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

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Date:	Tue, 25 Jan 2011 16:10:48 -0600
From:	Linas Vepstas <linasvepstas@...il.com>
To:	Eric Blake <eblake@...hat.com>
Cc:	Mike Frysinger <vapier@...too.org>, Arnd Bergmann <arnd@...db.de>,
	Paul Eggert <eggert@...ucla.edu>,
	Roland McGrath <roland@...hat.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
Subject: Re: [BUG] Generic syscalls -- chmod vs. fchmodat

Hi,

On 25 January 2011 15:32, Eric Blake <eblake@...hat.com> wrote:
>
> 7. fchmodat(AT_FDCWD, "") must fail with ENOENT.  Apparently, the kernel
> currently modifies ".", although point 1 says it might make more sense
> to fail with ENOENT so that glibc doesn't have to do as much work in
> chmod and fchmodat (if the kernel call does not change, then the burden
> is on glibc).

Interesting discussion, but I'm very embarrassed to say I started it
due to a mis-diagnosis of a bug :-(

In fact, the current kernels *do* check for an empty string, and do so
in order to maintain POSIX compliance, per Eric's points 5 and 7.
See fs/namei.c in do_getname, circa line 114,  where we read:

 * POSIX.1 2.4: an empty pathname is invalid (ENOENT).

This is backed up with code, circa line 129 ff:

   retval = strncpy_from_user(page, filename, len);
...
else if (!retval)
      retval = -ENOENT;
   return retval;

So an empty string will have length zero, and the ENOENT is indeed
set.

This handles cases 5 & 7.  I've no idea how any of the other cases
work out.

Note that getname() is used in a variety of places; its not just chmod,
but also in symlinks and many others.

--linas
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