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SubjectRe: new __write_overflow_field compiler warning
On Tue, May 17, 2022 at 02:03:52PM -0400, Jeff Layton wrote:
> Hi Kees,

Hi!

> I'm hoping you can help with this. I recently updated to Fedora 36,
> which has gcc v12, and I've started seeing this warning pop up when
> compiling the ceph.ko:
>
> 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);
> | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
> This doesn't seem to happen with gcc v11. It looks like the code is
> doing the right thing. Is there something we need to fix how the netfs
> context gets initialized or is this a compiler problem?
>
> FWIW: I'm using:
>
> gcc (GCC) 12.1.1 20220507 (Red Hat 12.1.1-1)

Yeah, GCC 12 got "smarter" about how deeply it can analyze object sizes.
Usually, this has been helpful. Other times, it's a bit weirder, like
here.

So this is resolving to:

static inline void netfs_i_context_init(struct inode *inode,
const struct netfs_request_ops *ops)
{
struct netfs_i_context *ctx = netfs_i_context(inode);

memset(ctx, 0, sizeof(*ctx));
...

In the sense that the compiler is having trouble understanding this
object, it's due to the same "unexpected" manipulations that manifest in
other areas (randstruct) which got fixed recently:
https://lore.kernel.org/lkml/20220503205503.3054173-2-keescook@chromium.org/

But it seems randstruct is happy to look the other way here after the
(void *) cast, where as __builtin_object_size() (the work-horse of the
memcpy checking) is not. Hmpf.

Ignoring the linked change above (which doesn't change the warning
here), GCC is effectively seeing:

static inline void netfs_i_context_init(struct inode *inode,
const struct netfs_request_ops *ops)
{
struct netfs_i_context *ctx = (struct netfs_i_context *)(inode + 1);

if (__builtin_object_size(ctx, 1) < sizeof(*ctx))
__write_overflow_field(...)

And __builtin_object_size() see "ctx" as pointing past the end of a single
"struct inode" (i.e. there are zero bytes left in the original
structure).

However, I think we can solve both the FORTIFY and the randstruct
concerns by wrapping the conversions in container_of(). This passes for
me with -next (i.e. on top of the above linked change):

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;
}

/**





--
Kees Cook

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