linux-ext4 - Re: [RESEND] [PATCH] lseek: change i

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: <6.0.0.20.2.20090116094630.06b202c0@172.19.0.2>
Date:	Fri, 16 Jan 2009 09:53:02 +0900
From:	Hisashi Hifumi <hifumi.hisashi@....ntt.co.jp>
To:	Andrew Morton <akpm@...ux-foundation.org>,
	Theodore Tso <tytso@....edu>
Cc:	matthew@....cx, linux-ext4@...r.kernel.org,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RESEND] [PATCH] lseek: change i_mutex usage.


At 09:40 09/01/16, Andrew Morton wrote:
>On Thu, 15 Jan 2009 09:21:13 -0500
>Theodore Tso <tytso@....edu> wrote:
>
>> On Thu, Jan 15, 2009 at 06:22:52AM -0700, Matthew Wilcox wrote:
>> > 
>> > Of course if you have multiple threads, they will share a struct file,
>> > and you're updating f_pos and f_version without locking.  Maybe that's
>> > OK, but it's soemthing you didn't discuss.
>> 
>> f_pos is updated by sys_write(), and friends without locking, so we're
>> fine on that front, or at least no worse off.
>
>bug ;)
>
>>  SUSv3 doesn't seem to
>> say one way or another what should happen if two threads try to
>> write() to a file at the same time using the same file descriptor in
>> terms of whether or not f_pos gets updated intelligently.  We've opted
>> for speed over determinism already.
>
>I think our thinking was that if two threads are racily updating f_pos
>with different values, then it should end up with one of those values.
>
>>From a quality-of-implementation POV (what _is_ that, anyway) it would
>be bad if the kernel were to set f_pos to the upper 32 bits of position
>A and the lower 32 bits of position B.  Which could happen if we remove
>the i_mutex protection on 32-bits.
>
>We could perhaps omit some locking if CONFIG_64BIT.  There's probably
>quite a bit of locking which could be omitted in that case.

Updating f_pos value on 32bit is not atomic, so we discussed about this
but we concluded that it does not matter whether f_pos is atomic or not
See,
Subject:[RESEND] [PATCH] VFS: make file->f_pos access atomic on 32bit
http://marc.info/?l=linux-fsdevel&m=122335627224515

I think even i_mutex is not needed. When we touch i_size, i_size_read is enough,
and we can remove i_mutex at all on lseek.


--
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