Re: [PATCHv2] net: bpf: reject invalid shifts

On Tue, Jan 12, 2016 at 06:11:38PM -0800, Eric Dumazet wrote:

On Tue, 2016-01-12 at 15:47 -0800, Alexei Starovoitov wrote:

quoted

I would agree if those loaded programs would do something sensible,
but they're broken. As shown arm and arm64 would execute them
differently without JIT, because HW treats such shifts differently.
I also checked that libpcap is sane and doesn't generate broken shifts.
imo we're not breaking backward compatiblity here.

How did you prove a particular code path was even taken in a BPF
program ? This is new to me.

Simple. I only found absolute constants for shift instructions
in libpcap source.

As I said, it is possible some guys never noticed their BPF program were
'broken' because this invalid shift was hidden in a dead code part.

So a program might appear as 'weak' when in fact its behavior was
absolutely correct.

You assume everybody uses libpcap, this is wrong, and for very obvious
reasons.

I didn't imply that.
Obviously there is chromium, libsecomp, lxd, dhclient, nmap and tons
of other apps. The point was for the library the most frequently
associated with classic bpf.

I think adding pr_err_once() to bpf_check_classic() as you
suggested makes the most sense to me at this point.
If anyone wants to submit a patch that masks K &= 31, I would ok with
it as well, but imo it's a disservice to classic bpf users.
Leaving it as-is and waiting for other jits to blow up is not an option.