linux-kernel - Re: [PATCH 2/9] vfs: export do_splice

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:	Sat, 23 Mar 2013 04:41:40 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	"J. R. Okajima" <hooanon05@...oo.co.jp>
Cc:	David Howells <dhowells@...hat.com>,
	Miklos Szeredi <miklos@...redi.hu>, jack@...e.cz,
	torvalds@...ux-foundation.org, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org, hch@...radead.org,
	akpm@...ux-foundation.org, apw@...onical.com, nbd@...nwrt.org,
	neilb@...e.de, jordipujolp@...il.com, ezk@....cs.sunysb.edu,
	sedat.dilek@...glemail.com, mszeredi@...e.cz
Subject: Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

On Sat, Mar 23, 2013 at 11:49:11AM +0900, J. R. Okajima wrote:
> 
> Al Viro:
> > The scenario, BTW, looks so:
> > process A does sb_start_write() (on your upper layer)
> > process B tries to freeze said upper layer and blocks, waiting for A to finish
> > process C grabs ->i_mutex in your upper layer
> > process C does vfs_write(), which blocks, since there's a pending attempt to
> > freeze
> > process A tries to grab ->i_mutex held by C and blocks
> 
> According to latest mm/filemap.c:generic_file_aio_write(),
> 	sb_start_write(inode->i_sb);
> 	mutex_lock(&inode->i_mutex);
> 	ret = __generic_file_aio_write(iocb, iov, nr_segs, &iocb->ki_pos);
> 	mutex_unlock(&inode->i_mutex);
> 	:::
> 	sb_end_write(inode->i_sb);
> 
> Process C would block *BEFORE* i_mutex by sb_start_write()? No?

Different ->i_mutex; you are holding one on the parent directory already.

That's the problem - you have ->i_mutex nested both inside that sucker (as
it ought to) and outside.  Which tends to do bad things, obviously, in
particular because something like mkdir(2) will do sb_start_write() (from
mnt_want_write(), called by kern_path_create()) before grabbing directory
->i_mutex.

Thus the activity with lifting the bastard out of ->aio_write(), etc. in
vfs.git#experimental - *any* union-like variant will need the ability to
pull sb_start_write() outside of locking the parent directory on copyup.
And yes, it's a common prerequisite to anything doing copyups - aufs is
in the same boat as overlayfs and unionmount.  Same deadlock for all three
of them.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/