opensnoop on ARM64 fails to read filename

BCC's eBPF-based opensnoop tool [1] installs a
kprobe on do_sys_open to monitor calls to the open syscall globally.

do_sys_open() has prototype:

long do_sys_open(int dfd, const char __user *filename, int flags, umode_t mode);

This causes a "blank" filename to be displayed by opensnoop when I run it on
my Pixel 3 (arm64), possibly because this is a user pointer. However, it
works fine on x86-64.

So it seems to me that on arm64, reading user pointers directly still doesn't
work even if there is a distinction between user/kernel addresses. In that
case reading the user pointer using user accessors (possibly using
bpf_probe_user_read helper) should be needed to fix this issue

Start at the BCC opensnoop implementation and its do_sys_open kprobe entry point; compare how the filename argument is read on ARM64 versus x86-64. Reproduce the issue on a Pixel 3 or another ARM64 host, then verify that opensnoop displays nonblank filenames without regressing x86-64 behavior.

Written by the indexing model from the issue text.