linux-kernel - Re: [PATCH] x86_64: fix delayed signals

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Date:	Sun, 13 Jul 2008 11:36:02 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Oleg Nesterov <oleg@...sign.ru>
cc:	Denys Vlasenko <vda.linux@...glemail.com>,
	T?r?k Edwin <edwintorok@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Roland McGrath <roland@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Elias Oltmanns <eo@...ensachen.de>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH] x86_64: fix delayed signals

On Sun, 13 Jul 2008, Oleg Nesterov wrote:
> 
> SETFL_MASK doesn't have O_RDWR, and in any case setfl() changes ->f_flags,
> not ->f_mode.

I think you can still read a write-only file descriptor on some systems: 
doing a mmap( PROT_WRITE ) on it can sometimes do it.

The reason - PROT_WRITE often implies PROT_READ (because on many CPU's you 
have to do unaligned writes with a read-modify-write cycle - on some like 
old alphas you have to do even normal byte and word writes that way).

So if the OS checks just the asked-for protections against the file 
descriptor protections, you'll essentially get read access for free when 
you do the mmap.

At least current versions of Linux won't ever allow a file mapping of a 
non-readable fd, but I won't guarantee we always did that, nor that other 
OS's always do it.

Anyway, I misunderstood Edwins idea to actually throw away the writes if 
the file isn't linked anywhere and is only open for writing. I guess we 
could optimize that, but it's such an unrealistic specialc case that it 
really isn't worth it except for some benchmark-gaming kind of thing 
(which I don't like doing anyway).

				Linus
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