linux-kernel - Re: [PATCH] ptrdiff_t is not uintptr

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Date:	Sun, 19 Aug 2007 20:26:24 -0700
From:	David Brownell <david-b@...bell.net>
To:	Satyam Sharma <satyam@...radead.org>
Cc:	Anton Altaparmakov <aia21@....ac.uk>,
	Al Viro <viro@....linux.org.uk>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ptrdiff_t is not uintptr_t, damnit

On Sunday 19 August 2007, Satyam Sharma wrote:
> > > On a 32-bit arch "unsigned long" is 32-bit and pointers are 32-bit.
> > > 
> > > On a 64-bit archi "unsigned long" is 64-bit and pointers are 64-bit.
> > 
> > So with 32 bit userspace "unsigned long long" is the type to use
> > when talking to a 64-bit kernel; and with pure 64-bit code, it's
> > enough to write "unsigned long".
> > 
> > I'm fairly sure that's the root cause of the pain I recall here;
> > but I'd have to run experiments again to verify that.  
> 
> I suspect the root cause of the pain was that you used "int" or "long"
> to talk between kernel and userspace in the first place. You shouldn't,
> we have __u32 / __u64 / etc for that.

Nope; the relevant code was always with "__u64".  The issue was
warning when turning that into a __user pointer.


> The reason I ask is that gcc will also complain (understandably so) with
> "warning: cast from pointer to integer of different size" i.e. even if
> it's a conversion from smaller size to greater size, and not really a
> case of truncation.

ISTR the warning was the other way around:   about "cast from integer
to pointer of a different size".  The __u64 came from userspace and
the kernel pointer was only 32 bits.  Not really truncation, but GCC
could not know that directly ... ergo the extra non-pointer cast.


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