netfs: Fix gcc-12 warning by embedding vfs inode in netfs_i_context

While randstruct was satisfied with using an open-coded "void *" offset
cast for the netfs_i_context <-> inode casting, __builtin_object_size() as
used by FORTIFY_SOURCE was not as easily fooled.  This was causing the
following complaint[1] from gcc v12:

  In file included from include/linux/string.h:253,
                   from include/linux/ceph/ceph_debug.h:7,
                   from fs/ceph/inode.c:2:
  In function 'fortify_memset_chk',
      inlined from 'netfs_i_context_init' at include/linux/netfs.h:326:2,
      inlined from 'ceph_alloc_inode' at fs/ceph/inode.c:463:2:
  include/linux/fortify-string.h:242:25: warning: call to '__write_overflow_field' declared with attribute warning: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wattribute-warning]
    242 |                         __write_overflow_field(p_size_field, size);
        |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Fix this by embedding a struct inode into struct netfs_i_context (which
should perhaps be renamed to struct netfs_inode).  The struct inode
vfs_inode fields are then removed from the 9p, afs, ceph and cifs inode
structs and vfs_inode is then simply changed to "netfs.inode" in those
filesystems.

Further, rename netfs_i_context to netfs_inode, get rid of the
netfs_inode() function that converted a netfs_i_context pointer to an
inode pointer (that can now be done with &ctx->inode) and rename the
netfs_i_context() function to netfs_inode() (which is now a wrapper
around container_of()).

Most of the changes were done with:

  perl -p -i -e 's/vfs_inode/netfs.inode/'g \
        `git grep -l 'vfs_inode' -- fs/{9p,afs,ceph,cifs}/*.[ch]`

Kees suggested doing it with a pair structure[2] and a special
declarator to insert that into the network filesystem's inode
wrapper[3], but I think it's cleaner to embed it - and then it doesn't
matter if struct randomisation reorders things.

Dave Chinner suggested using a filesystem-specific VFS_I() function in
each filesystem to convert that filesystem's own inode wrapper struct
into the VFS inode struct[4].

Version #2:
 - Fix a couple of missed name changes due to a disabled cifs option.
 - Rename nfs_i_context to nfs_inode
 - Use "netfs" instead of "nic" as the member name in per-fs inode wrapper
   structs.

[ This also undoes commit 507160f46c55 ("netfs: gcc-12: temporarily
  disable '-Wattribute-warning' for now") that is no longer needed ]

Fixes: bc899ee1c898 ("netfs: Add a netfs inode context")
Reported-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
cc: Jonathan Corbet <corbet@lwn.net>
cc: Eric Van Hensbergen <ericvh@gmail.com>
cc: Latchesar Ionkov <lucho@ionkov.net>
cc: Dominique Martinet <asmadeus@codewreck.org>
cc: Christian Schoenebeck <linux_oss@crudebyte.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Ilya Dryomov <idryomov@gmail.com>
cc: Steve French <smfrench@gmail.com>
cc: William Kucharski <william.kucharski@oracle.com>
cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
cc: Dave Chinner <david@fromorbit.com>
cc: linux-doc@vger.kernel.org
cc: v9fs-developer@lists.sourceforge.net
cc: linux-afs@lists.infradead.org
cc: ceph-devel@vger.kernel.org
cc: linux-cifs@vger.kernel.org
cc: samba-technical@lists.samba.org
cc: linux-fsdevel@vger.kernel.org
cc: linux-hardening@vger.kernel.org
Link: https://lore.kernel.org/r/d2ad3a3d7bdd794c6efb562d2f2b655fb67756b9.camel@kernel.org/ [1]
Link: https://lore.kernel.org/r/20220517210230.864239-1-keescook@chromium.org/ [2]
Link: https://lore.kernel.org/r/20220518202212.2322058-1-keescook@chromium.org/ [3]
Link: https://lore.kernel.org/r/20220524101205.GI2306852@dread.disaster.area/ [4]
Link: https://lore.kernel.org/r/165296786831.3591209.12111293034669289733.stgit@warthog.procyon.org.uk/ # v1
Link: https://lore.kernel.org/r/165305805651.4094995.7763502506786714216.stgit@warthog.procyon.org.uk # v2
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 18 insertions, 19 deletions

diff --git a/Documentation/filesystems/netfs_library.rst b/Documentation/filesystems/netfs_library.rst
index a80a59941d2f..3276c3d55142 100644
--- a/Documentation/filesystems/netfs_library.rst
+++ b/Documentation/filesystems/netfs_library.rst
@@ -37,30 +37,31 @@ The network filesystem helper library needs a place to store a bit of state for
 its use on each netfs inode it is helping to manage.  To this end, a context
 structure is defined::
 
-	struct netfs_i_context {
+	struct netfs_inode {
+		struct inode inode;
 		const struct netfs_request_ops *ops;
-		struct fscache_cookie	*cache;
+		struct fscache_cookie *cache;
 	};
 
-A network filesystem that wants to use netfs lib must place one of these
-directly after the VFS ``struct inode`` it allocates, usually as part of its
-own struct.  This can be done in a way similar to the following::
+A network filesystem that wants to use netfs lib must place one of these in its
+inode wrapper struct instead of the VFS ``struct inode``.  This can be done in
+a way similar to the following::
 
 	struct my_inode {
-		struct {
-			/* These must be contiguous */
-			struct inode		vfs_inode;
-			struct netfs_i_context  netfs_ctx;
-		};
+		struct netfs_inode netfs; /* Netfslib context and vfs inode */
 		...
 	};
 
-This allows netfslib to find its state by simple offset from the inode pointer,
-thereby allowing the netfslib helper functions to be pointed to directly by the
-VFS/VM operation tables.
+This allows netfslib to find its state by using ``container_of()`` from the
+inode pointer, thereby allowing the netfslib helper functions to be pointed to
+directly by the VFS/VM operation tables.
 
 The structure contains the following fields:
 
+ * ``inode``
+
+   The VFS inode structure.
+
  * ``ops``
 
    The set of operations provided by the network filesystem to netfslib.
@@ -78,14 +79,12 @@ To help deal with the per-inode context, a number helper functions are
 provided.  Firstly, a function to perform basic initialisation on a context and
 set the operations table pointer::
 
-	void netfs_i_context_init(struct inode *inode,
-				  const struct netfs_request_ops *ops);
+	void netfs_inode_init(struct inode *inode,
+			      const struct netfs_request_ops *ops);
 
-then two functions to cast between the VFS inode structure and the netfs
-context::
+then a function to cast from the VFS inode structure to the netfs context::
 
-	struct netfs_i_context *netfs_i_context(struct inode *inode);
-	struct inode *netfs_inode(struct netfs_i_context *ctx);
+	struct netfs_inode *netfs_node(struct inode *inode);
 
 and finally, a function to get the cache cookie pointer from the context
 attached to an inode (or NULL if fscache is disabled)::