linux-kernel - Re: [RESEND] [PATCH] VFS: make file->f

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]

Message-Id: <200810080327.44530.nickpiggin@yahoo.com.au>
Date:	Wed, 8 Oct 2008 03:27:44 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit arch

On Tuesday 07 October 2008 21:29, Andi Kleen wrote:
> > Maybe cmpxchg8b is good for i486 or later x86, but i386 or other
> > architectures that do not have similar instruction needs some locking
> > primitive. I think lazy
>
> We have a cmpxchg emulation on 386. That works because only UP 386s are
> supported, so it can be done in software.
>
> > seqlock is one option for making file->f_pos access atomic.
>
> The question is if it's the right option. At least all the common
> operations on fds (read/write) are all writers, not readers.

Common operations are read, do something, write. So seqlocks then cost
one atomic operation, a couple of memory barriers (all noops on x86),
and some predictable branches etc.

cmpxchg based would require 2 lock ; cmpxchg8b on 32-bit. Fairly heavy.
Also I don't think we have generic accessors to do this, so I think
that is for another project.

Anyway, I think importantly this creates some usable accessors for the
f_pos problem. I think we actually need to touch a _lot_ of code to
cover all f_pos accesses in the kernel, but I guess this gets the ball
rolling.

So.. is everyone agreed that corrupting f_pos is a bad thing? (serious
question) If so, then we should get something like this merged sooner
rather than later.
--
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/