Re: [PATCH] kernfs: avoid READ_ONCE() on a kgid_t in kernfs_new_node()

[Tejun Heo <tj@kernel.org>]

On Tue, Oct 27, 2026 at 07:32:47AM -0700, Shakeel Butt wrote:
> kernfs_new_node() reads the parent's gid with READ_ONCE(attrs->ia_gid).
> That gid is a kgid_t, which is a struct.
> 
> On SMP alpha, READ_ONCE() ends by casting the value it loaded back to
> the type of its argument. Here that is a cast from a struct to the same
> struct. GCC accepts that as an extension, but ISO C does not allow a
> cast to or from a struct type, so sparse warns:
> 
>   fs/kernfs/dir.c:751:31: sparse: sparse: cast to non-scalar
>   fs/kernfs/dir.c:751:31: sparse: sparse: cast from non-scalar
> 
> Read the gid_t inside the kgid_t instead, and wrap it back up with
> KGIDT_INIT(). This is still a single marked load of the same four bytes.
> 
> There is no functional change. With GCC 12 for alpha (the config from
> the report) and GCC 14 for x86-64 (with and without KCSAN),
> fs/kernfs/dir.o has the same object code before and after this change.
> 
> Fixes: 81bdc85b4c08 ("kernfs: fix up the unlocked attribute reads on the creation paths")
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202610261608.vItiXCGW-lkp@intel.com/
> Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>

Acked-by: Tejun Heo <tj@kernel.org>

Thanks.

-- 
tejun