linux-kernel - Re: [PATCH 6/6] vfs: introduce FMODE_NEG_OFFSET for allowing negative f

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Date:	Mon, 18 Jan 2010 12:22:14 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	"Wu, Fengguang" <fengguang.wu@...el.com>
Cc:	OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Christoph Hellwig <hch@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Eric Paris <eparis@...hat.com>, Nick Piggin <npiggin@...e.de>,
	Andi Kleen <andi@...stfloor.org>,
	David Howells <dhowells@...hat.com>,
	Jonathan Corbet <corbet@....net>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 6/6] vfs: introduce FMODE_NEG_OFFSET for allowing
 negative f_pos

On Mon, 18 Jan 2010 11:15:38 +0800
"Wu, Fengguang" <fengguang.wu@...el.com> wrote:

> Hi,
> 
> [replying from webmail, sorry for top-posting]
> 
> memory_lseek() calls force_successful_syscall_return() to force success on negative vals.
> However that is a no-op for x86.
> 
> My experiment shows that lseek() does return negative pos. However,
> manual says that "a value of (off_t) -1 is returned" on error. So it's OK
> as long as your program is written as "err == -1" instead of "err < 0".
> 
On error, the kernel returns -EOVERFLOW (via %eax) and libc hides
it by
  errno = EOVERFLOW
  ret = -1

The problem discussed here is the kernel's return value. So, the kernel's
lseek should check that, I think.

Anyway, this lseek problem is not related to this patch itself and has
existed for very long time. Fixing it later by another patch is not very
bad, I think.
(I'm sorry I myself is not ready for writing a patch...)

Thaks,
-Kame

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