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SubjectRe: [PATCH v2] kernel.h: handle pointers to arrays better in container_of()
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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@mev.co.uk> )=-
-=( Web: http://www.mev.co.uk/ )=-

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