linux-kernel - Re: [Bug Report] bpf: incorrectly pruning runtime execution path

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Date: Fri, 15 Dec 2023 18:22:45 +0200
From: Eduard Zingerman <eddyz87@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <alexei.starovoitov@...il.com>, Hao Sun
 <sunhao.th@...il.com>, Alexei Starovoitov <ast@...nel.org>, Andrii Nakryiko
 <andrii@...nel.org>, Daniel Borkmann <daniel@...earbox.net>, bpf
 <bpf@...r.kernel.org>, Linux Kernel Mailing List
 <linux-kernel@...r.kernel.org>
Subject: Re: [Bug Report] bpf: incorrectly pruning runtime execution path

On Thu, 2023-12-14 at 21:20 -0800, Andrii Nakryiko wrote:
[...]
> > > can we detect that any register link is broken and force checkpoint here?
> > 
> > Should be possible. I'll try this in the morning and check veristat results.

{still working on this}

> > By the way, I added some stats collection for find_equal_scalars() and see
> > the following results when run on ./test_progs:
> > - maximal number of registers with same id per call: 3
> > - average number of registers with same id per call: 1.4
> 
> What if we keep 8 extra bytes in jump/instruction history and encode
> up to 8 linked registers/slots:
> 
> 1. 1 bit to mark whether it's a src_reg set, or dst_reg set
> 2. 1 bit to mark whether it's a stack slot or register
> 3. 6 bits (0..63 values) to record register or slot number
> 
> If we ever need more than 8 linked registers, we can just forcefully
> some "links" by resetting some IDs?

That should work as well.
Probably don't need src/dst bit, as backtracker marks both as precise
when processing conditional jump.

You mean "just forcefully [breaking] some "links" by resetting ...", right?

> BTW, is it only conditional jumps that need to record this linked
> register sets? Did we previously discuss why we don't need this for
> any other operation?

Don't think that we discussed it.
Here is my reasoning: the range transfer happens at find_equal_scalars()
which is called only from check_cond_jmp_op().
I think there are no other effects IDs have for scalar values.
Thus, covering conditional jumps seems sufficient.