linux-kernel - Re: [BUG] Generic syscalls -- chmod vs. fchmodat

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Message-Id: <20110125183437.7C6C2180999@magilla.sf.frob.com>
Date:	Tue, 25 Jan 2011 10:34:37 -0800 (PST)
From:	Roland McGrath <roland@...hat.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	linasvepstas@...il.com, Chris Metcalf <cmetcalf@...era.com>,
	GLIBC Devel <libc-alpha@...rceware.org>,
	linux-kernel@...r.kernel.org, libc-ports@...rceware.org,
	linux-api@...r.kernel.org, Mike Frysinger <vapier@...too.org>
Subject: Re: [BUG] Generic syscalls -- chmod vs. fchmodat

> On Tuesday 25 January 2011 18:45:15 Roland McGrath wrote:
> >  I know of no reason to
> > think that the current treatment of the empty string was ever intended at
> > the creation of the *at interfaces.
> 
> I always assumed that this was done so that the *at syscalls can replace
> both the ones that take a file descriptor (e.g. fstat) and the ones that
> take a pathname (e.g. stat), which is sensible for the non-AT_FDCWD case,
> although not documented in the man pages.

I see your point.  That is, having the empty string relative to a file
descriptor work means it can replace f* calls on non-directories, whereas
the standard method of passing "." for descriptor-relative resolution can
only work on a file descriptor open on a directory.  Is that what you mean?

I don't think this was part of the original intent when the calls were
added, but I suppose it makes sense.

> Treating the empty string special for AT_FDCWD is rather pointless, but
> at least consistent.

I agree about the consistency point.  However, one could also call it
consistent if the empty string fails to resolve when operating on either a
directory file descriptor or AT_FDCWD but works on a non-directory file
descriptor.  POSIX does not mandate that *at calls fail with ENOTDIR when
passed a non-directory file descriptor (it's a "may fail" error, not a
"shall fail" error).  So that behavior would be consistent both with the
POSIX requirements as I read them, and with the desire you mentioned to let
the fblahat system call serve to implement fblah as well as blah.  Then
libc would not have to wrap the *at calls with any special check to conform
to POSIX.

Thanks,
Roland
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