linux-kernel - [PATCH 1/6] netfs: Eliminate Clang randstruct warning

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Date:   Tue,  3 May 2022 13:54:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Bill Wendling <morbo@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        David Howells <dhowells@...hat.com>,
        Jeff Layton <jlayton@...nel.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org,
        linux-hardening@...r.kernel.org, llvm@...ts.linux.dev
Subject: [PATCH 1/6] netfs: Eliminate Clang randstruct warning

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@...hat.com>
Cc: Jeff Layton <jlayton@...nel.org>
Signed-off-by: Kees Cook <keescook@...omium.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);
 }
 
 /**
-- 
2.32.0