Re: [PATCH] loop: reject binding to procfs and sysfs files

[kernel test robot <lkp@intel.com>]

Hi Tetsuo,

kernel test robot noticed the following build errors:

[auto build test ERROR on axboe/for-next]
[also build test ERROR on linus/master v7.1-rc5 next-20260529]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch#_base_tree_information]

url:    https://github.com/intel-lab-lkp/linux/commits/Tetsuo-Handa/loop-reject-binding-to-procfs-and-sysfs-files/20260530-214900
base:   https://git.kernel.org/pub/scm/linux/kernel/git/axboe/linux.git for-next
patch link:    https://lore.kernel.org/r/148efba2-a0b6-47d7-ac76-b19d2f4b696c%40I-love.SAKURA.ne.jp
patch subject: [PATCH] loop: reject binding to procfs and sysfs files
config: um-x86_64_defconfig (https://download.01.org/0day-ci/archive/20260531/202605310318.dbidMe6W-lkp@intel.com/config)
compiler: clang version 23.0.0git (https://github.com/llvm/llvm-project 9409c07de6378507397ecdb6f05f628f58110112)
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260531/202605310318.dbidMe6W-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/202605310318.dbidMe6W-lkp@intel.com/

All errors (new ones prefixed by >>):

>> drivers/block/loop.c:504:7: error: use of undeclared identifier 'PROC_SUPER_MAGIC'
     504 |         case PROC_SUPER_MAGIC: /* e.g. "losetup -f /proc/sys/kernel/version" */
         |              ^~~~~~~~~~~~~~~~
>> drivers/block/loop.c:505:7: error: use of undeclared identifier 'SYSFS_MAGIC'
     505 |         case SYSFS_MAGIC: /* e.g. "losetup -f /sys/power/state" */
         |              ^~~~~~~~~~~
   2 errors generated.


vim +/PROC_SUPER_MAGIC +504 drivers/block/loop.c

   478	
   479	static int loop_validate_file(struct file *file, struct block_device *bdev)
   480	{
   481		struct inode	*inode = file->f_mapping->host;
   482		struct file	*f = file;
   483	
   484		/* Avoid recursion */
   485		while (is_loop_device(f)) {
   486			struct loop_device *l;
   487	
   488			lockdep_assert_held(&loop_validate_mutex);
   489			if (f->f_mapping->host->i_rdev == bdev->bd_dev)
   490				return -EBADF;
   491	
   492			l = I_BDEV(f->f_mapping->host)->bd_disk->private_data;
   493			if (l->lo_state != Lo_bound)
   494				return -EINVAL;
   495			/* Order wrt setting lo->lo_backing_file in loop_configure(). */
   496			rmb();
   497			f = l->lo_backing_file;
   498		}
   499		if (S_ISBLK(inode->i_mode))
   500			return 0;
   501		if (!S_ISREG(inode->i_mode))
   502			return -EINVAL;
   503		switch (inode->i_sb->s_magic) {
 > 504		case PROC_SUPER_MAGIC: /* e.g. "losetup -f /proc/sys/kernel/version" */
 > 505		case SYSFS_MAGIC: /* e.g. "losetup -f /sys/power/state" */
   506			return -EINVAL;
   507		}
   508		return 0;
   509	}
   510	

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