linux-ext4 - Re: /fs/ext4/namei.c ext4_find_dest

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Date:   Tue, 5 May 2020 19:07:23 +0100
From:   Jonny Grant <jg@...k.org>
To:     "Theodore Y. Ts'o" <tytso@....edu>
Cc:     linux-ext4@...r.kernel.org
Subject: Re: /fs/ext4/namei.c ext4_find_dest_de()



On 04/05/2020 20:52, Theodore Y. Ts'o wrote:
> On Mon, May 04, 2020 at 08:38:33AM +0100, Jonny Grant wrote:
>>>> I noticed that mkdir() returns EEXIST if a directory already exists.
>>>> strerror(EEXIST) text is "File exists"
>>>>
>>>> Can ext4_find_dest_de() be amended to return EISDIR if a directory already
>>>> exists? This will make the error message clearer.
>>>
>>> No; this will confuse potentially a large number of existing programs.
>>> Also, the current behavior is required by POSIx and the Single Unix
>>> Specification standards.
>>>
>>> 	https://pubs.opengroup.org/onlinepubs/009695399/
>>>
>> Is it likely POSIX would introduce this change? It's a shame we're still
>> constrained by old standards (SVr4, BSD), but it's fine if they can be
>> updated.
> 
> No, because it has the potential to break existing Unix/Linux/Posix-compliant
> programs.  There may very well be C programs doing the following....
> 
> 	   if (mkdir(filename) < 0) {
> 	   	if (errno != EEXIST) {
> 			perror(filename);
> 			exit(1);
> 		}
> 	}
> 
> For example, there may very well be implementations of "mkdir -p" that
> do precisely this.
> 
> If we change the error returned by the mkdir system call as you
> propose, it would break these innocent, unsuspecting programs.  That's
> not something which will be allowed, because it falls into the
> category of a Bad Thing.

Thank you for your reply.

What's an appropriate solution to this problem?

To achieve the desired output. when a directory exists.

$ mkdir test
$ mkdir: cannot create directory ‘test’: Is a directory

Cheers, Jonny