linux-kernel - Re: [patch 7/8] fdmap v2

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Message-ID: <466BC0E3.4050600@cosmosbay.com>
Date:	Sun, 10 Jun 2007 11:14:11 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Al Viro <viro@....linux.org.uk>,
	Kyle Moffett <mrmacman_g4@....com>,
	Ulrich Drepper <drepper@...hat.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Theodore Tso <tytso@....edu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [patch 7/8] fdmap v2 - implement sys_socket2

Linus Torvalds a écrit :
> (And dammit, that _is_ a *real*issue*. No races necessary, no NR_OPEN 
> iterations, no even *halfway* suspect code. It's perfectly fine to do
> 
> 	close(0);
> 	close(1);
> 	close(2);
> 	.. generate filenames, whatever ..
> 	if (open(..) < 0 || open(..) < 0 || open(..) < 0)
> 		die("Couldn't redirect stdin/stdout/stderr");
> 
> and there's absolutely nothing wrong with this kind of setup, even if you 
> could obviously have done it other ways too (ie by using "dup2()" instead 
> of "close + open"),
> 

This kind of setup was OK 25 years ago, before multithreading era.
You cannot reasonably expect it to work in a multithreaded program.

Anyway, I would like to give an alternative idea of the double fdmap, and 
probably more *secure* .

Current fd API mandates integers (32 bits)

Lot of broken code consider a fd must be >= 0, so we currently are limited to 
31 bits.

With NR_OPEN = 1024*1024 = 2^20, that give us 11 bits that we could use as a 
signature.

That is, we could use O_NONSEQ as a indication to kernel to give us a 
composite fd : 20 low order bits give the slot in file table, then 11 bits can 
  be use to make sure the fd was not stolen by malicious code.

Legacy app, (without O_NONSEQ in flags) would get POSIX compatables fd in [0, 
2^20-1] range, with the lowest available fd.

If O_NONSEQ is given, kernel is free to give an fd in [0, 2^31 - 1], with a 
strategy that could be the one Davide gave in its patch (with a list of 
available slots). But instead of FIFO, we can use now LIFO, more cache friendly.

In fget()/fget_light()/close(), we can then use 20 bits to select the slot in 
the single fdmap.
And 11 bits to check the 'signature'.

So if open( O_NONSEQFD) gave us 0x77000010, we cannot do close(0x10) or 
read(0x10, ....)

Storage for these bits is already there in Davide fd_slot structure, where we 
currently use one long to store 3 bits 'only'.

This should work even bumping NR_OPEN to say... 8*1024*1024, and 8 bits signature.

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