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Message-ID: <CAFX2Jf=yktJ_Bz--c2VpoDmW13YyoY3MNSv5sJYSseWBGAV9Cw@mail.gmail.com>
Date: Wed, 19 Feb 2025 12:46:46 -0500
From: Anna Schumaker <anna@...nel.org>
To: Chuck Lever <chuck.lever@...cle.com>
Cc: Trond Myklebust <trondmy@...nel.org>, linux-nfs@...r.kernel.org,
linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
"Gustavo A. R. Silva" <gustavoars@...nel.org>, Dai Ngo <Dai.Ngo@...cle.com>,
Jeff Layton <jlayton@...nel.org>, Olga Kornievskaia <okorniev@...hat.com>, Tom Talpey <tom@...pey.com>,
Neil Brown <neilb@...e.de>
Subject: Re: [RESEND PATCH][next] fs: nfs: acl: Avoid -Wflex-array-member-not-at-end
warning
Hi Chuck,
On Tue, Feb 18, 2025 at 1:10 PM Chuck Lever <chuck.lever@...cle.com> wrote:
>
> On 2/10/25 2:48 PM, Chuck Lever wrote:
> > On 2/3/25 10:03 PM, 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.
> >>
> >> So, in order to avoid ending up with a flexible-array member in the
> >> middle of other structs, we use the `struct_group_tagged()` helper
> >> to create a new tagged `struct posix_acl_hdr`. This structure
> >> groups together all the members of the flexible `struct posix_acl`
> >> except the flexible array.
> >>
> >> As a result, the array is effectively separated from the rest of the
> >> members without modifying the memory layout of the flexible structure.
> >> We then change the type of the middle struct member currently causing
> >> trouble from `struct posix_acl` to `struct posix_acl_hdr`.
> >>
> >> We also want to ensure that when new members need to be added to the
> >> flexible structure, they are always included within the newly created
> >> tagged struct. For this, we use `static_assert()`. This ensures that the
> >> memory layout for both the flexible structure and the new tagged struct
> >> is the same after any changes.
> >>
> >> This approach avoids having to implement `struct posix_acl_hdr` as a
> >> completely separate structure, thus preventing having to maintain two
> >> independent but basically identical structures, closing the door to
> >> potential bugs in the future.
> >>
> >> We also use `container_of()` whenever we need to retrieve a pointer to
> >> the flexible structure, through which we can access the flexible-array
> >> member, if necessary.
> >>
> >> So, with these changes, fix the following warning:
> >>
> >> fs/nfs_common/nfsacl.c:45:26: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]
> >>
> >> Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> >> ---
> >> fs/nfs_common/nfsacl.c | 8 +++++---
> >> include/linux/posix_acl.h | 11 ++++++++---
> >> 2 files changed, 13 insertions(+), 6 deletions(-)
> >>
> >> diff --git a/fs/nfs_common/nfsacl.c b/fs/nfs_common/nfsacl.c
> >> index ea382b75b26c..e2eaac14fd8e 100644
> >> --- a/fs/nfs_common/nfsacl.c
> >> +++ b/fs/nfs_common/nfsacl.c
> >> @@ -42,7 +42,7 @@ struct nfsacl_encode_desc {
> >> };
> >>
> >> struct nfsacl_simple_acl {
> >> - struct posix_acl acl;
> >> + struct posix_acl_hdr acl;
> >> struct posix_acl_entry ace[4];
> >> };
> >>
> >> @@ -112,7 +112,8 @@ int nfsacl_encode(struct xdr_buf *buf, unsigned int base, struct inode *inode,
> >> xdr_encode_word(buf, base, entries))
> >> return -EINVAL;
> >> if (encode_entries && acl && acl->a_count == 3) {
> >> - struct posix_acl *acl2 = &aclbuf.acl;
> >> + struct posix_acl *acl2 =
> >> + container_of(&aclbuf.acl, struct posix_acl, hdr);
> >>
> >> /* Avoid the use of posix_acl_alloc(). nfsacl_encode() is
> >> * invoked in contexts where a memory allocation failure is
> >> @@ -177,7 +178,8 @@ bool nfs_stream_encode_acl(struct xdr_stream *xdr, struct inode *inode,
> >> return false;
> >>
> >> if (encode_entries && acl && acl->a_count == 3) {
> >> - struct posix_acl *acl2 = &aclbuf.acl;
> >> + struct posix_acl *acl2 =
> >> + container_of(&aclbuf.acl, struct posix_acl, hdr);
> >>
> >> /* Avoid the use of posix_acl_alloc(). nfsacl_encode() is
> >> * invoked in contexts where a memory allocation failure is
> >> diff --git a/include/linux/posix_acl.h b/include/linux/posix_acl.h
> >> index e2d47eb1a7f3..62d497763e25 100644
> >> --- a/include/linux/posix_acl.h
> >> +++ b/include/linux/posix_acl.h
> >> @@ -27,11 +27,16 @@ struct posix_acl_entry {
> >> };
> >>
> >> struct posix_acl {
> >> - refcount_t a_refcount;
> >> - unsigned int a_count;
> >> - struct rcu_head a_rcu;
> >> + /* New members MUST be added within the struct_group() macro below. */
> >> + struct_group_tagged(posix_acl_hdr, hdr,
> >> + refcount_t a_refcount;
> >> + unsigned int a_count;
> >> + struct rcu_head a_rcu;
> >> + );
> >> struct posix_acl_entry a_entries[] __counted_by(a_count);
> >> };
> >> +static_assert(offsetof(struct posix_acl, a_entries) == sizeof(struct posix_acl_hdr),
> >> + "struct member likely outside of struct_group_tagged()");
> >>
> >> #define FOREACH_ACL_ENTRY(pa, acl, pe) \
> >> for(pa=(acl)->a_entries, pe=pa+(acl)->a_count; pa<pe; pa++)
> >
> > Trond, Anna -
> >
> > Let me know if I need to take this one via the NFSD tree. If not,
> >
> > Acked-by: Chuck Lever <chuck.lever@...cle.com>
>
> Gentle ping: Still waiting for a response.
If you want it, it's yours! :)
I am planning a bugfixes pull request soon, so I don't mind taking it
if you don't have anything else planned for 6.14 at the moment.
Anna
>
>
> --
> Chuck Lever
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