On Wed, Jun 11, 2014 at 3:18 PM, H. Peter Anvin [off-list ref] wrote:

On 06/11/2014 02:56 PM, Andy Lutomirski wrote:

quoted

13ns is with the simplest nonempty filter.  I hope that empty filters
don't work.

Why wouldn't they?

Is it permissible to fall off the end of a BPF program?  I'm getting
EINVAL trying to install an actual empty filter.  The filter I tested
with was:

#include <unistd.h>
#include <linux/filter.h>
#include <linux/seccomp.h>
#include <sys/syscall.h>
#include <err.h>
#include <sys/prctl.h>
#include <stddef.h>
#include <stdio.h>

int main(int argc, char **argv)
{
    int rc;

    struct sock_filter filter[] = {
        BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
    };

    struct sock_fprog prog = {
        .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
        .filter = filter,
    };

    if (argc < 2) {
        printf("Usage: null_seccomp PATH ARGS...\n");
        return 1;
    }

    if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0))
        err(1, "PR_SET_NO_NEW_PRIVS");
    if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog))
        err(1, "PR_SET_SECCOMP");

    execv(argv[1], argv + 1);
    err(1, argv[1]);
}


--Andy