linux-kernel - Re: [PATCH 2/5][next] nfsd: avoid -Wflex-array-member-not-at-end warnings

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Message-id: <172912626140.81717.18213359690044489488@noble.neil.brown.name>
Date: Thu, 17 Oct 2024 11:51:01 +1100
From: "NeilBrown" <neilb@...e.de>
To: "Chuck Lever" <chuck.lever@...cle.com>
Cc: "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 "Jeff Layton" <jlayton@...nel.org>, "Olga Kornievskaia" <okorniev@...hat.com>,
 "Dai Ngo" <Dai.Ngo@...cle.com>, "Tom Talpey" <tom@...pey.com>,
 linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-hardening@...r.kernel.org, "Kees Cook" <kees@...nel.org>
Subject:
 Re: [PATCH 2/5][next] nfsd: avoid -Wflex-array-member-not-at-end warnings

On Thu, 17 Oct 2024, Chuck Lever wrote:
> On Tue, Oct 15, 2024 at 06:29:31PM -0600, Gustavo A. R. Silva wrote:
> > -Wflex-array-member-not-at-end was introduced in GCC-14, and we are
> > getting ready to enable it, globally.
> > 
> > Address the following warnings by changing the type of the middle struct
> > members in `struct nfsd_genl_rqstp`, which are currently causing trouble,
> > from `struct sockaddr` to `struct sockaddr_legacy`. Note that the latter
> > struct doesn't contain a flexible-array member.
> > 
> > fs/nfsd/nfsd.h:74:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > fs/nfsd/nfsd.h:75:33: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> > 
> > Also, update some related code, accordingly.
> > 
> > No binary differences are present after these changes.
> > 
> > Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> > ---
> >  fs/nfsd/nfsctl.c | 4 ++--
> >  fs/nfsd/nfsd.h   | 4 ++--
> >  2 files changed, 4 insertions(+), 4 deletions(-)
> > 
> > diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> > index 3adbc05ebaac..884bfdc7a255 100644
> > --- a/fs/nfsd/nfsctl.c
> > +++ b/fs/nfsd/nfsctl.c
> > @@ -1599,9 +1599,9 @@ int nfsd_nl_rpc_status_get_dumpit(struct sk_buff *skb,
> >  			genl_rqstp.rq_stime = rqstp->rq_stime;
> >  			genl_rqstp.rq_opcnt = 0;
> >  			memcpy(&genl_rqstp.rq_daddr, svc_daddr(rqstp),
> > -			       sizeof(struct sockaddr));
> > +			       sizeof(struct sockaddr_legacy));
> >  			memcpy(&genl_rqstp.rq_saddr, svc_addr(rqstp),
> > -			       sizeof(struct sockaddr));
> > +			       sizeof(struct sockaddr_legacy));
> >  
> >  #ifdef CONFIG_NFSD_V4
> >  			if (rqstp->rq_vers == NFS4_VERSION &&
> > diff --git a/fs/nfsd/nfsd.h b/fs/nfsd/nfsd.h
> > index 004415651295..44be32510595 100644
> > --- a/fs/nfsd/nfsd.h
> > +++ b/fs/nfsd/nfsd.h
> > @@ -71,8 +71,8 @@ struct readdir_cd {
> >  #define NFSD_MAX_OPS_PER_COMPOUND	50
> >  
> >  struct nfsd_genl_rqstp {
> > -	struct sockaddr		rq_daddr;
> > -	struct sockaddr		rq_saddr;
> > +	struct sockaddr_legacy	rq_daddr;
> > +	struct sockaddr_legacy	rq_saddr;
> 
> I was hoping to find a struct definition that was simply a union of
> sockaddr_in and sockaddr_in6, which we have in nfs-utils but I guess
> not here in the kernel.
> 
> Can we use "struct sockaddr_storage" safely here? Then above,
> replace the raw memcpy() with rpc_copy_addr() ?

sockaddr_storage is conceptually the right solution.  It is 128 bytes so
2 of them will waste about 200 bytes on the stack.  Does that matter
these days?  Probably not.

I do like using rpc_copy_addr().

NeilBrown


> 
> 
> >  	unsigned long		rq_flags;
> >  	ktime_t			rq_stime;
> >  	__be32			rq_xid;
> > -- 
> > 2.34.1
> > 
> 
> -- 
> Chuck Lever
>