linux-kernel - Re: General flags to turn things off (getrandom, pid lookup, etc)

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Date:	Wed, 30 Jul 2014 11:41:41 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>
Cc:	Andy Lutomirski <luto@...capital.net>,
	Paolo Bonzini <pbonzini@...hat.com>,
	linux-crypto@...r.kernel.org,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	James Morris <james.l.morris@...cle.com>,
	LSM List <linux-security-module@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Linux API <linux-api@...r.kernel.org>,
	Julien Tinnes <jln@...gle.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Paul Moore <paul@...l-moore.com>,
	David Drysdale <drysdale@...gle.com>,
	Kees Cook <keescook@...omium.org>,
	Meredydd Luff <meredydd@...atehouse.org>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: General flags to turn things off (getrandom, pid lookup, etc)

One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk> writes:

>> Andy you seem to be arguing here for two system calls.
>> get_urandom() and get_random().
>> 
>> Where get_urandom only blocks if there is not enough starting entropy,
>> and get_random(GRND_RANDOM) blocks if there is currently not enough
>> entropy.
>> 
>> That would allow -ENOSYS to be the right return value and it would
>> simply things for everyone.
>
> So you replace the "no file handle" special case with the "unsupported or
> disabled syscall" special case, which is even harder to test.
>
> Interfaces have failure modes. People who can't deal with that shouldn't
> be writing code that does anything important in languages which don't
> handle it for them.

Perhaps I misread the earlier conversation but it what I have read of
this discussion people want to disable some of get_random() modes with
seccomp.  Today get_random does not have any failure codes define except
-ENOSYS.

get_random(0) succeeding and get_random(GRND_RANDOM) returning -ENOSYS
has every chance of causing applications to legitimately assume the
get_random system call is not available in any mode.

So the code either needs a defined error code for bad flags (-EINVAL) or
we need to split the syscall in two.  Now that I think about it having
the seccomp filter return -EINVAL if it doesn't like the parameter is
better that splitting a syscall.  Presumably that is what
get_random(UNSUPPORTED_FLAG) returns.

Eric
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