bcachefs: Fix sb-downgrade validation

Superblock downgrade entries are only two byte aligned, but section
sizes are 8 byte aligned, which means we have to be careful about
overrun checks; an entry that crosses the end of the section is allowed
(and ignored) as long as it has zero errors.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

diff --git a/fs/bcachefs/sb-downgrade.c b/fs/bcachefs/sb-downgrade.c
index 390a1bbd2567ac271f3c69b505753f969d0508eb..3fb23e399ffb3e6c021b8825fe3403e5a048ba4c 100644 (file)
--- a/fs/bcachefs/sb-downgrade.c
+++ b/fs/bcachefs/sb-downgrade.c
@@ -146,10 +146,17 @@ static int bch2_sb_downgrade_validate(struct bch_sb *sb, struct bch_sb_field *f,
        for (const struct bch_sb_field_downgrade_entry *i = e->entries;
             (void *) i < vstruct_end(&e->field);
             i = downgrade_entry_next_c(i)) {
+               /*
+                * Careful: sb_field_downgrade_entry is only 2 byte aligned, but
+                * section sizes are 8 byte aligned - an empty entry spanning
+                * the end of the section is allowed (and ignored):
+                */
+               if ((void *) &i->errors[0] > vstruct_end(&e->field))
+                       break;
+
                if (flags & BCH_VALIDATE_write &&
-                   ((void *) &i->errors[0] > vstruct_end(&e->field) ||
-                    (void *) downgrade_entry_next_c(i) > vstruct_end(&e->field))) {
-                       prt_printf(err, "downgrade entry overruns end of superblock section)");
+                   (void *) downgrade_entry_next_c(i) > vstruct_end(&e->field)) {
+                       prt_printf(err, "downgrade entry overruns end of superblock section");
                        return -BCH_ERR_invalid_sb_downgrade;
                }