linux-kernel - Re: [PATCH 2/4] seccomp: rework seccomp_prepare

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Date:	Thu, 30 Apr 2015 14:46:37 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Nicolas Schichan <nschichan@...ebox.fr>,
	Kees Cook <keescook@...omium.org>,
	Andy Lutomirski <luto@...capital.net>,
	Will Drewry <wad@...omium.org>, linux-kernel@...r.kernel.org,
	ast@...mgrid.com, davem@...emloft.net
Subject: Re: [PATCH 2/4] seccomp: rework seccomp_prepare_filter().

On 04/30/2015 02:27 PM, Nicolas Schichan wrote:
...
> I'll take more care about the receiver list for the v2 of this serie.

Ok, cool.

>> I see, you need that to make it available to the old bpf_jit_compile()
>> for probing on classic JITs. Actually, I really would prefer, if instead
>> of duplicating that code, you could export bpf_prepare_filter() and
>> pass seccomp_check_filter() as an argument to bpf_prepare_filter().
>
> Just to be sure you want me to pass a pointer to seccomp_check_filter to
> bpf_prepare_filter so that it can run it between bpf_check_classic() and
> bpf_jit_compile ?

For example, what comes to mind is something along these lines:

struct bpf_prog *
bpf_prepare_filter(struct bpf_prog *fp,
		   int (*aux_trans_classic)(struct sock_filter *filter,
					    unsigned int flen))
{
	int err;

	fp->bpf_func = NULL;
	fp->jited = false;

	err = bpf_check_classic(fp->insns, fp->len);
	if (err) {
		__bpf_prog_release(fp);
		return ERR_PTR(err);
	}

	/* There might be additional checks and transformations
	 * needed on classic filters, f.e. in case of seccomp.
	 */
	if (aux_trans_classic) {
		err = aux_trans_classic(fp->insns,
					fp->len);
		if (err) {
			__bpf_prog_release(fp);
			return ERR_PTR(err);
		}
	}

	/* Probe if we can JIT compile the filter and if so, do
	 * the compilation of the filter.
	 */
	bpf_jit_compile(fp);

	/* JIT compiler couldn't process this filter, so do the
	 * internal BPF translation for the optimized interpreter.
	 */
	if (!fp->jited)
		fp = bpf_migrate_filter(fp);

	return fp;
}

 From seccomp side, you invoke:

... bpf_prepare_filter(fp, seccomp_check_filter);

I would actually like to see that customized code in seccomp_prepare_filter()
further reduced instead of increased, would certainly make it easier to
maintain and understand.

Do you think something like the above is possible?

>> Otherwise, in case bpf_prepare_filter() changes, people will easily
>> forget to update seccomp related code, really.
>
> Fair point.
>
> Thanks,

Thanks a lot,
Daniel
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