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

[ebiederm@xmission.com (Eric W. Biederman)]

One Thousand Gnomes <gnomes@lxorguk.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