linux-kernel - Re: replace() system call needed (was Re: EXT4-ish "fixes" in UBIFS)

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] [day] [month] [year] [list]

Date:	Thu, 2 Apr 2009 00:58:20 +0200 (CEST)
From:	Bodo Eggert <7eggert@....de>
To:	Pavel Machek <pavel@....cz>
cc:	Theodore Tso <tytso@....edu>, Bodo Eggert <7eggert@....de>,
	Artem Bityutskiy <dedekind@...dex.ru>,
	Artem Bityutskiy <Artem.Bityutskiy@...ia.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: replace() system call needed (was Re: EXT4-ish "fixes" in
 UBIFS)

On Wed, 1 Apr 2009, Pavel Machek wrote:
> On Tue 2009-03-31 20:06:57, Theodore Tso wrote:
> > On Tue, Mar 31, 2009 at 11:27:33PM +0200, Bodo Eggert wrote:

> > > This can be done using implicit logic:
> > > 
> > > ->E.g. on close(), mark inodes without being sync()ed as poisoned.
> > > (I can think of more sophisticated logic, but ...)
> > > ->On completing the inode with the delayed allocations, unpoison it.
> > > ->Don't commit rename()s if the corresponding inode is poisoned.
> > 
> > Send us patches if you think it's that easy to do what you are
> > proposing.   I assure it's not easy.
> 
> Well, implementing replace() syscall would be quite easy. Would you
> want such a patch?

You'd need - minus setting the flag on close - about the same logic:
- detecting if the inode are dirty
- detecting when the inode gets into a clean state
- delaying the commit of the rename() until then
- not replaying parts of the journal

As I understand, the replace() should not behave different from rename(),
except for making a guaranty. Since this guaranty is desired anyway, I 
think making it automatically would be a good thing. Especially until
all applications are rewritten ...

Off cause if you call replace, you know it's going to be OK, but you could 
also use pathconf($configdir, _PC_SYNC_BEFORE_RENAME)
(== 0: Sync or barrier implicit, (data = ordered)
    1: small chance of data loss due to not syncing, (vfat?)
    2: guarantee of data loss without sync (ext4))

// userspace.c
static inline int sync_before_rename(char * where, int is_important_data)
{
	#ifndef _PC_SYNC_BEFORE_RENAME
		// be optimistic for unimportant data
		return is_important_data;
	#else
		int ret = pathconf(where, _PC_SYNC_BEFORE_RENAME);
		if (is_important_data)
			return ret;
		return (ret > 1);
	#endif
}
-- 
How do I set my laser printer on stun?
--
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/