Re: [PATCH v2] kernel.h: handle pointers to arrays better in container_of()

[Ian Abbott]

On 23/05/17 13:02, Ian Abbott wrote:
> On 23/05/17 12:24, Peter Zijlstra wrote:
> > On Tue, May 23, 2017 at 11:32:02AM +0100, Ian Abbott wrote:
> >
> > > > #define container_of(ptr, type, member) (                \
> > > >     _Static_assert(__builtin_types_compatible_p(            \
> > > >             typeof(*ptr), typeof( ((type *)0)->member )), "WUT"),    \
> > > >     ((type *)((char *)(ptr) - offsetof(type, member)));        \
> > > > )
> > >
> > > It's a fine suggestion (if more parentheses are added), but
> > > _Static_assert
> > > is a C11 feature, and I thought the kernel was using gnu89 (unless
> > > it's been
> > > updated since).
> >
> > We have BUILD_BUG_ON() that should be similar in functionality.
>
> Yes, I think BUILD_BUG_ON_ZERO() will be better in this case to avoid
> some baggage.  Also, __same_type() can be used for type checking.

BUILD_BUG_ON_ZERO() in the left-hand side of a comma expression produced 
lots of warnings about it having no effect, so I tried using 
BUILD_BUG_ON() instead as follows:

#define container_of(ptr, type, member) ({			\
	BUILD_BUG_ON(!__same_type((ptr), &((type *)0)->member));\
	((type *)((char *)(ptr) - offsetof(type, member)));})

Unfortunately, this does result in compiler failures in places where the 
types do not in fact match, for example `slab_show()` in 
"mm/slab_common.c" (`list_entry()` uses `container_of()`):

static int slab_show(struct seq_file *m, void *p)
{
	struct kmem_cache *s = list_entry(p, struct kmem_cache, root_caches_node);


I then changed it to allow `ptr` to be a `void *` too:

#define container_of(ptr, type, member) ({			\
	BUILD_BUG_ON(!__same_type((ptr), &((type *)0)->member) &&\
		     !__same_type((ptr), void *));		\
	((type *)((char *)(ptr) - offsetof(type, member)));})

Now it fails on `external_name()` in "fs/dcache.c":

static inline struct external_name *external_name(struct dentry *dentry)
{
	return container_of(dentry->d_name.name, struct external_name, name[0]);
}

Here, `dentry->d_name.name` is of type `const unsigned char *` and 
`name[0]` in `struct external_name` is of type `unsigned char`.  The 
error is:

error: call to ‘__compiletime_assert_258’ declared with attribute error: 
BUILD_BUG_ON failed: !__same_type((dentry->d_name.name), &((struct 
external_name *)0)->name[0]) && !__same_type((dentry->d_name.name), void *)
  _compiletime_assert(condition, msg, __compiletime_assert_, __LINE__)
                                      ^

I'm not quite sure why that is failing yet.

--
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@xxxxxxxxx> )=-
-=(                          Web: http://www.mev.co.uk/  )=-