netdev - Re: [PATCH 2/3] bpf: Track alignment of MAP pointers in verifier.

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Message-Id: <20170516.150842.1909732065431538554.davem@davemloft.net>
Date:   Tue, 16 May 2017 15:08:42 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     daniel@...earbox.net
Cc:     ast@...com, alexei.starovoitov@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH 2/3] bpf: Track alignment of MAP pointers in verifier.

From: Daniel Borkmann <daniel@...earbox.net>
Date: Mon, 15 May 2017 23:55:47 +0200

> I'm actually wondering about the min_align/aux_off/aux_off_align and
> given this is not really related to varlen_map_access and we currently
> just skip this.
> 
> We should make sure that when env->strict_alignment is false that we
> ignore any difference in min_align/aux_off/aux_off_align, afaik, the
> min_align would also be set on regs other than ptr_to_pkt.

Ok I see what you are saying, alignment related register state has to
be taken into consideration during pruning but only when
env->strict_alignment is true.

->min_align is set on any register upon which a calculation is
performed.

> What about compare_ptrs_to_packet() for when env->strict_alignment is
> true in ptr_to_pkt case?

Yes we need to do something there, and yes we do need testcases.

You also remind me that I was thinking about whether we should
propagate alignment state through branches.  For example on
the taken path of a JEQ we can set both arms of the test to
have the largest of the two arms alignment.