linux-kernel - Re: unlink(nonexistent): EROFS or ENOENT?

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Date:	Mon, 06 Jun 2011 21:18:45 +0400
From:	Michael Tokarev <mjt@....msk.ru>
To:	Ted Ts'o <tytso@....edu>,
	Linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: unlink(nonexistent): EROFS or ENOENT?

06.06.2011 21:13, Michael Tokarev wrote:
> Thank you for the answer.  I thought noone will reply... ;)
> 
> 06.06.2011 07:39, Ted Ts'o wrote:
>> On Sun, May 29, 2011 at 08:08:55PM +0400, Michael Tokarev wrote:
>>> Hello.
>>>
>>> Just noticed that at least on ext4, unlinking a
>>> non-existing file from a read-only filesystem
>>> results in EROFS instead of ENOENT.  I'd expect
>>> it return ENOENT - it is more logical, at least
>>> in my opinion.
>>>
>>> For one, (readonly) NFS mount returns ENOENT in
>>> this case.
>>
>> Um, it doesn't for me.   Testing on v3.0-rc1:
>>
>> # ls /test/foo; rm /test/foo
>> ls: cannot access /test/foo: No such file or directory
>> rm: cannot remove `/test/foo': No such file or directory
> 
> This is a hack in coreutils rm to work around this
> kernel change.  The comment at
>  http://git.savannah.gnu.org/cgit/coreutils.git/tree/src/remove.c#n450
> says:
> 
>   /* The unlinkat from kernels like linux-2.6.32 reports EROFS even for
>      nonexistent files.  When the file is indeed missing, map that to ENOENT,
>      so that rm -f ignores it, as required.  Even without -f, this is useful
>      because it makes rm print the more precise diagnostic.  */
> 
> so that rm(1) calls stat(2) to see if the file actually
> exist if unlinkat() returned EROFS, and turns this errno
> into ENOENT.

And another followup to this, -- the original case when I actually
noticed the problem.   A readonly-mounted root filesystem with /etc
in git (the repository is in /var, symlinked from /etc/.git).  I
deleted a few files from /etc (when it was readwrite), and noticed
that I forgot to commit the change.  So I used `git rm oldfiles' and
voila, git, for the first time, refused to commit stuff for me in
this configuration, -- before, I was always able to _commit_ the
changes even if the working tree is read-only.  It works for
everything but not for unlinks.

> That is, rm(1) output is not a good indicator.  Use
> 
>   strace rm -f /test/foo 2>&1 | grep unlink
> 
> to see the actual errno reported by the kernel.
> 
> Here's the POSIX description of unlink (and unlinkat) again:
> http://pubs.opengroup.org/onlinepubs/009695399/functions/unlink.html
> 
> Thanks!
> 
> /mjt
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