[PATCH 0/2] landlock: truncate(2) support

[Mickaël Salaün]

On 04/08/2022 18:10, Günther Noack wrote:
> On Fri, Jul 29, 2022 at 01:58:17PM +0200, Mickaël Salaün wrote:

[...]

>>>> While we may question whether a dedicated access right should be added for
>>>> the Landlock use case, two arguments are in favor of this approach:
>>>> - For compatibility reasons, the kernel must follow the semantic of a
>>>> specific Landlock ABI, otherwise it could break user space. We could still
>>>> backport this patch and merge it with the ABI 1 and treat it as a bug, but
>>>> the initial version of Landlock was meant to be an MVP, hence this lack of
>>>> access right.
>>>> - There is a specific access right for Capsicum (CAP_FTRUNCATE) that could
>>>> makes more sense in the future.
>>>>
>>>> Following the Capsicum semantic, I think it would be a good idea to also
>>>> check for the O_TRUNC open flag:
>>>> https://www.freebsd.org/cgi/man.cgi?query=rights
>>>
>>> open() with O_TRUNC was indeed a case I had not thought about - thanks
>>> for pointing it out.
>>>
>>> I started adding some tests for it, and found to my surprise that
>>> open() *is* already checking security_path_truncate() when it is
>>> truncating files. So there is a chance that we can get away without a
>>> special check for O_TRUNC in the security_file_open hook.
>>>
>>> The exact semantics might be slightly different to Capsicum though -
>>> in particular, the creat() call (= open with O_TRUNC|O_CREAT|O_WRONLY)
>>> will require the Landlock truncate right when it's overwriting an
>>> existing regular file, but it will not require the Landlock truncate
>>> right when it's creating a new file.
>>
>> Is the creat() check really different from what is done by Capsicum?
> 
> TBH, I'm not sure, it might also do the same thing. I don't have a
> FreeBSD machine at hand and am not familiar with Capsicum in detail.
> Let me know if you think we should go to the effort of ensuring the
> compatibility down to that level.

I'll take a look at the code, but it makes sense to implement it like 
you did.