netdev - Re: [RFC PATCH 2/4] bpf: verifier, do explicit u32 bounds tracking

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Date:   Sun, 8 Mar 2020 22:39:52 -0700
From:   Yonghong Song <yhs@...com>
To:     John Fastabend <john.fastabend@...il.com>,
        <alexei.starovoitov@...il.com>, <daniel@...earbox.net>
CC:     <netdev@...r.kernel.org>, <bpf@...r.kernel.org>
Subject: Re: [RFC PATCH 2/4] bpf: verifier, do explicit u32 bounds tracking



On 3/6/20 4:22 PM, John Fastabend wrote:
> John Fastabend wrote:
>> It is not possible for the current verifier to track u32 alu ops and jmps
>> correctly. This can result in the verifier aborting with errors even though
>> the program should be verifiable. Cilium code base has hit this but worked
>> around it by changing int variables to u64 variables and marking a few
>> things volatile. It would be better to avoid these tricks.
> 
> Quick bit of clarification, originally I tried to just track u32 hence
> the title and above u32 reference. After runnning some programs I realized
> this wasn't really enough to handle all cases so I added the signed 32-bit
> bounds tracker. If I missed some spots in the descriptions that was just
> because I missed it in the proof reading here. u32 above should be 32-bit
> subreg.
> 
> I also forgot to give Yonhong credit. Sorry Yonghong! The original alu ops
> tracking patch came from him.

John, thanks for working on this! Hopefully this will resolve tricky 
32bit subreg tracking issues. I will look at the RFC in the next couple 
of days!