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

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Message-Id: <20110125174515.C1DC2183C19@magilla.sf.frob.com>
Date:	Tue, 25 Jan 2011 09:45:15 -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

> My feeling is that it should be in glibc: as Mike mentioned, we don't normally
> change the behavior of existing system calls unless they are obviously
> broken to start with. If we want to keep fchmodat getting the implicit
> "." directory, and at the same time keep fchmod returning an error, the fchmod
> wrapper around fchmodat is the only place that can enforce this.

My point was that it's quite arguable that the *at syscall interfaces were
broken to begin with.  I've never seen anything suggesting their intent was
other than to permit relative pathnames, and the empty string has never
been a valid relative pathname.  To fit the POSIX requirements as I read
them, the *at functions must refuse to resolve the empty string.  So if the
kernel does not change and my interpretation of POSIX stands, then libc
must wrap all the *at syscalls with a function that checks for the empty
string and fails with ENOENT as a special case.

I don't have any strong opinion about this subject, but it makes the most
sense to me for the kernel's behavior to change.  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.

Thanks,
Roland
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