ext4: don't use kfree() on rcu protected pointer sbi->s_qf_names

During ext4 mount api rework the commit e6e268cb ("ext4: move quota
configuration out of handle_mount_opt()") introduced a bug where we
would kfree(sbi->s_qf_names[i]) before assigning the new quota name in
ext4_apply_quota_options().

This is wrong because we're using kfree() on rcu prointer that could be
simultaneously accessed from ext4_show_quota_options() during remount.
Fix it by using rcu_replace_pointer() to replace the old qname with the
new one and then kfree_rcu() the old quota name.

Also use get_qf_name() instead of sbi->s_qf_names in strcmp() to silence
the sparse warning.

Fixes: e6e268cb ("ext4: move quota configuration out of handle_mount_opt()")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Link: https://lore.kernel.org/r/20220104143518.134465-1-lczerner@redhat.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -2636,8 +2636,10 @@ static void ext4_apply_quota_options(struct fs_context *fc,
 
 			qname = ctx->s_qf_names[i]; /* May be NULL */
 			ctx->s_qf_names[i] = NULL;
-			kfree(sbi->s_qf_names[i]);
-			rcu_assign_pointer(sbi->s_qf_names[i], qname);
+			qname = rcu_replace_pointer(sbi->s_qf_names[i], qname,
+						lockdep_is_held(&sb->s_umount));
+			if (qname)
+				kfree_rcu(qname);
 			set_opt(sb, QUOTA);
 		}
 	}
@@ -2691,7 +2693,7 @@ static int ext4_check_quota_consistency(struct fs_context *fc,
 				goto err_jquota_change;
 
 			if (sbi->s_qf_names[i] && ctx->s_qf_names[i] &&
-			    strcmp(sbi->s_qf_names[i],
+			    strcmp(get_qf_name(sb, sbi, i),
 				   ctx->s_qf_names[i]) != 0)
 				goto err_jquota_specified;
 		}