Re: [PATCH -v5] random: introduce getrandom(2) system call

On Thu, Jul 24, 2014 at 4:24 PM, Theodore Ts'o [off-list ref] wrote:

On Thu, Jul 24, 2014 at 05:30:19PM -0300, Henrique de Moraes Holschuh wrote:

quoted

quoted

I wouldn't add the error to the man page until we actually modify the
kernel to add such a restriction.

By then, it might be too late.  It would be really sad to find ourselves
forced to return ENOSYS to getrandom(GRND_RANDOM) when we actually wanted to
return EPERM/EACCES.

I wouldn't worry about.  The reality is that anyone using GRND_RANDOM
has to be checking for error codes anyway, and if they do something
stupid because the system call returns EPERM/EACCESS when they weren't
expecting it, again, they are much more likely to be making many other
fatal mistakes anyway.

In general, all system calls can return errno's other than the ones
documented in the man page.  This is certainly true for open(2), and
read(2) if you are using a network file system such as NFS.  Someone
who assumes that the only errors that they have to handle is the list
in the man page, and assumes that this list is an exhaustive listing
of all possible errors, is going to be in a *world* of hurt.

I don't think it's necessary to add a sentence that other errors can
be returned in the future, and users much check for other errors, but
if you really think people are that stupid that we need to say
something which is true for every single system call in Linux, we can
do that....

I think that people might do:

try getrandom(GRND_RANDOM)
fall back to /dev/random
fall back to something intensely stupid

We want them to at least attempt the fallback from GRND_RANDOM to !GRND_RANDOM.

--Andy