linux-ext4 - Re: [PATCH] ext4: check missed return value ext4_sync

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 17 Mar 2010 12:24:57 +0100
From:	Jan Kara <jack@...e.cz>
To:	Dmitry Monakhov <dmonakhov@...nvz.org>
Cc:	Jan Kara <jack@...e.cz>, linux-ext4@...r.kernel.org,
	Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH] ext4: check missed return value ext4_sync_file

> > Jan Kara <jack@...e.cz> writes:
> > >> 
> > >> If jbd2_log_start_commit return 0 then it means that transaction
> > >> was already committed. So we don't have to issue barrier for
> > >> ordered mode, because it was already done during commit.
> > >   Umm, we have to - when a file has just been rewritten (i.e. no block
> > > allocation), then i_datasync_tid is not updated and thus we won't commit
> > > any transaction as a part of fdatasync (and that is correct because there
> > > are no metadata that need to be written for that fdatasync). But we still
> > > have to flush disk caches with data submitted by filemap_fdatawrite_and_wait.
> > Yepp. I've missed that. i thought that transaction id updated even in that
> > case.  The most unpleasant part in ext4_sync_file implementation is that
> > barrier is issued on each fsync() call.  So some bad user may perform:
> > while(1) fsync(fd); which result in bad system performance. And since barrier
> > request is empty it is hard to detect the reason of troubles.
>   Actually, you'll be able to see the barrier requests in the blktrace dump
> so it won't be that hard to detect.
> 
> > Off course we may solve it by introducing some sort of dirty flag which is
> > set in write_page, and clear in fsync. But it looks as ugly workaround.
>   I agree that sending barrier request on each fsync isn't very nice but
> in common case, I'd assume that an application calls fsync only if it has
> written something to the file previously. So I wouldn't invest much into
> solving this until I see a realistic use case where it matters...
> 
> > >> By unknown reason we ignored ret val from jbd2_log_wait_commit()
> > >> so even in case of EIO fsync will succeed.
> > >   I just forgot jbd2_log_wait_commit can return a failure...
> > In respect to previous comments the patch reduced to simple missed
> > error check fix.
>   I guess you can resend the fix to Ted directly to catch his attention.
> 
> > BTW: While investigating similar code in ext3 i've found what fsync is broken
> > in case of external journal.
>   Yes, I've noticed this recently as well. So will you send a fix or should
> I go and backport ext4 fixes of this?
  Oops, sorry, I've notice you sent the patches to the list already...

								Honza
-- 
Jan Kara <jack@...e.cz>
SuSE CR Labs
--
To unsubscribe from this list: send the line "unsubscribe linux-ext4" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html