 include/linux/skbuff.h   |  8 +++---
 net/core/skbuff.c        | 65 +++++++++++++++++++++++++++++++-----------------
 net/ipv4/ah4.c           |  8 ++++--
 net/ipv4/esp4.c          | 20 +++++++++------
 net/ipv6/ah6.c           |  8 ++++--
 net/ipv6/esp6.c          | 20 +++++++++------
 net/rxrpc/rxkad.c        | 13 +++++++---
 9 files changed, 112 insertions(+), 52 deletions(-)

-- 
2.13.0

[PATCH v8 1/5] skbuff: return -EMSGSIZE in skb_to_sgvec to prevent overflow

[Jason A. Donenfeld]

This is a defense-in-depth measure in response to bugs like
4d6fa57b4dab ("macsec: avoid heap overflow in skb_to_sgvec"). There's
not only a potential overflow of sglist items, but also a stack overflow
potential, so we fix this by limiting the amount of recursion this function
is allowed to do. Not actually providing a bounded base case is a future
disaster that we can easily avoid here.

As a small matter of house keeping, we take this opportunity to move the
documentation comment over the actual function the documentation is for.

While this could be implemented by using an explicit stack of skbuffs,
when implementing this, the function complexity increased considerably,
and I don't think such complexity and bloat is actually worth it. So,
instead I built this and tested it on x86, x86_64, ARM, ARM64, and MIPS,
and measured the stack usage there. I also reverted the recent MIPS
changes that give it a separate IRQ stack, so that I could experience
some worst-case situations. I found that limiting it to 24 layers deep
yielded a good stack usage with room for safety, as well as being much
deeper than any driver actually ever creates.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: David Howells <dhowells@redhat.com>
Cc: Sabrina Dubroca <sd@queasysnail.net>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
---
 include/linux/skbuff.h |  8 +++----
 net/core/skbuff.c      | 65 ++++++++++++++++++++++++++++++++------------------
 2 files changed, 46 insertions(+), 27 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index a098d95b3d84..f351df28942d 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1001,10 +1001,10 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb,
 				     unsigned int headroom);
 struct sk_buff *skb_copy_expand(const struct sk_buff *skb, int newheadroom,
 				int newtailroom, gfp_t priority);
-int skb_to_sgvec_nomark(struct sk_buff *skb, struct scatterlist *sg,
-			int offset, int len);
-int skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset,
-		 int len);
+int __must_check skb_to_sgvec_nomark(struct sk_buff *skb, struct scatterlist *sg,
+				     int offset, int len);
+int __must_check skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg,
+			      int offset, int len);
 int skb_cow_data(struct sk_buff *skb, int tailbits, struct sk_buff **trailer);
 int skb_pad(struct sk_buff *skb, int pad);
 #define dev_kfree_skb(a)	consume_skb(a)
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 346d3e85dfbc..ec33811559e3 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -3482,24 +3482,18 @@ void __init skb_init(void)
 						NULL);
 }
 
-/**
- *	skb_to_sgvec - Fill a scatter-gather list from a socket buffer
- *	@skb: Socket buffer containing the buffers to be mapped
- *	@sg: The scatter-gather list to map into
- *	@offset: The offset into the buffer's contents to start mapping
- *	@len: Length of buffer space to be mapped
- *
- *	Fill the specified scatter-gather list with mappings/pointers into a
- *	region of the buffer space attached to a socket buffer.
- */
 static int
-__skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
+__skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len,
+	       unsigned int recursion_level)
 {
 	int start = skb_headlen(skb);
 	int i, copy = start - offset;
 	struct sk_buff *frag_iter;
 	int elt = 0;
 
+	if (unlikely(recursion_level >= 24))
+		return -EMSGSIZE;
+
 	if (copy > 0) {
 		if (copy > len)
 			copy = len;
@@ -3518,6 +3512,8 @@ __skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
 		end = start + skb_frag_size(&skb_shinfo(skb)->frags[i]);
 		if ((copy = end - offset) > 0) {
 			skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
+			if (unlikely(elt && sg_is_last(&sg[elt - 1])))
+				return -EMSGSIZE;
 
 			if (copy > len)
 				copy = len;
@@ -3532,16 +3528,22 @@ __skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
 	}
 
 	skb_walk_frags(skb, frag_iter) {
-		int end;
+		int end, ret;
 
 		WARN_ON(start > offset + len);
 
 		end = start + frag_iter->len;
 		if ((copy = end - offset) > 0) {
+			if (unlikely(elt && sg_is_last(&sg[elt - 1])))
+				return -EMSGSIZE;
+
 			if (copy > len)
 				copy = len;
-			elt += __skb_to_sgvec(frag_iter, sg+elt, offset - start,
-					      copy);
+			ret = __skb_to_sgvec(frag_iter, sg+elt, offset - start,
+					      copy, recursion_level + 1);
+			if (unlikely(ret < 0))
+				return ret;
+			elt += ret;
 			if ((len -= copy) == 0)
 				return elt;
 			offset += copy;
@@ -3552,6 +3554,31 @@ __skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
 	return elt;
 }
 
+/**
+ *	skb_to_sgvec - Fill a scatter-gather list from a socket buffer
+ *	@skb: Socket buffer containing the buffers to be mapped
+ *	@sg: The scatter-gather list to map into
+ *	@offset: The offset into the buffer's contents to start mapping
+ *	@len: Length of buffer space to be mapped
+ *
+ *	Fill the specified scatter-gather list with mappings/pointers into a
+ *	region of the buffer space attached to a socket buffer. Returns either
+ *	the number of scatterlist items used, or -EMSGSIZE if the contents
+ *	could not fit.
+ */
+int skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
+{
+	int nsg = __skb_to_sgvec(skb, sg, offset, len, 0);
+
+	if (nsg <= 0)
+		return nsg;
+
+	sg_mark_end(&sg[nsg - 1]);
+
+	return nsg;
+}
+EXPORT_SYMBOL_GPL(skb_to_sgvec);
+
 /* As compared with skb_to_sgvec, skb_to_sgvec_nomark only map skb to given
  * sglist without mark the sg which contain last skb data as the end.
  * So the caller can mannipulate sg list as will when padding new data after
@@ -3574,19 +3601,11 @@ __skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
 int skb_to_sgvec_nomark(struct sk_buff *skb, struct scatterlist *sg,
 			int offset, int len)
 {
-	return __skb_to_sgvec(skb, sg, offset, len);
+	return __skb_to_sgvec(skb, sg, offset, len, 0);
 }
 EXPORT_SYMBOL_GPL(skb_to_sgvec_nomark);
 
-int skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
-{
-	int nsg = __skb_to_sgvec(skb, sg, offset, len);
 
-	sg_mark_end(&sg[nsg - 1]);
-
-	return nsg;
-}
-EXPORT_SYMBOL_GPL(skb_to_sgvec);
 
 /**
  *	skb_cow_data - Check that a socket buffer's data buffers are writable
-- 
2.13.0

[PATCH v8 2/5] ipsec: check return value of skb_to_sgvec always

[Jason A. Donenfeld]

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
---
 net/ipv4/ah4.c  |  8 ++++++--
 net/ipv4/esp4.c | 20 +++++++++++++-------
 net/ipv6/ah6.c  |  8 ++++++--
 net/ipv6/esp6.c | 20 +++++++++++++-------
 4 files changed, 38 insertions(+), 18 deletions(-)

diff --git a/net/ipv4/ah4.c b/net/ipv4/ah4.c
index 22377c8ff14b..e8f862358518 100644
--- a/net/ipv4/ah4.c
+++ b/net/ipv4/ah4.c
@@ -220,7 +220,9 @@ static int ah_output(struct xfrm_state *x, struct sk_buff *skb)
 	ah->seq_no = htonl(XFRM_SKB_CB(skb)->seq.output.low);
 
 	sg_init_table(sg, nfrags + sglists);
-	skb_to_sgvec_nomark(skb, sg, 0, skb->len);
+	err = skb_to_sgvec_nomark(skb, sg, 0, skb->len);
+	if (unlikely(err < 0))
+		goto out_free;
 
 	if (x->props.flags & XFRM_STATE_ESN) {
 		/* Attach seqhi sg right after packet payload */
@@ -393,7 +395,9 @@ static int ah_input(struct xfrm_state *x, struct sk_buff *skb)
 	skb_push(skb, ihl);
 
 	sg_init_table(sg, nfrags + sglists);
-	skb_to_sgvec_nomark(skb, sg, 0, skb->len);
+	err = skb_to_sgvec_nomark(skb, sg, 0, skb->len);
+	if (unlikely(err < 0))
+		goto out_free;
 
 	if (x->props.flags & XFRM_STATE_ESN) {
 		/* Attach seqhi sg right after packet payload */
diff --git a/net/ipv4/esp4.c b/net/ipv4/esp4.c
index 65cc02bd82bc..392432860bb9 100644
--- a/net/ipv4/esp4.c
+++ b/net/ipv4/esp4.c
@@ -374,9 +374,11 @@ int esp_output_tail(struct xfrm_state *x, struct sk_buff *skb, struct esp_info *
 	esp->esph = esph;
 
 	sg_init_table(sg, esp->nfrags);
-	skb_to_sgvec(skb, sg,
-		     (unsigned char *)esph - skb->data,
-		     assoclen + ivlen + esp->clen + alen);
+	err = skb_to_sgvec(skb, sg,
+		           (unsigned char *)esph - skb->data,
+		           assoclen + ivlen + esp->clen + alen);
+	if (unlikely(err < 0))
+		goto error;
 
 	if (!esp->inplace) {
 		int allocsize;
@@ -400,9 +402,11 @@ int esp_output_tail(struct xfrm_state *x, struct sk_buff *skb, struct esp_info *
 		spin_unlock_bh(&x->lock);
 
 		sg_init_table(dsg, skb_shinfo(skb)->nr_frags + 1);
-		skb_to_sgvec(skb, dsg,
-			     (unsigned char *)esph - skb->data,
-			     assoclen + ivlen + esp->clen + alen);
+		err = skb_to_sgvec(skb, dsg,
+			           (unsigned char *)esph - skb->data,
+			           assoclen + ivlen + esp->clen + alen);
+		if (unlikely(err < 0))
+			goto error;
 	}
 
 	if ((x->props.flags & XFRM_STATE_ESN))
@@ -687,7 +691,9 @@ static int esp_input(struct xfrm_state *x, struct sk_buff *skb)
 	esp_input_set_header(skb, seqhi);
 
 	sg_init_table(sg, nfrags);
-	skb_to_sgvec(skb, sg, 0, skb->len);
+	err = skb_to_sgvec(skb, sg, 0, skb->len);
+	if (unlikely(err < 0))
+		goto out;
 
 	skb->ip_summed = CHECKSUM_NONE;
 
diff --git a/net/ipv6/ah6.c b/net/ipv6/ah6.c
index dda6035e3b84..755f38271dd5 100644
--- a/net/ipv6/ah6.c
+++ b/net/ipv6/ah6.c
@@ -423,7 +423,9 @@ static int ah6_output(struct xfrm_state *x, struct sk_buff *skb)
 	ah->seq_no = htonl(XFRM_SKB_CB(skb)->seq.output.low);
 
 	sg_init_table(sg, nfrags + sglists);
-	skb_to_sgvec_nomark(skb, sg, 0, skb->len);
+	err = skb_to_sgvec_nomark(skb, sg, 0, skb->len);
+	if (unlikely(err < 0))
+		goto out_free;
 
 	if (x->props.flags & XFRM_STATE_ESN) {
 		/* Attach seqhi sg right after packet payload */
@@ -606,7 +608,9 @@ static int ah6_input(struct xfrm_state *x, struct sk_buff *skb)
 	ip6h->hop_limit   = 0;
 
 	sg_init_table(sg, nfrags + sglists);
-	skb_to_sgvec_nomark(skb, sg, 0, skb->len);
+	err = skb_to_sgvec_nomark(skb, sg, 0, skb->len);
+	if (unlikely(err < 0))
+		goto out_free;
 
 	if (x->props.flags & XFRM_STATE_ESN) {
 		/* Attach seqhi sg right after packet payload */
diff --git a/net/ipv6/esp6.c b/net/ipv6/esp6.c
index 1fe99ba8066c..2ede4e459c4e 100644
--- a/net/ipv6/esp6.c
+++ b/net/ipv6/esp6.c
@@ -346,9 +346,11 @@ int esp6_output_tail(struct xfrm_state *x, struct sk_buff *skb, struct esp_info
 	esph = esp_output_set_esn(skb, x, ip_esp_hdr(skb), seqhi);
 
 	sg_init_table(sg, esp->nfrags);
-	skb_to_sgvec(skb, sg,
-		     (unsigned char *)esph - skb->data,
-		     assoclen + ivlen + esp->clen + alen);
+	err = skb_to_sgvec(skb, sg,
+		           (unsigned char *)esph - skb->data,
+		           assoclen + ivlen + esp->clen + alen);
+	if (unlikely(err < 0))
+		goto error;
 
 	if (!esp->inplace) {
 		int allocsize;
@@ -372,9 +374,11 @@ int esp6_output_tail(struct xfrm_state *x, struct sk_buff *skb, struct esp_info
 		spin_unlock_bh(&x->lock);
 
 		sg_init_table(dsg, skb_shinfo(skb)->nr_frags + 1);
-		skb_to_sgvec(skb, dsg,
-			     (unsigned char *)esph - skb->data,
-			     assoclen + ivlen + esp->clen + alen);
+		err = skb_to_sgvec(skb, dsg,
+			           (unsigned char *)esph - skb->data,
+			           assoclen + ivlen + esp->clen + alen);
+		if (unlikely(err < 0))
+			goto error;
 	}
 
 	if ((x->props.flags & XFRM_STATE_ESN))
@@ -618,7 +622,9 @@ static int esp6_input(struct xfrm_state *x, struct sk_buff *skb)
 	esp_input_set_header(skb, seqhi);
 
 	sg_init_table(sg, nfrags);
-	skb_to_sgvec(skb, sg, 0, skb->len);
+	ret = skb_to_sgvec(skb, sg, 0, skb->len);
+	if (unlikely(ret < 0))
+		goto out;
 
 	skb->ip_summed = CHECKSUM_NONE;
 
-- 
2.13.0

[PATCH v8 3/5] rxrpc: check return value of skb_to_sgvec always

[Jason A. Donenfeld]

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: David Howells <dhowells@redhat.com>
---
 net/rxrpc/rxkad.c | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/net/rxrpc/rxkad.c b/net/rxrpc/rxkad.c
index 1bb9b2ccc267..ecab9334e3c1 100644
--- a/net/rxrpc/rxkad.c
+++ b/net/rxrpc/rxkad.c
@@ -227,7 +227,9 @@ static int rxkad_secure_packet_encrypt(const struct rxrpc_call *call,
 	len &= ~(call->conn->size_align - 1);
 
 	sg_init_table(sg, nsg);
-	skb_to_sgvec(skb, sg, 0, len);
+	err = skb_to_sgvec(skb, sg, 0, len);
+	if (unlikely(err < 0))
+		goto out;
 	skcipher_request_set_crypt(req, sg, sg, len, iv.x);
 	crypto_skcipher_encrypt(req);
 
@@ -342,7 +344,8 @@ static int rxkad_verify_packet_1(struct rxrpc_call *call, struct sk_buff *skb,
 		goto nomem;
 
 	sg_init_table(sg, nsg);
-	skb_to_sgvec(skb, sg, offset, 8);
+	if (unlikely(skb_to_sgvec(skb, sg, offset, 8) < 0))
+		goto nomem;
 
 	/* start the decryption afresh */
 	memset(&iv, 0, sizeof(iv));
@@ -434,7 +437,11 @@ static int rxkad_verify_packet_2(struct rxrpc_call *call, struct sk_buff *skb,
 	}
 
 	sg_init_table(sg, nsg);
-	skb_to_sgvec(skb, sg, offset, len);
+	if (unlikely(skb_to_sgvec(skb, sg, offset, len) < 0)) {
+		if (sg != _sg)
+			kfree(sg);
+		goto nomem;
+	}
 
 	/* decrypt from the session key */
 	token = call->conn->params.key->payload.data[0];
-- 
2.13.0

[PATCH v8 4/5] macsec: check return value of skb_to_sgvec always

[Jason A. Donenfeld]

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: Sabrina Dubroca <sd@queasysnail.net>
---
 drivers/net/macsec.c | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/drivers/net/macsec.c b/drivers/net/macsec.c
index cdc347be68f2..dfcb1e9d2ab2 100644
--- a/drivers/net/macsec.c
+++ b/drivers/net/macsec.c
@@ -742,7 +742,12 @@ static struct sk_buff *macsec_encrypt(struct sk_buff *skb,
 	macsec_fill_iv(iv, secy->sci, pn);
 
 	sg_init_table(sg, ret);
-	skb_to_sgvec(skb, sg, 0, skb->len);
+	ret = skb_to_sgvec(skb, sg, 0, skb->len);
+	if (unlikely(ret < 0)) {
+		macsec_txsa_put(tx_sa);
+		kfree_skb(skb);
+		return ERR_PTR(ret);
+	}
 
 	if (tx_sc->encrypt) {
 		int len = skb->len - macsec_hdr_len(sci_present) -
@@ -952,7 +957,11 @@ static struct sk_buff *macsec_decrypt(struct sk_buff *skb,
 	macsec_fill_iv(iv, sci, ntohl(hdr->packet_number));
 
 	sg_init_table(sg, ret);
-	skb_to_sgvec(skb, sg, 0, skb->len);
+	ret = skb_to_sgvec(skb, sg, 0, skb->len);
+	if (unlikely(ret < 0)) {
+		kfree_skb(skb);
+		return ERR_PTR(ret);
+	}
 
 	if (hdr->tci_an & MACSEC_TCI_E) {
 		/* confidentiality: ethernet + macsec header
-- 
2.13.0

[PATCH v8 5/5] virtio_net: check return value of skb_to_sgvec always

[Jason A. Donenfeld]

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
---
 drivers/net/virtio_net.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 9320d96a1632..13fbe4b349c2 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -1150,7 +1150,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
 	struct virtio_net_hdr_mrg_rxbuf *hdr;
 	const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;
 	struct virtnet_info *vi = sq->vq->vdev->priv;
-	unsigned num_sg;
+	int num_sg;
 	unsigned hdr_len = vi->hdr_len;
 	bool can_push;
 
@@ -1177,11 +1177,16 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb)
 	if (can_push) {
 		__skb_push(skb, hdr_len);
 		num_sg = skb_to_sgvec(skb, sq->sg, 0, skb->len);
+                if (unlikely(num_sg < 0))
+			return num_sg;
 		/* Pull header back to avoid skew in tx bytes calculations. */
 		__skb_pull(skb, hdr_len);
 	} else {
 		sg_set_buf(sq->sg, hdr, hdr_len);
-		num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len) + 1;
+		num_sg = skb_to_sgvec(skb, sq->sg + 1, 0, skb->len);
+		if (unlikely(num_sg < 0))
+			return num_sg;
+		num_sg++;
 	}
 	return virtqueue_add_outbuf(sq->vq, sq->sg, num_sg, skb, GFP_ATOMIC);
 }
-- 
2.13.0

Re: [PATCH v8 3/5] rxrpc: check return value of skb_to_sgvec always

[David Howells]

Jason A. Donenfeld <Jason@zx2c4.com> wrote:

> +	if (unlikely(skb_to_sgvec(skb, sg, offset, 8) < 0))
> +		goto nomem;
> ...
> +	if (unlikely(skb_to_sgvec(skb, sg, offset, len) < 0)) {
> +		if (sg != _sg)
> +			kfree(sg);
> +		goto nomem;

skb_to_sgvec() can return -EMSGSIZE in some circumstances.  You shouldn't
return -ENOMEM here in such a case.

David

Re: [PATCH v8 3/5] rxrpc: check return value of skb_to_sgvec always

[Jason A. Donenfeld]

On Mon, May 15, 2017 at 3:11 PM, David Howells <dhowells@redhat.com> wrote:
> skb_to_sgvec() can return -EMSGSIZE in some circumstances.  You shouldn't
> return -ENOMEM here in such a case.

Noted. I'll fix this up for the next round.

Re: [PATCH v8 1/5] skbuff: return -EMSGSIZE in skb_to_sgvec to prevent overflow

[David Howells]

Jason A. Donenfeld <Jason@zx2c4.com> wrote:

> +int skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)
> ...
> -int skb_to_sgvec(struct sk_buff *skb, struct scatterlist *sg, int offset, int len)

Is there a reason you moved skb_to_sgvec() in the file rather than just moving
the comment to it (since you moved the comment anyway)?

David

Re: [PATCH v8 1/5] skbuff: return -EMSGSIZE in skb_to_sgvec to prevent overflow

[Jason A. Donenfeld]

On Mon, May 15, 2017 at 3:12 PM, David Howells <dhowells@redhat.com> wrote:
> Is there a reason you moved skb_to_sgvec() in the file rather than just moving
> the comment to it (since you moved the comment anyway)?

1) Because it's easier to understand skb_to_sgvec_nomark as a variant
of skb_to_sgvec, so I'd rather skb_to_sgvec to be first when reading.
2) Because skb_to_sgvec relies on the return value of __skb_to_sgvec,
and so when assessing it, it's sometimes nice to be able to look at
why it will return different things. In that case, it's easier to have
both functions within the same view without scrolling.

It's the little things that make life easier sometimes.