[PATCH] netfs: Use container_of() for offset casting

[PATCH] netfs: Use container_of() for offset casting

[Kees Cook]

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. Switch to using
an internally defined netfs_i_context/inode struct for doing a full
container_of() casting. This keeps both randstruct and __bos() happy
under GCC 12. Silences:

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);
      |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Reported-by: Jeff Layton <jlayton@kernel.org>
Link: https://lore.kernel.org/lkml/d2ad3a3d7bdd794c6efb562d2f2b655fb67756b9.camel@kernel.org
Cc: Jeff Layton <jlayton@kernel.org>
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
If this looks good I can add it to my hardening tree, or if you want to
carry it, I can respin this without the earlier randstruct changes and
drop that patch from my tree?
---
 include/linux/netfs.h | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/linux/netfs.h b/include/linux/netfs.h
index 0c33b715cbfd..cce5a9b53a8a 100644
--- a/include/linux/netfs.h
+++ b/include/linux/netfs.h
@@ -286,6 +286,17 @@ extern void netfs_put_subrequest(struct netfs_io_subrequest *subreq,
 				 bool was_async, enum netfs_sreq_ref_trace what);
 extern void netfs_stats_show(struct seq_file *);
 
+/*
+ * The struct netfs_i_context instance must always follow the VFS inode,
+ * but existing users want to avoid a substructure name space, so just
+ * use this internally to perform the needed container_of() offset
+ * casting, which will keep both FORTIFY_SOURCE and randstruct happy.
+ */
+struct netfs_i_c_pair {
+	struct inode inode;
+	struct netfs_i_context ctx;
+};
+
 /**
  * netfs_i_context - Get the netfs inode context from the inode
  * @inode: The inode to query
@@ -295,7 +306,7 @@ extern void netfs_stats_show(struct seq_file *);
  */
 static inline struct netfs_i_context *netfs_i_context(struct inode *inode)
 {
-	return (void *)inode + sizeof(*inode);
+	return &container_of(inode, struct netfs_i_c_pair, inode)->ctx;
 }
 
 /**
@@ -307,7 +318,7 @@ static inline struct netfs_i_context *netfs_i_context(struct inode *inode)
  */
 static inline struct inode *netfs_inode(struct netfs_i_context *ctx)
 {
-	return (void *)ctx - sizeof(struct inode);
+	return &container_of(ctx, struct netfs_i_c_pair, ctx)->inode;
 }
 
 /**
-- 
2.32.0

Re: [PATCH] netfs: Use container_of() for offset casting

[Jeff Layton]

On Tue, 2022-05-17 at 14:02 -0700, Kees Cook wrote:
> 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. Switch to using
> an internally defined netfs_i_context/inode struct for doing a full
> container_of() casting. This keeps both randstruct and __bos() happy
> under GCC 12. Silences:
> 
> 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);
>       |                         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> 
> Reported-by: Jeff Layton <jlayton@kernel.org>
> Link: https://lore.kernel.org/lkml/d2ad3a3d7bdd794c6efb562d2f2b655fb67756b9.camel@kernel.org
> Cc: Jeff Layton <jlayton@kernel.org>
> Cc: David Howells <dhowells@redhat.com>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
> If this looks good I can add it to my hardening tree, or if you want to
> carry it, I can respin this without the earlier randstruct changes and
> drop that patch from my tree?
> ---
>  include/linux/netfs.h | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/netfs.h b/include/linux/netfs.h
> index 0c33b715cbfd..cce5a9b53a8a 100644
> --- a/include/linux/netfs.h
> +++ b/include/linux/netfs.h
> @@ -286,6 +286,17 @@ extern void netfs_put_subrequest(struct netfs_io_subrequest *subreq,
>  				 bool was_async, enum netfs_sreq_ref_trace what);
>  extern void netfs_stats_show(struct seq_file *);
>  
> +/*
> + * The struct netfs_i_context instance must always follow the VFS inode,
> + * but existing users want to avoid a substructure name space, so just
> + * use this internally to perform the needed container_of() offset
> + * casting, which will keep both FORTIFY_SOURCE and randstruct happy.
> + */
> +struct netfs_i_c_pair {
> +	struct inode inode;
> +	struct netfs_i_context ctx;
> +};
> +
>  /**
>   * netfs_i_context - Get the netfs inode context from the inode
>   * @inode: The inode to query
> @@ -295,7 +306,7 @@ extern void netfs_stats_show(struct seq_file *);
>   */
>  static inline struct netfs_i_context *netfs_i_context(struct inode *inode)
>  {
> -	return (void *)inode + sizeof(*inode);
> +	return &container_of(inode, struct netfs_i_c_pair, inode)->ctx;
>  }
>  
>  /**
> @@ -307,7 +318,7 @@ static inline struct netfs_i_context *netfs_i_context(struct inode *inode)
>   */
>  static inline struct inode *netfs_inode(struct netfs_i_context *ctx)
>  {
> -	return (void *)ctx - sizeof(struct inode);
> +	return &container_of(ctx, struct netfs_i_c_pair, ctx)->inode;
>  }
>  
>  /**

This patch didn't apply cleanly for me to a recent tree, but I was able
to wiggle it into place and it seemed to work.

Tested-by: Jeff Layton <jlayton@kernel.org>

Re: [PATCH] netfs: Use container_of() for offset casting

[David Howells]

I wonder if it would be worth making this explicit in the inode wrappers of
the users of netfslib.  In afs, for instance, there is:

	struct afs_vnode {
		struct {
			/* These must be contiguous */
			struct inode	vfs_inode;
			struct netfs_i_context netfs_ctx;
		};
		...
	};

would it be worth making that:

	struct afs_vnode {
		union {
			struct netfs_i_c_pair netfs_inode;
			struct {
				/* These must be contiguous */
				struct inode	vfs_inode;
				struct netfs_i_context netfs_ctx;
			};
		};
		...
	};

I don't want to do the following, say:


	struct afs_vnode {
		struct netfs_i_c_pair ni;
		...
	};

as that would then require a lot of s/->vfs_inode/->ni.vfs_inode/, but maybe
it would be better to include a struct inode in struct netfs_i_context, and
then do:

	struct afs_vnode {
		union {
			struct netfs_i_context netfs_ctx;
			struct inode vfs_inode;
		};
		...
	};

and perhaps rename netfs_i_context to netfs_inode (though that looks a bit
close to nfs_inode).  It's just a shame I can't do:

	struct netfs_inode : inode {
		...
	};

	struct afs_vnode : netfs_inode {
		...
	};

right? ;-)

On the other hand:

	warthog>git grep '[>.]vfs_inode' -- fs/{9p,afs,ceph,cifs,nfs} | wc -l
	181

so maybe a mass change to, say:

	struct netfs_inode {
		struct inode vfs_inode;
		...
	};

	struct afs_vnode {
		struct netfs_inode ni;
		...
	};

wouldn't be so bad.

David

RE: [PATCH] netfs: Use container_of() for offset casting

[David Laight]

From: David Howells 
> Sent: 18 May 2022 09:05
> 
> I wonder if it would be worth making this explicit in the inode wrappers of
> the users of netfslib.  In afs, for instance, there is:
> 
> 	struct afs_vnode {
> 		struct {
> 			/* These must be contiguous */
> 			struct inode	vfs_inode;
> 			struct netfs_i_context netfs_ctx;
> 		};
> 		...
> 	};
> 
> would it be worth making that:
> 
> 	struct afs_vnode {
> 		union {
> 			struct netfs_i_c_pair netfs_inode;
> 			struct {
> 				/* These must be contiguous */
> 				struct inode	vfs_inode;
> 				struct netfs_i_context netfs_ctx;
> 			};
> 		};
> 		...
> 	};
> 

Can't you just name the structure so it is:

	struct afs_vnode {
 		struct netfs_i_c_pair {
 			/* These must be contiguous */
 			struct inode	vfs_inode;
 			struct netfs_i_context netfs_ctx;
 		};
 		...
 	};

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)

Re: [PATCH] netfs: Use container_of() for offset casting

[David Howells]

David Laight <David.Laight@ACULAB.COM> wrote:

> Can't you just name the structure so it is:
> 
> 	struct afs_vnode {
>  		struct netfs_i_c_pair {
>  			/* These must be contiguous */
>  			struct inode	vfs_inode;
>  			struct netfs_i_context netfs_ctx;
>  		};
>  		...
>  	};

No.  C won't let you do that (the same thing has to be done in 9p, ceph as
well, and at some point hopefully cifs and nfs too).

David

Re: [PATCH] netfs: Use container_of() for offset casting

[Kees Cook]

On Wed, May 18, 2022 at 09:05:14AM +0100, David Howells wrote:
> [...]
> I don't want to do the following, say:
> 
> 
> 	struct afs_vnode {
> 		struct netfs_i_c_pair ni;
> 		...
> 	};
> 
> as that would then require a lot of s/->vfs_inode/->ni.vfs_inode/, but maybe
> it would be better to include a struct inode in struct netfs_i_context, and

Right; that's why I kept the struct internal -- the implicit ordering of
inode and netfs_i_context is already present in all the users.

> On the other hand:
> 
> 	warthog>git grep '[>.]vfs_inode' -- fs/{9p,afs,ceph,cifs,nfs} | wc -l
> 	181

That seems painful. Maybe _new_ users of netfs could be written to use
the proposed netfs_inode:

> so maybe a mass change to, say:
> 
> 	struct netfs_inode {
> 		struct inode vfs_inode;
> 		...
> 	};

Better yet, netfs can define a macro helper. I'll send a v2...

-- 
Kees Cook