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Message-ID: <20150128145410.GH4574@brightrain.aerifal.cx>
Date: Wed, 28 Jan 2015 09:54:10 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: getrandom syscall
On Tue, Jan 27, 2015 at 11:12:46PM +0100, Daniel Cegiełka wrote:
> best regards,
> Daniel
Thanks. I've been wanting to get this added as well as a getentropy
function (the other API for the same thing).
> #include <stddef.h>
> #include <errno.h>
> #include "syscall.h"
>
> int getrandom(void *buf, size_t len)
> {
> int ret, pre_errno = errno;
There's no need to save/restore errno here. errno is only meaningful
after a function returns an error code. On success it should not be
inspected and could contain junk.
> if (len > 256) {
> errno = EIO;
> return -1;
> }
Could you explain the motivation for this part?
> do {
> ret = syscall(SYS_getrandom, buf, len, 0);
> } while (ret == -1 && errno == EINTR);
This would be more efficient (and avoid your errno issue entirely) if
you use __syscall which returns -errcode rather than storing errcode
in errno. It allows the whole loop to be inlined with no function
call. Something like:
while ((ret = __syscall(SYS_getrandom, buf, len, 0)) == -EINTR);
Of course there's the question of whether it should loop on EINTR
anyway; I don't know. Also if this can block there's the question of
whether it should be cancellable, but that can be decided later.
Finally, I wonder if it would make sense to use other fallbacks in the
case where the syscall is not supported -- perhaps the aux vector
AT_RANDOM or even /dev/urandom? (But I'd rather avoid doing anything
that depends on fds, which would make a function that appears to
always-work but actually fails on resource exhaustion.)
Rich
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