Subject: Re: lockup and kernel panic in linux-next-202505{09,12} when compiled with clang

From: Bert Karwatzki <[email-masked]>
Date: Sun, 18 May 2025 16:15:15 +0200

Am Sonntag, dem 18.05.2025 um 14:43 +0200 schrieb Bert Karwatzki:
> Am Sonntag, dem 18.05.2025 um 14:12 +0200 schrieb Bert Karwatzki:
> > > > > 
> > 
> > I even tried this version of your patch, to keep the offset of skc_refcnt at 128,
> > but it doesn't  work, either.
> > 
> > commit fca84c5cde713be480544a64ed6680afc3319670
> > Author: Bert Karwatzki <[email-masked]>
> > Date:   Sun May 18 13:32:36 2025 +0200
> > 
> >     include: net: sock: move skc_flags out of the union
> >     
> >     Signed-off-by: Bert Karwatzki <[email-masked]>
> > 
> > diff --git a/include/net/sock.h b/include/net/sock.h
> > index 3e15d7105ad2..e73929a4da6e 100644
> > --- a/include/net/sock.h
> > +++ b/include/net/sock.h
> > @@ -195,7 +195,6 @@ struct sock_common {
> >  	 * for different kind of 'sockets'
> >  	 */
> >  	union {
> > -		unsigned long	skc_flags;
> >  		struct sock	*skc_listener; /* request_sock */
> >  		struct inet_timewait_death_row *skc_tw_dr; /* inet_timewait_sock */
> >  	};
> > @@ -221,6 +220,9 @@ struct sock_common {
> >  	};
> >  
> >  	refcount_t		skc_refcnt;
> > +
> > +	/* place skc_flags here to keep offset(struct sock, sk_refcnt) == 128 */
> > +	unsigned long	skc_flags;
> >  	/* private: */
> >  	int                     skc_dontcopy_end[0];
> >  	union {
> > 
> 
> In the patch above I accidently put skc_flags in the part of struct sock_common
> which does not get copied, but putting it below skc_dontcopy_end[0] does not work,
> either:
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 3e15d7105ad2..6d69753a205a 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -195,7 +195,6 @@ struct sock_common {
>          * for different kind of 'sockets'
>          */
>         union {
> -               unsigned long   skc_flags;
>                 struct sock     *skc_listener; /* request_sock */
>                 struct inet_timewait_death_row *skc_tw_dr; /* inet_timewait_sock */
>         };
> @@ -221,8 +220,12 @@ struct sock_common {
>         };
>  
>         refcount_t              skc_refcnt;
> +
>         /* private: */
>         int                     skc_dontcopy_end[0];
> +       /* place skc_flags here to keep offset(struct sock, sk_refcnt) == 128 
> +        * Also place it below skc_dontcopy_end[0] */
> +       unsigned long   skc_flags;
>         union {
>                 u32             skc_rxhash;
>                 u32             skc_window_clamp;
> 
> This locks up as usual.
> 
> Bert Karwatzki

So I did some more monitoring and found that even though skc_flags is removed from the union
it can take strange values, e.g.:

Here the value is not even a pointer (perhaps unitialized memory?):
[  T572] ieee80211_8023_xmit_clang_debug_helper: skb->sk = ffff88fc2abf4cc0 skb->sk->sk_flags = 0xa00f7fe57b16f7e1
These could be pointers, but as pointers would only be aligned to a 2-byte boundary ...
[  T572] ieee80211_8023_xmit_clang_debug_helper: skb->sk = ffff88fbd0bd3210 skb->sk->sk_flags = 0xffffc0f1c62dcc4e
[  T572] ieee80211_8023_xmit_clang_debug_helper: skb->sk = ffff88fbd0bd3210 skb->sk->sk_flags = 0xffffc0f1c62dcc4e

Bert Karwatzki