fs/quota: add kernel_quotactl() helper; remove in-kernel call to syscall

Using the fs-internal kernel_quotactl() helper allows us to get rid of
the fs-internal call to the sys_quotactl() syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Acked-by: Jan Kara <jack@suse.cz>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

fs/quota/compat.c

@@ -59,7 +59,7 @@ asmlinkage long sys32_quotactl(unsigned int cmd, const char __user *special,
 	case Q_GETQUOTA:
 		dqblk = compat_alloc_user_space(sizeof(struct if_dqblk));
 		compat_dqblk = addr;
-		ret = sys_quotactl(cmd, special, id, dqblk);
+		ret = kernel_quotactl(cmd, special, id, dqblk);
 		if (ret)
 			break;
 		if (copy_in_user(compat_dqblk, dqblk, sizeof(*compat_dqblk)) ||
@@ -75,12 +75,12 @@ asmlinkage long sys32_quotactl(unsigned int cmd, const char __user *special,
 			get_user(data, &compat_dqblk->dqb_valid) ||
 			put_user(data, &dqblk->dqb_valid))
 			break;
-		ret = sys_quotactl(cmd, special, id, dqblk);
+		ret = kernel_quotactl(cmd, special, id, dqblk);
 		break;
 	case Q_XGETQSTAT:
 		fsqstat = compat_alloc_user_space(sizeof(struct fs_quota_stat));
 		compat_fsqstat = addr;
-		ret = sys_quotactl(cmd, special, id, fsqstat);
+		ret = kernel_quotactl(cmd, special, id, fsqstat);
 		if (ret)
 			break;
 		ret = -EFAULT;
@@ -113,7 +113,7 @@ asmlinkage long sys32_quotactl(unsigned int cmd, const char __user *special,
 		ret = 0;
 		break;
 	default:
-		ret = sys_quotactl(cmd, special, id, addr);
+		ret = kernel_quotactl(cmd, special, id, addr);
 	}
 	return ret;
 }

fs/quota/quota.c

@@ -833,8 +833,8 @@ static struct super_block *quotactl_block(const char __user *special, int cmd)
  * calls. Maybe we need to add the process quotas etc. in the future,
  * but we probably should use rlimits for that.
  */
-SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
-		qid_t, id, void __user *, addr)
+int kernel_quotactl(unsigned int cmd, const char __user *special,
+		    qid_t id, void __user *addr)
 {
 	uint cmds, type;
 	struct super_block *sb = NULL;
@@ -885,3 +885,9 @@ out:
 		path_put(pathp);
 	return ret;
 }
+
+SYSCALL_DEFINE4(quotactl, unsigned int, cmd, const char __user *, special,
+		qid_t, id, void __user *, addr)
+{
+	return kernel_quotactl(cmd, special, id, addr);
+}

include/linux/quotaops.h

@@ -27,6 +27,9 @@ static inline bool is_quota_modification(struct inode *inode, struct iattr *ia)
 		(ia->ia_valid & ATTR_GID && !gid_eq(ia->ia_gid, inode->i_gid));
 }
 
+int kernel_quotactl(unsigned int cmd, const char __user *special,
+		    qid_t id, void __user *addr);
+
 #if defined(CONFIG_QUOTA)
 
 #define quota_error(sb, fmt, args...) \