Date: Mon, 31 Aug 2015 09:37:18 +0200
From: Jiri Olsa <jolsa@redhat.com>
To: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>, lkml <linux-kernel@vger.kernel.org>,
        David Ahern <dsahern@gmail.com>, Ingo Molnar <mingo@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Matt Fleming <matt@codeblueprint.co.uk>,
        =?iso-8859-1?Q?Rapha=EBl?= Beamonte <raphael.beamonte@gmail.com>
Subject: Re: [PATCH 01/11] tools: Add err.h with ERR_PTR PTR_ERR interface
Message-ID: <20150831073718.GF1787@krava.brq.redhat.com>
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 <20150828162139.GE11407@kernel.org>
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On Fri, Aug 28, 2015 at 01:21:39PM -0300, Arnaldo Carvalho de Melo wrote:
> Em Wed, Aug 26, 2015 at 03:46:43PM +0200, Jiri Olsa escreveu:
> > Adding part of the kernel's <linux/err.h> interface:
> >   inline void * __must_check ERR_PTR(long error);
> >   inline long   __must_check PTR_ERR(__force const void *ptr);
> >   inline bool   __must_check IS_ERR(__force const void *ptr);
> > 
> > it will be used to propagate error through pointers
> > in following patches.
> > 
> > Link: http://lkml.kernel.org/n/tip-ufgnyf683uab69anmmrabgdf@git.kernel.org
> > Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> > ---
> >  tools/include/linux/err.h | 28 ++++++++++++++++++++++++++++
> >  1 file changed, 28 insertions(+)
> >  create mode 100644 tools/include/linux/err.h
> > 
> > diff --git a/tools/include/linux/err.h b/tools/include/linux/err.h
> > new file mode 100644
> > index 000000000000..2df92df55cfe
> > --- /dev/null
> > +++ b/tools/include/linux/err.h
> > @@ -0,0 +1,28 @@
> > +#ifndef __TOOLS_LINUX_ERR_H
> > +#define __TOOLS_LINUX_ERR_H
> > +
> > +#include <linux/compiler.h>
> > +#include <linux/types.h>
> > +
> > +#include <asm/errno.h>
> > +
> 
> You deleted the comment in the kernel sources at this point:

right.. I did not want to bring too much attention :-))

> 
> /*
>  * Kernel pointers have redundant information, so we can use a
>  * scheme where we can return either an error code or a normal
>  * pointer with the same return value.
>  *
>  * This should be a per-architecture thing, to allow different
>  * error and pointer decisions.
>  */
> 
> > +#define MAX_ERRNO	4095
> 
> Now we're dealing with user pointers, are we completely sure we can use
> this trick here?

it's safe for user as well, because 'error' pointers
fall down to the unused hole:

Documentation/x86/x86_64/mm.txt:
ffffffffffe00000 - ffffffffffffffff (=2 MB) unused hole

haven't checked for other archs, but since it's used
within generic code, it should be ok

I'll put the comment back with additional explanation
wrt user space in v2

jirka
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