Messages in this thread Patch in this message | | | Date | Tue, 17 May 2022 13:53:45 -0700 | From | Kees Cook <> | Subject | Re: new __write_overflow_field compiler warning |
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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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