linux-kernel - [PATCH] bcachefs: fix size used to calculate `nr` of devices in member_to

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Message-ID: <20250612081935.161598-1-sperezglz@gmail.com>
Date: Thu, 12 Jun 2025 02:19:26 -0600
From: Sergio Perez Gonzalez <sperezglz@...il.com>
To: kent.overstreet@...ux.dev,
	linux-bcachefs@...r.kernel.org
Cc: Sergio Perez Gonzalez <sperezglz@...il.com>,
	linux-kernel@...r.kernel.org,
	shuah@...nel.org,
	syzbot+e577022d4fba380653be@...kaller.appspotmail.com
Subject: [PATCH] bcachefs: fix size used to calculate `nr` of devices in member_to_text()

In bch2_sb_members_v1_to_text() there is an incorrect size used to
calculate the number of devices.

The correct size should be `BCH_MEMBER_V1_BYTES` as implied by:

static struct bch_member *members_v1_get_mut(struct bch_sb_field_members_v1 *mi, int i)
{
	return (void *) mi->_members + (i * BCH_MEMBER_V1_BYTES);
}

Also invert the pointers used in the calculation, given that they yield a
negative number.

Reported-by: syzbot+e577022d4fba380653be@...kaller.appspotmail.com
Signed-off-by: Sergio Perez Gonzalez <sperezglz@...il.com>
---
 fs/bcachefs/sb-members.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/bcachefs/sb-members.c b/fs/bcachefs/sb-members.c
index c673e76ca27f..cc51313ebcf6 100644
--- a/fs/bcachefs/sb-members.c
+++ b/fs/bcachefs/sb-members.c
@@ -325,10 +325,10 @@ static void bch2_sb_members_v1_to_text(struct printbuf *out, struct bch_sb *sb,
 {
 	struct bch_sb_field_members_v1 *mi = field_to_type(f, members_v1);
 	struct bch_sb_field_disk_groups *gi = bch2_sb_field_get(sb, disk_groups);
-	int nr = (vstruct_end(&mi->field) - (void *) &gi->entries[0]) / sizeof(gi->entries[0]);
+	int nr = ((void *) &gi->entries[0] - vstruct_end(&mi->field)) / BCH_MEMBER_V1_BYTES;
 
 	if (nr != sb->nr_devices)
-		prt_printf(out, "nr_devices mismatch: have %i entries, should be %u", nr, sb->nr_devices);
+		prt_printf(out, "nr_devices mismatch: have %i entries, should be %u\n", nr, sb->nr_devices);
 
 	for (int i = 0; i < nr; i++)
 		member_to_text(out, members_v1_get(mi, i), gi, sb, i);
@@ -344,10 +344,10 @@ static void bch2_sb_members_v2_to_text(struct printbuf *out, struct bch_sb *sb,
 {
 	struct bch_sb_field_members_v2 *mi = field_to_type(f, members_v2);
 	struct bch_sb_field_disk_groups *gi = bch2_sb_field_get(sb, disk_groups);
-	int nr = (vstruct_end(&mi->field) - (void *) &gi->entries[0]) / le16_to_cpu(mi->member_bytes);
+	int nr = ((void *) &gi->entries[0] - vstruct_end(&mi->field)) / le16_to_cpu(mi->member_bytes);
 
 	if (nr != sb->nr_devices)
-		prt_printf(out, "nr_devices mismatch: have %i entries, should be %u", nr, sb->nr_devices);
+		prt_printf(out, "nr_devices mismatch: have %i entries, should be %u\n", nr, sb->nr_devices);
 
 	/*
 	 * We call to_text() on superblock sections that haven't passed
-- 
2.43.0