Re: [RFC,PATCH 1/2] seccomp_filters: system call filtering using BPF

[Kees Cook <keescook@chromium.org>]

On Mon, Jan 16, 2012 at 10:46 PM, Indan Zupancic <indan@nul.nu> wrote:
> So call it once and store the value in a long. Then copy the low half
> to the right place and then the upper half when on 64 bits. It may not
> look too pretty, but the compiler should be able to optimise almost all
> overhead away and end up with 6 (or 12) int copies. Something like this:
>
> struct bpf_data {
>        uint32 syscall_nr;
>        uint32 arg_low[MAX_SC_ARGS];
>        uint32 arg_high[MAX_SC_ARGS];
> };
>
> void fill_bpf_data(struct task_struct *t, struct pt_regs *r, struct bpf_data *d)
> {
>        int i;
>        unsigned long arg;
>
>        d->syscall_nr = syscall_get_nr(t, r);
>        for (i = 0; i < MAX_SC_ARGS; ++i){
>                syscall_get_arguments(t, r, i, 1, &arg);
>                d->arg_low[i] = arg;
>                d->arg_high[i] = arg >> 32;
>        }
> }

If this turns out to be expensive, it might be possible to break it up
and load the arguments on demand (and cache them); i.e. have
load_pointer() or similar notice when it is about to access something
other than bpf_data.syscall_nr.

-Kees

-- 
Kees Cook
ChromeOS Security