linux-kernel - Re: [PATCH] fcntl.h: define AT

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Message-ID: <20100419215711.GR10984@baikonur.stro.at>
Date:	Mon, 19 Apr 2010 23:57:11 +0200
From:	maximilian attems <max@...o.at>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	Ulrich Drepper <drepper@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Herbert Xu <herbert@...dor.hengli.com.au>
Subject: Re: [PATCH] fcntl.h: define AT_EACCESS

On Mon, Apr 19, 2010 at 02:47:29PM -0700, Andrew Morton wrote:
> On Fri, 16 Apr 2010 05:08:00 +0200
> maximilian attems <max@...o.at> wrote:
> 
> > noticed on a klibc build of dash that someone had left out that def:
> > usr/dash/bltin/test.c:490: error: ___AT_EACCESS___ undeclared (first use in thiction)
> > 
> > Cc: stable@...nel.org
> > Cc: Ulrich Drepper <drepper@...hat.com>
> > Cc: H. Peter Anvin <hpa@...or.com>
> > Cc: Herbert Xu <herbert@...dor.apana.org.au>
> > Signed-off-by: maximilian attems <max@...o.at>
> > ---
> >  include/linux/fcntl.h |    2 ++
> >  1 files changed, 2 insertions(+), 0 deletions(-)
> > 
> > diff --git a/include/linux/fcntl.h b/include/linux/fcntl.h
> > index 8603740..8bb001d 100644
> > --- a/include/linux/fcntl.h
> > +++ b/include/linux/fcntl.h
> > @@ -39,6 +39,8 @@
> >  #define AT_REMOVEDIR		0x200   /* Remove directory instead of
> >                                             unlinking file.  */
> >  #define AT_SYMLINK_FOLLOW	0x400   /* Follow symbolic links.  */
> > +#define AT_EACCESS		0x200	/* Test access permitted for
> > +					   effective IDs, not real IDs.  */
> >  
> 
> I'm all confused.
> 
> The affects sys_faccesat(), yes?  But sys_faccesat() never gets passed
> a `flags' argument so how does the behaviour which the FACCESSAT(2)
> manpage describes get implemented?
> 
> This patch doesn't actually change kernel behaviour, so how can setting
> AT_EACCESS change any syscall's actions?
> 
> It's a bit of a worry that the proposed value for AT_EACCESS duplicates
> AT_REMOVEDIR.  I guess that, despite apeparances, they're different
> namespaces.  Any thoughts on the implications of this?

glibc fcntl.h defines AT_EACCESS in the same way as aboves patch,
concerning the implementation, others should know better.

the dash code calling faccessat has the 4 params,
klibc faccessat had only 3 args, guess nobody had used it before.

the relevant dash code reads:
#ifdef HAVE_FACCESSAT
static int test_file_access(const char *path, int mode)
{
	return !faccessat(AT_FDCWD, path, mode, AT_EACCESS);
}
#else  /* HAVE_FACCESSAT */
 
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