Date: Sat, 16 Jun 2018 19:31:44 +0200 From: Fabian Freyer <fabian.freyer@physik.tu-berlin.de> To: James Gritton <jamie@freebsd.org>, freebsd-jail@freebsd.org Cc: freebsd-hackers@freebsd.org Subject: Re: sizeof jail parameter values Message-ID: <d33a0f84-ca48-9f35-6ab0-61247dff2cb3@physik.tu-berlin.de> In-Reply-To: <e7b27a24a56dd883777804e058e31737@freebsd.org> References: <e7b27a24a56dd883777804e058e31737@freebsd.org>
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[reordered parts of the reply for better reading flow]
On 05/18/2018 18:49, James Gritton wrote:
> I would recommend skipping out on jail_getv(), which is really only
> good for getting a few well-known parameters, and instead use the more
> complete but more complex jailparam_init/get/export/free.
Thanks! I ended up writing wrappers around the jail_get(2) and
jail_set(2) interfaces and constructing the iovectors myself, which
ended up quite a bit cleaner. The jailparam_{init,get,export,free} APIs
are unnecessarily complex and don't seem to be a good fit (writing
wrappers around wrappers around wrappers etc...).
> There is a way to find the length of a string parameter, though there
> isn't a good library interface for it. The security.jail.param.*
> sysctls describe the form of the parameters, giving the type. The
> "contents" of these sysctls are generally unused (and set to zero), but
> for string parameters there's actually the max length of the string
> (itself in string form).
Thanks, this works great for strings!
> For non-string parameters, the length in
> string form depends on the type of the parameters, so for an int you'll
> need as long as the string representation of an ant can be, etc. I
> don't know how much good C code will do for you for Rust work, but you
> might want to take a look at jailparam_type() in the libjail source code.
> It gets more complicated with array parameters, those that can hold an
> arbitrary number of values. The IP addresses are the best example of
> that.
I've now hit that snag. I see that the security.jail.param.ip4.addr and
security.jail.param.ip6.addr sysctls contain the sizes of an in_addr_t
and an in6_addr_t, respectively. How would I now determine the number of
IPv4 and IPv6 addresses, or should I just allocate
security.jail.jail_max_af_ips per family? I've tried to go through how
libjail does it, but don't quite understand it, nor the implied race
conditions (?) it attempts to mitigate by reading the vector multiple times:
lib/libjail/jail.c:
/*
* Get the prison. If there are array elements, retry a few times
* in case their sizes changed from under us.
*/
for (sanity = 0;; sanity++) {
[...]
Fabian
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