On Tue, 2022-05-03 at 13:54 -0700, Kees Cook wrote:
> Clang's structure layout randomization feature gets upset when it sees
> struct inode (which is randomized) cast to struct netfs_i_context. This
> is due to seeing the inode pointer as being treated as an array of inodes,
> rather than "something else, following struct inode".
>
> Since netfs can't use container_of() (since it doesn't know what the
> true containing struct is), it uses this direct offset instead. Adjust
> the code to better reflect what is happening: an arbitrary pointer is
> being adjusted and cast to something else: use a "void *" for the math.
> The resulting binary output is the same, but Clang no longer sees an
> unexpected cross-structure cast:
>
> In file included from ../fs/nfs/inode.c:50:
> In file included from ../fs/nfs/fscache.h:15:
> In file included from ../include/linux/fscache.h:18:
> ../include/linux/netfs.h:298:9: error: casting from randomized structure pointer type 'struct inode *' to 'struct netfs_i_context *'
> return (struct netfs_i_context *)(inode + 1);
> ^
> 1 error generated.
>
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: Jeff Layton <jlayton@xxxxxxxxxx>
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> include/linux/netfs.h | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/include/linux/netfs.h b/include/linux/netfs.h
> index c7bf1eaf51d5..0c33b715cbfd 100644
> --- a/include/linux/netfs.h
> +++ b/include/linux/netfs.h
> @@ -295,7 +295,7 @@ extern void netfs_stats_show(struct seq_file *);
> */
> static inline struct netfs_i_context *netfs_i_context(struct inode *inode)
> {
> - return (struct netfs_i_context *)(inode + 1);
> + return (void *)inode + sizeof(*inode);
> }
>
> /**
> @@ -307,7 +307,7 @@ static inline struct netfs_i_context *netfs_i_context(struct inode *inode)
> */
> static inline struct inode *netfs_inode(struct netfs_i_context *ctx)
> {
> - return ((struct inode *)ctx) - 1;
> + return (void *)ctx - sizeof(struct inode);
> }
>
> /**
Looks reasonable.
Reviewed-by: Jeff Layton <jlayton@xxxxxxxxxx>
