[v1] bcachefs: fix last_seq and last_empty_seq in bch2_fs_journal_start()

[PATCH] bcachefs: fix last_seq and last_empty_seq in bch2_fs_journal_start()

Posted by Camila Alvarez 1 year, 9 months ago

Values were left as the next possible sequence number when there were no
entries.

The fix involves updating the last_seq initial value and
setting last_empty_seq to cur_seq - 1.

Reported-by: syzbot+10b936c5eaee2819b49b@syzkaller.appspotmail.com
Signed-off-by: Camila Alvarez <cam.alvarez.i@gmail.com>
---
 fs/bcachefs/journal.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/bcachefs/journal.c b/fs/bcachefs/journal.c
index adec8e1ea73e..3835c458eec9 100644
--- a/fs/bcachefs/journal.c
+++ b/fs/bcachefs/journal.c
@@ -1196,7 +1196,7 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
 	struct journal_replay *i, **_i;
 	struct genradix_iter iter;
 	bool had_entries = false;
-	u64 last_seq = cur_seq, nr, seq;
+	u64 last_seq = cur_seq - 1, nr, seq;
 
 	genradix_for_each_reverse(&c->journal_entries, iter, _i) {
 		i = *_i;
@@ -1256,7 +1256,7 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
 	}
 
 	if (!had_entries)
-		j->last_empty_seq = cur_seq;
+		j->last_empty_seq = cur_seq - 1;
 
 	spin_lock(&j->lock);
 
-- 
2.34.1

Re: [PATCH] bcachefs: fix last_seq and last_empty_seq in bch2_fs_journal_start()

Posted by Kent Overstreet 1 year, 9 months ago

On Wed, May 15, 2024 at 11:19:20PM -0400, Camila Alvarez wrote:
> Values were left as the next possible sequence number when there were no
> entries.
> 
> The fix involves updating the last_seq initial value and
> setting last_empty_seq to cur_seq - 1.

I think this is correct, but we should try to come up with some better
assertions or something to make the code clearer; we don't want off by
ones to lurk so easily.

Could you give it some thought?

> 
> Reported-by: syzbot+10b936c5eaee2819b49b@syzkaller.appspotmail.com
> Signed-off-by: Camila Alvarez <cam.alvarez.i@gmail.com>
> ---
>  fs/bcachefs/journal.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/bcachefs/journal.c b/fs/bcachefs/journal.c
> index adec8e1ea73e..3835c458eec9 100644
> --- a/fs/bcachefs/journal.c
> +++ b/fs/bcachefs/journal.c
> @@ -1196,7 +1196,7 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
>  	struct journal_replay *i, **_i;
>  	struct genradix_iter iter;
>  	bool had_entries = false;
> -	u64 last_seq = cur_seq, nr, seq;
> +	u64 last_seq = cur_seq - 1, nr, seq;
>  
>  	genradix_for_each_reverse(&c->journal_entries, iter, _i) {
>  		i = *_i;
> @@ -1256,7 +1256,7 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
>  	}
>  
>  	if (!had_entries)
> -		j->last_empty_seq = cur_seq;
> +		j->last_empty_seq = cur_seq - 1;
>  
>  	spin_lock(&j->lock);
>  
> -- 
> 2.34.1
>

Re: [PATCH] bcachefs: fix last_seq and last_empty_seq in bch2_fs_journal_start()

Posted by Camila Alvarez Inostroza 1 year, 9 months ago


On Sun, 19 May 2024, Kent Overstreet wrote:

> On Wed, May 15, 2024 at 11:19:20PM -0400, Camila Alvarez wrote:
>> Values were left as the next possible sequence number when there were no
>> entries.
>>
>> The fix involves updating the last_seq initial value and
>> setting last_empty_seq to cur_seq - 1.
>
> I think this is correct, but we should try to come up with some better
> assertions or something to make the code clearer; we don't want off by
> ones to lurk so easily.
>
> Could you give it some thought?
>
You're right. I think the code is written in a confusing way. In 
particular it seems that cur_seq - 1 is used all over the place.
I believe we can abstract cur_seq - 1 in an independent variable (since 
it represents the actual last sequence number), that should make it 
clearer.
I'll share an updated version of the patch.
Thanks for the response!
>>
>> Reported-by: syzbot+10b936c5eaee2819b49b@syzkaller.appspotmail.com
>> Signed-off-by: Camila Alvarez <cam.alvarez.i@gmail.com>
>> ---
>>  fs/bcachefs/journal.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/bcachefs/journal.c b/fs/bcachefs/journal.c
>> index adec8e1ea73e..3835c458eec9 100644
>> --- a/fs/bcachefs/journal.c
>> +++ b/fs/bcachefs/journal.c
>> @@ -1196,7 +1196,7 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
>>  	struct journal_replay *i, **_i;
>>  	struct genradix_iter iter;
>>  	bool had_entries = false;
>> -	u64 last_seq = cur_seq, nr, seq;
>> +	u64 last_seq = cur_seq - 1, nr, seq;
>>
>>  	genradix_for_each_reverse(&c->journal_entries, iter, _i) {
>>  		i = *_i;
>> @@ -1256,7 +1256,7 @@ int bch2_fs_journal_start(struct journal *j, u64 cur_seq)
>>  	}
>>
>>  	if (!had_entries)
>> -		j->last_empty_seq = cur_seq;
>> +		j->last_empty_seq = cur_seq - 1;
>>
>>  	spin_lock(&j->lock);
>>
>> --
>> 2.34.1
>>
>