netdev - Re: [PATCH v2 net-next 1/5] selftests/bpf: add a test for a bug in liveness-based pruning

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Date:   Wed, 23 Aug 2017 07:43:04 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Edward Cree <ecree@...arflare.com>, <davem@...emloft.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>
CC:     <netdev@...r.kernel.org>,
        iovisor-dev <iovisor-dev@...ts.iovisor.org>
Subject: Re: [PATCH v2 net-next 1/5] selftests/bpf: add a test for a bug in
 liveness-based pruning

On 8/23/17 7:09 AM, Edward Cree wrote:
> Writes in straight-line code should not prevent reads from propagating
>  along jumps.  With current verifier code, the jump from 3 to 5 does not
>  add a read mark on 3:R0 (because 5:R0 has a write mark), meaning that
>  the jump from 1 to 3 gets pruned as safe even though R0 is NOT_INIT.
>
> Verifier output:
> 0: (61) r2 = *(u32 *)(r1 +0)
> 1: (35) if r2 >= 0x0 goto pc+1
>  R1=ctx(id=0,off=0,imm=0) R2=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
> 2: (b7) r0 = 0
> 3: (35) if r2 >= 0x0 goto pc+1
>  R0=inv0 R1=ctx(id=0,off=0,imm=0) R2=inv(id=0,umax_value=4294967295,var_off=(0x0; 0xffffffff)) R10=fp0
> 4: (b7) r0 = 0
> 5: (95) exit
>
> from 3 to 5: safe
>
> from 1 to 3: safe
> processed 8 insns, stack depth 0
>
> Signed-off-by: Edward Cree <ecree@...arflare.com>

Acked-by: Alexei Starovoitov <ast@...nel.org>