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Date:   Sat, 13 Apr 2019 18:38:36 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Kirill Smelkov <kirr@nexedi.com>, Arnd Bergmann <arnd@arndb.de>,
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        Rasmus Villemoes <linux@rasmusvillemoes.dk>
Subject: Re: [PATCH 1/2] vfs: pass ppos=NULL to .read()/.write() of
 FMODE_STREAM files
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On Sat, Apr 13, 2019 at 10:27:00AM -0700, Linus Torvalds wrote:

> And I'd really like to have people (Al?) look at this and go "yeah,
> makes sense". I do think that moving to a model where we wither have a
> (properly locked) file position or no pos pointer at all is the right
> model (ie I'd really like to get rid of the mixed case), but there
> might be some practical problem that makes it impractical.
>
> Because the *real* problem with the mixed case is not "insane people
> who do bad things might get odd results". No, the real problem with
> the mixed case is that it could be a security issue (ie: one process
> intentionally changes the file position just as another process is
> going a 'read' and then avoids some limit test because the limit test
> was done using the old 'pos' value but the actual IO was done using
> the new one).
> 
> So I suspect that we will have to either
> 
>  - get rid of the mixed case entirely (and do only properly locked
> f_pos accesses or pass is a NULL f_pos)

Good luck.  Character devices
	get no exclusion
	often do use position
	are many
	might be buried behind several layers of indirection
	... and often left unmaintained for a decade or two

IOW, I don't see how you'd go about eliminating the mixed case...