Subject: Re: [PATCH 09/44] ipv6: __ip6_append_data() don't abuse max_t() casts

From: David Laight <[email-masked]>
Date: Thu, 20 Nov 2025 11:16:23 +0000

On Thu, 20 Nov 2025 00:32:34 +0000 (UTC)
[email-masked] wrote:

> > diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
> > index f904739e9..6fecf2f2c 100644
> > --- a/net/ipv6/ip6_output.c
> > +++ b/net/ipv6/ip6_output.c
> > @@ -1440,7 +1440,7 @@ static int __ip6_append_data(struct sock *sk,
> >  	struct sk_buff *skb, *skb_prev = NULL;
> >  	struct inet_cork *cork = &cork_full->base;
> >  	struct flowi6 *fl6 = &cork_full->fl.u.ip6;
> > -	unsigned int maxfraglen, fragheaderlen, mtu, orig_mtu, pmtu;
> > +	unsigned int maxfraglen, fragheaderlen, mtu, orig_mtu;  
> 
> [ ... ]
> 
> > @@ -1504,9 +1504,10 @@ static int __ip6_append_data(struct sock *sk,
> >  		maxnonfragsize = mtu;
> >
> >  	if (cork->length + length > maxnonfragsize - headersize) {
> > +		int pmtu;
> >  emsgsize:
> > -		pmtu = max_t(int, mtu - headersize + sizeof(struct ipv6hdr), 0);
> > -		ipv6_local_error(sk, EMSGSIZE, fl6, pmtu);
> > +		pmtu = mtu - headersize + (int)sizeof(struct ipv6hdr);
> > +		ipv6_local_error(sk, EMSGSIZE, fl6, max(pmtu, 0));
> >  		return -EMSGSIZE;
> >  	}  
> 
> Can the variable pmtu be used uninitialized here?

If your AI bot doesn't understand code don't run it.

	David

> 
> When reaching the emsgsize label via goto statements from earlier in
> __ip6_append_data() (the checks at lines around mtu <= fragheaderlen,
> headersize + transhdrlen > mtu, and the dontfrag check), the int pmtu
> declaration is skipped. This leaves pmtu uninitialized when it's used
> in the calculation that's passed to ipv6_local_error().
> 
> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/19519124312