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From:   Vasily Averin <vvs@virtuozzo.com>
Subject: [PATCH NET v4 1/7] skbuff: introduce skb_expand_head()
To:     "David S. Miller" <davem@davemloft.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        David Ahern <dsahern@kernel.org>,
        Jakub Kicinski <kuba@kernel.org>,
        Eric Dumazet <eric.dumazet@gmail.com>
Cc:     netdev@vger.kernel.org, Joerg Reuter <jreuter@yaina.de>,
        Ralf Baechle <ralf@linux-mips.org>, linux-hams@vger.kernel.org,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <kafai@fb.com>,
        Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
        KP Singh <kpsingh@kernel.org>, bpf@vger.kernel.org,
        linux-kernel@vger.kernel.org, kernel@openvz.org,
        Julian Wiedmann <jwi@linux.ibm.com>
References: <ccce7edb-54dd-e6bf-1e84-0ec320d8886c@linux.ibm.com>
 <cover.1628235065.git.vvs@virtuozzo.com>
Message-ID: <5290ec1e-72e5-06dc-4886-ffc5255a162a@virtuozzo.com>
Date:   Fri, 6 Aug 2021 10:49:57 +0300
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Precedence: bulk

Like skb_realloc_headroom(), new helper increases headroom of specified skb.
Unlike skb_realloc_headroom(), it does not allocate a new skb if possible;
copies skb->sk on new skb when as needed and frees original skb in case
of failures.

This helps to simplify ip[6]_finish_output2() and a few other similar cases.

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
---
 include/linux/skbuff.h |  1 +
 net/core/skbuff.c      | 42 ++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index b2db9cd..ec8a783 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1179,6 +1179,7 @@ static inline struct sk_buff *__pskb_copy(struct sk_buff *skb, int headroom,
 int pskb_expand_head(struct sk_buff *skb, int nhead, int ntail, gfp_t gfp_mask);
 struct sk_buff *skb_realloc_headroom(struct sk_buff *skb,
 				     unsigned int headroom);
+struct sk_buff *skb_expand_head(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 __must_check skb_to_sgvec_nomark(struct sk_buff *skb, struct scatterlist *sg,
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index fc7942c..0c70b2b 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -1786,6 +1786,48 @@ struct sk_buff *skb_realloc_headroom(struct sk_buff *skb, unsigned int headroom)
 EXPORT_SYMBOL(skb_realloc_headroom);
 
 /**
+ *	skb_expand_head - reallocate header of &sk_buff
+ *	@skb: buffer to reallocate
+ *	@headroom: needed headroom
+ *
+ *	Unlike skb_realloc_headroom, this one does not allocate a new skb
+ *	if possible; copies skb->sk to new skb as needed
+ *	and frees original skb in case of failures.
+ *
+ *	It expect increased headroom and generates warning otherwise.
+ */
+
+struct sk_buff *skb_expand_head(struct sk_buff *skb, unsigned int headroom)
+{
+	int delta = headroom - skb_headroom(skb);
+
+	if (WARN_ONCE(delta <= 0,
+		      "%s is expecting an increase in the headroom", __func__))
+		return skb;
+
+	/* pskb_expand_head() might crash, if skb is shared */
+	if (skb_shared(skb)) {
+		struct sk_buff *nskb = skb_clone(skb, GFP_ATOMIC);
+
+		if (likely(nskb)) {
+			if (skb->sk)
+				skb_set_owner_w(nskb, skb->sk);
+			consume_skb(skb);
+		} else {
+			kfree_skb(skb);
+		}
+		skb = nskb;
+	}
+	if (skb &&
+	    pskb_expand_head(skb, SKB_DATA_ALIGN(delta), 0, GFP_ATOMIC)) {
+		kfree_skb(skb);
+		skb = NULL;
+	}
+	return skb;
+}
+EXPORT_SYMBOL(skb_expand_head);
+
+/**
  *	skb_copy_expand	-	copy and expand sk_buff
  *	@skb: buffer to copy
  *	@newheadroom: new free bytes at head
-- 
1.8.3.1