linux-kernel - Re: why "probe_kernel_address()", not "probe_user

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Message-ID: <Pine.LNX.4.64.0610281246160.2537@localhost.localdomain>
Date:	Sat, 28 Oct 2006 12:52:13 -0400 (EDT)
From:	"Robert P. J. Day" <rpjday@...dspring.com>
To:	Andrew Morton <akpm@...l.org>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: why "probe_kernel_address()", not "probe_user_address()"?

On Sat, 28 Oct 2006, Andrew Morton wrote:

> On Sat, 28 Oct 2006 11:56:24 -0400 (EDT)
> "Robert P. J. Day" <rpjday@...dspring.com> wrote:
>
> >
> >   it seems odd that the purpose of the "probe_kernel_address()" macro
> > is, in fact, to probe a *user* address (from linux/uaccess.h):
> >
> > #define probe_kernel_address(addr, retval)              \
> >         ({                                              \
> >                 long ret;                               \
> >                                                         \
> >                 inc_preempt_count();                    \
> >                 ret = __get_user(retval, addr);         \
> >                 dec_preempt_count();                    \
> >                 ret;                                    \
> >         })
> >
> >   given that that routine is referenced only 5 places in the
> > entire source tree, wouldn't it be more meaningful to use a more
> > appropriate name?
>
> You'll notice that all callers are indeed probing kernel addresses.
> The function _could_ be used for user addresses and could perhaps be
> called probe_address().
>
> One of the reasons this wrapper exists is to communicate that the
> __get_user() it is in fact not being used to access user memory.

um ... ok.  i think.  i agree that "probe_address()" would be a more
appropriate name, but i'm still a bit confused as to why
"__get_user()" would be used to access something *not* in user memory,
given this seemingly unambiguous explanation in
include/asm-i386/uaccess.h:

=====
 * get_user: - Get a simple variable from user space.
 * @x:   Variable to store result.
 * @ptr: Source address, in user space.
 *
 * Context: User context only.  This function may sleep.
 *
 * This macro copies a single simple variable from user space to kernel
 * space.  It supports simple types like char and int, but not larger
 * data types like structures or arrays.
  ...
=====

  so having probe_kernel_address() invoke __get_user() does seem to be
just a wee bit confusing for us newbies.  in any event, i'll leave the
clarification for someone much higher up the food chain.

rday
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