Re: [bug report] net: ipv4: Move ip_options_fragment() out of loop

[yajun.deng@linux.dev]

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August 27, 2021 4:49 PM, "Dan Carpenter" <dan.carpenter@oracle.com> wrote:

> Hello Yajun Deng,
> 
> This is a semi-automatic email about new static checker warnings.

Can you test the attached?

Thanks.
> 
> The patch faf482ca196a: "net: ipv4: Move ip_options_fragment() out of 
> loop" from Aug 23, 2021, leads to the following Smatch complaint:
> 
> net/ipv4/ip_output.c:833 ip_do_fragment()
> warn: variable dereferenced before check 'iter.frag' (see line 828)
> 
> net/ipv4/ip_output.c
> 827 ip_fraglist_init(skb, iph, hlen, &iter);
> ^^^^^
> iter.frag is set here.
> 
> 828 ip_options_fragment(iter.frag);
> ^^^^^^^^^
> The patch introduces a new dereference here
> 
> 829 
> 830 for (;;) {
> 831 /* Prepare header of the next frame,
> 832 * before previous one went down. */
> 833 if (iter.frag) {
> ^^^^^^^^^
> But the old code assumed that "iter.frag" could be NULL.
> 
> 834 IPCB(iter.frag)->flags = IPCB(skb)->flags;
> 835 ip_fraglist_prepare(skb, &iter);
> 
> regards,
> dan carpenter

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From 19fda71966403608c3b2dc981b238a1c25865b83 Mon Sep 17 00:00:00 2001
From: Yajun Deng <yajun.deng@linux.dev>
Date: Fri, 27 Aug 2021 17:14:43 +0800
Subject: [PATCH net-next] net: ipv4: Fix the warning for dereference

Dan Carpenter report:
The patch faf482ca196a: "net: ipv4: Move ip_options_fragment() out of
loop" from Aug 23, 2021, leads to the following Smatch complaint:

    net/ipv4/ip_output.c:833 ip_do_fragment()
    warn: variable dereferenced before check 'iter.frag' (see line 828)

Add a if statements to avoid the warning.

Fixes: faf482ca196a ("net: ipv4: Move ip_options_fragment() out of loop")
Signed-off-by: Yajun Deng <yajun.deng@linux.dev>
---
 net/ipv4/ip_output.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
index 9a8f05d5476e..9bca57ef8b83 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -825,7 +825,9 @@ int ip_do_fragment(struct net *net, struct sock *sk, struct sk_buff *skb,
 
 		/* Everything is OK. Generate! */
 		ip_fraglist_init(skb, iph, hlen, &iter);
-		ip_options_fragment(iter.frag);
+
+		if (iter.frag)
+			ip_options_fragment(iter.frag);
 
 		for (;;) {
 			/* Prepare header of the next frame,
-- 
2.32.0