Re: [PATCH] nfsd: fix NULL pointer dereference in check_export()

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

Hi Jeff,

kernel test robot noticed the following build warnings:

[auto build test WARNING on 983d014aafb14ee5e4915465bf8948e8f3a723b5]

url:    https://github.com/intel-lab-lkp/linux/commits/Jeff-Layton/nfsd-fix-NULL-pointer-dereference-in-check_export/20260117-022519
base:   983d014aafb14ee5e4915465bf8948e8f3a723b5
patch link:    https://lore.kernel.org/r/20260116-nfsd-fixes-v1-1-019689b72747%40kernel.org
patch subject: [PATCH] nfsd: fix NULL pointer dereference in check_export()
config: parisc-defconfig (https://download.01.org/0day-ci/archive/20260117/202601171021.4DQZyXiU-lkp@intel.com/config)
compiler: hppa-linux-gcc (GCC) 15.2.0
reproduce (this is a W=1 build): (https://download.01.org/0day-ci/archive/20260117/202601171021.4DQZyXiU-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/202601171021.4DQZyXiU-lkp@intel.com/

All warnings (new ones prefixed by >>):

   fs/nfsd/export.c: In function 'check_export':
>> fs/nfsd/export.c:408:47: warning: initialization discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
     408 |         struct export_operations *export_op = inode->i_sb->s_export_op;
         |                                               ^~~~~


vim +/const +408 fs/nfsd/export.c

   404	
   405	static int check_export(const struct path *path, int *flags, unsigned char *uuid)
   406	{
   407		struct inode *inode = d_inode(path->dentry);
 > 408		struct export_operations *export_op = inode->i_sb->s_export_op;
   409	
   410		/*
   411		 * We currently export only dirs, regular files, and (for v4
   412		 * pseudoroot) symlinks.
   413		 */
   414		if (!S_ISDIR(inode->i_mode) &&
   415		    !S_ISLNK(inode->i_mode) &&
   416		    !S_ISREG(inode->i_mode))
   417			return -ENOTDIR;
   418	
   419		/*
   420		 * Mountd should never pass down a writeable V4ROOT export, but,
   421		 * just to make sure:
   422		 */
   423		if (*flags & NFSEXP_V4ROOT)
   424			*flags |= NFSEXP_READONLY;
   425	
   426		/* There are four requirements on a filesystem to be exportable:
   427		 * 1: It must define sb->s_export_op
   428		 * 2: We must be able to identify the filesystem from a number.
   429		 *       either a device number (so FS_REQUIRES_DEV needed)
   430		 *       or an FSID number (so NFSEXP_FSID or ->uuid is needed).
   431		 * 3: We must be able to find an inode from a filehandle.
   432		 *       This means that s_export_op must be set.
   433		 * 4: We must not currently be on an idmapped mount.
   434		 */
   435		if (!export_op) {
   436			dprintk("%s: fs doesn't define export_operations!\n", __func__);
   437			return -EINVAL;
   438		}
   439	
   440		if (!(inode->i_sb->s_type->fs_flags & FS_REQUIRES_DEV) &&
   441		    !(*flags & NFSEXP_FSID) &&
   442		    uuid == NULL) {
   443			dprintk("exp_export: export of non-dev fs without fsid\n");
   444			return -EINVAL;
   445		}
   446	
   447		if (!exportfs_can_decode_fh(export_op)) {
   448			dprintk("exp_export: export of invalid fs type.\n");
   449			return -EINVAL;
   450		}
   451	
   452		if (is_idmapped_mnt(path->mnt)) {
   453			dprintk("exp_export: export of idmapped mounts not yet supported.\n");
   454			return -EINVAL;
   455		}
   456	
   457		if (export_op->flags & EXPORT_OP_NOSUBTREECHK &&
   458		    !(*flags & NFSEXP_NOSUBTREECHECK)) {
   459			dprintk("%s: %s does not support subtree checking!\n",
   460				__func__, inode->i_sb->s_type->name);
   461			return -EINVAL;
   462		}
   463		return 0;
   464	}
   465	

-- 
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