linux-kernel - [PATCH 5.15 239/244] xfs: remove infinite loop when reserving free block pool

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Date:   Tue, 23 Aug 2022 10:26:38 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Brian Foster <bfoster@...hat.com>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Dave Chinner <dchinner@...hat.com>,
        Leah Rumancik <leah.rumancik@...il.com>
Subject: [PATCH 5.15 239/244] xfs: remove infinite loop when reserving free block pool

From: "Darrick J. Wong" <djwong@...nel.org>

[ Upstream commit 15f04fdc75aaaa1cccb0b8b3af1be290e118a7bc ]

Infinite loops in kernel code are scary.  Calls to xfs_reserve_blocks
should be rare (people should just use the defaults!) so we really don't
need to try so hard.  Simplify the logic here by removing the infinite
loop.

Cc: Brian Foster <bfoster@...hat.com>
Signed-off-by: Darrick J. Wong <djwong@...nel.org>
Reviewed-by: Dave Chinner <dchinner@...hat.com>
Signed-off-by: Leah Rumancik <leah.rumancik@...il.com>
Acked-by: Darrick J. Wong <djwong@...nel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 fs/xfs/xfs_fsops.c |   50 ++++++++++++++++++++------------------------------
 1 file changed, 20 insertions(+), 30 deletions(-)

--- a/fs/xfs/xfs_fsops.c
+++ b/fs/xfs/xfs_fsops.c
@@ -430,46 +430,36 @@ xfs_reserve_blocks(
 	 * If the request is larger than the current reservation, reserve the
 	 * blocks before we update the reserve counters. Sample m_fdblocks and
 	 * perform a partial reservation if the request exceeds free space.
+	 *
+	 * The code below estimates how many blocks it can request from
+	 * fdblocks to stash in the reserve pool.  This is a classic TOCTOU
+	 * race since fdblocks updates are not always coordinated via
+	 * m_sb_lock.
 	 */
-	error = -ENOSPC;
-	do {
-		free = percpu_counter_sum(&mp->m_fdblocks) -
+	free = percpu_counter_sum(&mp->m_fdblocks) -
 						xfs_fdblocks_unavailable(mp);
-		if (free <= 0)
-			break;
-
-		delta = request - mp->m_resblks;
-		lcounter = free - delta;
-		if (lcounter < 0)
-			/* We can't satisfy the request, just get what we can */
-			fdblks_delta = free;
-		else
-			fdblks_delta = delta;
-
+	delta = request - mp->m_resblks;
+	if (delta > 0 && free > 0) {
 		/*
 		 * We'll either succeed in getting space from the free block
-		 * count or we'll get an ENOSPC. If we get a ENOSPC, it means
-		 * things changed while we were calculating fdblks_delta and so
-		 * we should try again to see if there is anything left to
-		 * reserve.
-		 *
-		 * Don't set the reserved flag here - we don't want to reserve
-		 * the extra reserve blocks from the reserve.....
+		 * count or we'll get an ENOSPC.  Don't set the reserved flag
+		 * here - we don't want to reserve the extra reserve blocks
+		 * from the reserve.
 		 */
+		fdblks_delta = min(free, delta);
 		spin_unlock(&mp->m_sb_lock);
 		error = xfs_mod_fdblocks(mp, -fdblks_delta, 0);
 		spin_lock(&mp->m_sb_lock);
-	} while (error == -ENOSPC);
 
-	/*
-	 * Update the reserve counters if blocks have been successfully
-	 * allocated.
-	 */
-	if (!error && fdblks_delta) {
-		mp->m_resblks += fdblks_delta;
-		mp->m_resblks_avail += fdblks_delta;
+		/*
+		 * Update the reserve counters if blocks have been successfully
+		 * allocated.
+		 */
+		if (!error) {
+			mp->m_resblks += fdblks_delta;
+			mp->m_resblks_avail += fdblks_delta;
+		}
 	}
-
 out:
 	if (outval) {
 		outval->resblks = mp->m_resblks;