linux-kernel - Re: [PATCH v2] kernel.h: handle pointers to arrays better in container

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Date:   Tue, 23 May 2017 14:23:47 +0100
From:   Ian Abbott <abbotti@....co.uk>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Michal Nazarewicz <mina86@...a86.com>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Hidehiro Kawai <hidehiro.kawai.ez@...achi.com>,
        Borislav Petkov <bp@...e.de>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Johannes Berg <johannes.berg@...el.com>,
        Alexander Potapenko <glider@...gle.com>
Subject: Re: [PATCH v2] kernel.h: handle pointers to arrays better in
 container_of()

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@....co.uk> )=-
-=(                          Web: http://www.mev.co.uk/  )=-