[PATCH v2] xfs: return default quota limits for IDs without a dquot

[Ravi Singh <ravising@redhat.com>]

When an ID has no dquot on disk, Q_XGETQUOTA returns -ENOENT even
though default quota limits are configured and enforced against that
ID.  This means unprivileged users who have never used any resources
cannot see the limits that apply to them.

When xfs_qm_dqget() returns -ENOENT for a non-zero ID, return a
zero-usage response with the default limits filled in from
m_quotainfo rather than propagating the error.  This is consistent
with the enforcement behavior in xfs_qm_adjust_dqlimits(), which
pushes the same default limits into a dquot when it is first
allocated.

Signed-off-by: Ravi Singh <ravising@redhat.com>
---
 v2:
  - Moved fix from VFS (fs/quota/quota.c) to XFS
    (fs/xfs/xfs_qm_syscalls.c) per review feedback
  - Return default limits on ENOENT instead of granting
    unprivileged access to ID 0's dquot

 fs/xfs/xfs_qm_syscalls.c | 36 +++++++++++++++++++++++++++++++++++-
 1 file changed, 35 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_qm_syscalls.c b/fs/xfs/xfs_qm_syscalls.c
index d50b7318c..2176dc617 100644
--- a/fs/xfs/xfs_qm_syscalls.c
+++ b/fs/xfs/xfs_qm_syscalls.c
@@ -391,6 +391,30 @@ xfs_qm_scall_setqlim(
 	return error;
 }
 
+/*
+ * Fill out the default quota limits for an ID that has no dquot on disk.
+ * The default limits are enforced against such IDs by
+ * xfs_qm_adjust_dqlimits() when a dquot is first allocated.
+ */
+static void
+xfs_qm_scall_getquota_fill_defaults(
+	struct xfs_mount	*mp,
+	xfs_dqtype_t		type,
+	struct qc_dqblk		*dst)
+{
+	struct xfs_def_quota	*defq;
+
+	defq = xfs_get_defquota(mp->m_quotainfo, type);
+
+	memset(dst, 0, sizeof(*dst));
+	dst->d_spc_softlimit = XFS_FSB_TO_B(mp, defq->blk.soft);
+	dst->d_spc_hardlimit = XFS_FSB_TO_B(mp, defq->blk.hard);
+	dst->d_ino_softlimit = defq->ino.soft;
+	dst->d_ino_hardlimit = defq->ino.hard;
+	dst->d_rt_spc_softlimit = XFS_FSB_TO_B(mp, defq->rtb.soft);
+	dst->d_rt_spc_hardlimit = XFS_FSB_TO_B(mp, defq->rtb.hard);
+}
+
 /* Fill out the quota context. */
 static void
 xfs_qm_scall_getquota_fill_qc(
@@ -451,8 +475,18 @@ xfs_qm_scall_getquota(
 	 * set doalloc. If it doesn't exist, we'll get ENOENT back.
 	 */
 	error = xfs_qm_dqget(mp, id, type, false, &dqp);
-	if (error)
+	if (error) {
+		/*
+		 * If there is no dquot for this ID and it is not ID 0,
+		 * return the default limits with zero usage so that
+		 * unprivileged users can see what limits apply to them.
+		 */
+		if (error == -ENOENT && id != 0) {
+			xfs_qm_scall_getquota_fill_defaults(mp, type, dst);
+			return 0;
+		}
 		return error;
+	}
 
 	/*
 	 * If everything's NULL, this dquot doesn't quite exist as far as
-- 
2.49.0