[PATCH] bcachefs: Fix shift-out-of-bounds in bch2_blacklist_entries_gc

[Pei Li <peili.dev@gmail.com>]

This series fix the shift-out-of-bounds issue in
bch2_blacklist_entries_gc().

Instead of passing 0 to eytzinger0_first() when iterating the entries,
we explicitly check 0 and initialize i to be 0.

syzbot has tested the proposed patch and the reproducer did not trigger
any issue:

Reported-and-tested-by: syzbot+835d255ad6bc7f29ee12@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=835d255ad6bc7f29ee12
Signed-off-by: Pei Li <peili.dev@gmail.com>
---
Syzbot reported the following issue:
UBSAN: shift-out-of-bounds in ./include/linux/log2.h:67:13
shift exponent 4294967295 is too large for 64-bit type 'long unsigned int'

This is because 0 is passed to __rounddown_pow_of_two(), and -1 is
returned as an unsigned integer. In 32 bit system, it will become
4294967295.

This patch fixes the issue by adding check in
bch2_blacklist_entries_gc() to avoid passing 0 into eytzinger0_first().
If we found out t->nr equals to 0, we directly use 0 to access the root
of the list.

Tested on:

commit:         55027e68 Merge tag 'input-for-v6.10-rc5' of git://git...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1191153e980000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d6b9ee98d841760c
dashboard link: https://syzkaller.appspot.com/bug?extid=835d255ad6bc7f29ee12
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
patch:          https://syzkaller.appspot.com/x/patch.diff?x=15ca4301980000
---
 fs/bcachefs/journal_seq_blacklist.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/bcachefs/journal_seq_blacklist.c b/fs/bcachefs/journal_seq_blacklist.c
index ed4846709611..1f25c111c54c 100644
--- a/fs/bcachefs/journal_seq_blacklist.c
+++ b/fs/bcachefs/journal_seq_blacklist.c
@@ -232,7 +232,7 @@ bool bch2_blacklist_entries_gc(struct bch_fs *c)
 	BUG_ON(nr != t->nr);
 
 	unsigned i;
-	for (src = bl->start, i = eytzinger0_first(t->nr);
+	for (src = bl->start, i = t->nr == 0 ? 0 : eytzinger0_first(t->nr);
 	     src < bl->start + nr;
 	     src++, i = eytzinger0_next(i, nr)) {
 		BUG_ON(t->entries[i].start	!= le64_to_cpu(src->start));

---
base-commit: 563a50672d8a86ec4b114a4a2f44d6e7ff855f5b
change-id: 20240625-bug1-42684cf9d7ed

Best regards,
-- 
Pei Li <peili.dev@gmail.com>