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[1/6] netfs: Eliminate Clang randstruct warning

Message ID 20220503205503.3054173-2-keescook@chromium.org (mailing list archive)
State New
Headers show
Series randstruct: Enable Clang support | expand

Commit Message

Kees Cook May 3, 2022, 8:54 p.m. UTC
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@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/netfs.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Jeff Layton May 3, 2022, 9:31 p.m. UTC | #1
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@redhat.com>
> Cc: Jeff Layton <jlayton@kernel.org>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> ---
>  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@kernel.org>
diff mbox series

Patch

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);
 }
 
 /**