Re: [mic:next 16/24] include/linux/build_bug.h:78:41: error: static assertion failed: "sizeof(typeof_member(union access_masks_all, masks)) == sizeof(typeof_member(union access_masks_all, all))"

["Günther Noack" <gnoack@google.com>]

On Thu, Mar 26, 2026 at 02:42:56PM +0800, kernel test robot wrote:
> tree:   https://git.kernel.org/pub/scm/linux/kernel/git/mic/linux.git next
> head:   42d105958e98c6ae8fd6d62b38727444d25e735d
> commit: 8dcfb7e52f844e73413d7f743945e135747a1048 [16/24] landlock: Control pathname UNIX domain socket resolution by path
> config: m68k-allmodconfig (https://download.01.org/0day-ci/archive/20260326/202603261438.jBx2DGNe-lkp@intel.com/config)
> compiler: m68k-linux-gcc (GCC) 15.2.0
> reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260326/202603261438.jBx2DGNe-lkp@intel.com/reproduce)
> 
> If you fix the issue in a separate patch/commit (i.e. not just a new version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: https://lore.kernel.org/oe-kbuild-all/202603261438.jBx2DGNe-lkp@intel.com/
> 
> All errors (new ones prefixed by >>):
> 
>    In file included from include/linux/bits.h:30,
>                     from security/landlock/setup.c:9:
> >> include/linux/build_bug.h:78:41: error: static assertion failed: "sizeof(typeof_member(union access_masks_all, masks)) == sizeof(typeof_member(union access_masks_all, all))"
>       78 | #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
>          |                                         ^~~~~~~~~~~~~~
>    include/linux/build_bug.h:77:34: note: in expansion of macro '__static_assert'
>       77 | #define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
>          |                                  ^~~~~~~~~~~~~~~
>    security/landlock/access.h:61:1: note: in expansion of macro 'static_assert'
>       61 | static_assert(sizeof(typeof_member(union access_masks_all, masks)) ==
>          | ^~~~~~~~~~~~~
> 
> 
> vim +78 include/linux/build_bug.h
> 
> bc6245e5efd70c Ian Abbott       2017-07-10  60  
> 6bab69c65013be Rasmus Villemoes 2019-03-07  61  /**
> 6bab69c65013be Rasmus Villemoes 2019-03-07  62   * static_assert - check integer constant expression at build time
> 6bab69c65013be Rasmus Villemoes 2019-03-07  63   *
> 6bab69c65013be Rasmus Villemoes 2019-03-07  64   * static_assert() is a wrapper for the C11 _Static_assert, with a
> 6bab69c65013be Rasmus Villemoes 2019-03-07  65   * little macro magic to make the message optional (defaulting to the
> 6bab69c65013be Rasmus Villemoes 2019-03-07  66   * stringification of the tested expression).
> 6bab69c65013be Rasmus Villemoes 2019-03-07  67   *
> 6bab69c65013be Rasmus Villemoes 2019-03-07  68   * Contrary to BUILD_BUG_ON(), static_assert() can be used at global
> 6bab69c65013be Rasmus Villemoes 2019-03-07  69   * scope, but requires the expression to be an integer constant
> 6bab69c65013be Rasmus Villemoes 2019-03-07  70   * expression (i.e., it is not enough that __builtin_constant_p() is
> 6bab69c65013be Rasmus Villemoes 2019-03-07  71   * true for expr).
> 6bab69c65013be Rasmus Villemoes 2019-03-07  72   *
> 6bab69c65013be Rasmus Villemoes 2019-03-07  73   * Also note that BUILD_BUG_ON() fails the build if the condition is
> 6bab69c65013be Rasmus Villemoes 2019-03-07  74   * true, while static_assert() fails the build if the expression is
> 6bab69c65013be Rasmus Villemoes 2019-03-07  75   * false.
> 6bab69c65013be Rasmus Villemoes 2019-03-07  76   */
> 6bab69c65013be Rasmus Villemoes 2019-03-07  77  #define static_assert(expr, ...) __static_assert(expr, ##__VA_ARGS__, #expr)
> 6bab69c65013be Rasmus Villemoes 2019-03-07 @78  #define __static_assert(expr, msg, ...) _Static_assert(expr, msg)
> 6bab69c65013be Rasmus Villemoes 2019-03-07  79  
> 07a368b3f55a79 Maxim Levitsky   2022-10-25  80  
> 
> :::::: The code at line 78 was first introduced by commit
> :::::: 6bab69c65013bed5fce9f101a64a84d0385b3946 build_bug.h: add wrapper for _Static_assert
> 
> :::::: TO: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> :::::: CC: Linus Torvalds <torvalds@linux-foundation.org>
> 
> -- 
> 0-DAY CI Kernel Test Service
> https://github.com/intel/lkp-tests/wiki

Hmm, intuitively, it might have been introduced by the Unix Connect
Control patch set which was recently added to mic-next:

https://git.kernel.org/pub/scm/linux/kernel/git/mic/linux.git/commit/?h=next&id=8dcfb7e52f844e73413d7f743945e135747a1048

m68k is a 32-bit architecture.

We are introducing the 17th filesystem access bit, and changing the
size of access_mask_t from u16 to u32 there,

The relevant code is:

```
/* Ruleset access masks. */
struct access_masks {
	access_mask_t fs : LANDLOCK_NUM_ACCESS_FS;    /* 17 */
	access_mask_t net : LANDLOCK_NUM_ACCESS_NET;  /*  2 */
	access_mask_t scope : LANDLOCK_NUM_SCOPE;     /*  2 */
};

union access_masks_all {
	struct access_masks masks;
	u32 all;
};

/* Makes sure all fields are covered. */
static_assert(sizeof(typeof_member(union access_masks_all, masks)) ==
	      sizeof(typeof_member(union access_masks_all, all)));
```

Intuitively, it is not clear why that static assertion suddenly fails;
17+2+2 is still well below 32.  I see that the C standard does not
give strong guarantees about the memory layout of this, though.

I'll need to have a closer look later. I can imagine that there is an
attribute or such that will hint the compiler to pack these bits
tighter.

—Günther